iOS 性能的优化

小鱼儿 2022-08-10 04:51 176阅读 0赞

## 初学者性能提升 ##

这个部分致力于一些能提高性能的基本改变。但所有层次的开发者都有可能会从这个记录了一些被忽视的项目的小小的性能备忘录里获得一些提升。

1. 用ARC管理内存

ARC(Automatic Reference Counting, 自动引用计数)和iOS5一起发布，它避免了最常见的也就是经常是由于我们忘记释放内存所造成的内存泄露。它自动为你管理retain和release的过程，所以你就不必去手动干预了。

下面是你会经常用来去创建一个View的代码段:

忘掉代码段结尾的release简直像记得吃饭一样简单。而ARC会自动在底层为你做这些工作。

除了帮你避免内存泄露，ARC还可以帮你提高性能，它能保证释放掉不再需要的对象的内存。这都啥年代了，你应该在你的所有项目里使用ARC!

这里有一些更多关于ARC的学习资源：

*  [Apple’s official documentation][Apple_s official documentation]
 *  Matthijs Hollemans’s [Beginning ARC in iOS Tutorial][]
 *  Tony Dahbura’s [How To Enable ARC in a Cocos2D 2.X Project][]
 *  If you still aren’t convinced of the benefits of ARC, check out this article on [eight myths about ARC][] to really convince you why you should be using it!

ARC当然不能为你排除所有内存泄露的可能性。由于阻塞, retain 周期, 管理不完善的CoreFoundation object(还有C结构)或者就是代码太烂依然能导致内存泄露。

这里有一篇很棒的介绍ARC不能做到以及我们该怎么做的文章 http://conradstoll.com/blog/2013/1/19/blocks-operations-and-retain-cycles.html。

2. 在正确的地方使用 reuseIdentifier[![ReuseIdentifier][]][ReuseIdentifier]

一个开发中常见的错误就是没有给UITableViewCells， UICollectionViewCells，甚至是UITableViewHeaderFooterViews设置正确的reuseIdentifier。

为了性能最优化，table view用 \`tableView:cellForRowAtIndexPath:\` 为rows分配cells的时候，它的数据应该重用自UITableViewCell。 一个table view维持一个队列的数据可重用的UITableViewCell对象。

不使用reuseIdentifier的话，每显示一行table view就不得不设置全新的cell。这对性能的影响可是相当大的，尤其会使app的滚动体验大打折扣。

自iOS6起，除了UICollectionView的cells和补充views，你也应该在header和footer views中使用reuseIdentifiers。

想要使用reuseIdentifiers的话，在一个table view中添加一个新的cell时在data source object中添加这个方法：

这个方法把那些已经存在的cell从队列中排除，或者在必要时使用先前注册的nib或者class创造新的cell。如果没有可重用的cell，你也没有注册一个class或者nib的话，这个方法返回nil。

3.尽量把views设置为不透明

[![Opaque][]][Opaque]

如果你有不透明的Views，你应该设置它们的opaque属性为YES。

原因是这会使系统用一个最优的方式渲染这些views。这个简单的属性在IB或者代码里都可以设定。

Apple的文档对于为图片设置不透明属性的描述是：

(opaque)这个属性给渲染系统提供了一个如何处理这个view的提示。如果设为YES， 渲染系统就认为这个view是完全不透明的，这使得渲染系统优化一些渲染过程和提高性能。如果设置为NO，渲染系统正常地和其它内容组成这个View。默认值是YES。

在相对比较静止的画面中，设置这个属性不会有太大影响。然而当这个view嵌在scroll view里边，或者是一个复杂动画的一部分，不设置这个属性的话会在很大程度上影响app的性能。

你可以在模拟器中用Debug\\Color Blended Layers选项来发现哪些view没有被设置为opaque。目标就是，能设为opaque的就全设为opaque!

4. 避免过于庞大的XIB

[![FatXIB][]][FatXIB]

iOS5中加入的Storyboards(分镜)正在快速取代XIB。然而XIB在一些场景中仍然很有用。比如你的app需要适应iOS5之前的设备，或者你有一个自定义的可重用的view,你就不可避免地要用到他们。

如果你不得不XIB的话，使他们尽量简单。尝试为每个Controller配置一个单独的XIB，尽可能把一个View Controller的view层次结构分散到单独的XIB中去。

需要注意的是，当你加载一个XIB的时候所有内容都被放在了内存里，包括任何图片。如果有一个不会即刻用到的view，你这就是在浪费宝贵的内存资源了。Storyboards就是另一码事儿了，storyboard仅在需要时实例化一个view controller.

当家在XIB是，所有图片都被chache，如果你在做OS X开发的话，声音文件也是。Apple在[相关文档中][Link 1]的记述是：

当你加载一个引用了图片或者声音资源的nib时，nib加载代码会把图片和声音文件写进内存。在OS X中，图片和声音资源被缓存在named cache中以便将来用到时获取。在iOS中，仅图片资源会被存进named caches。取决于你所在的平台，使用NSImage 或UIImage 的\`imageNamed:\`方法来获取图片资源。

很明显，同样的事情也发生在storyboards中，但我并没有找到任何支持这个结论的文档。如果你了解这个操作，写信给我！

想要了解更多关于storyboards的内容的话你可以看看 Matthijs Hollemans的Beginning Storyboards in iOS 5 [Part 1][] 和 [Part 2][]

5. 不要阻塞主线程

[![RainbowWheel][]][RainbowWheel]

永远不要使主线程承担过多。因为UIKit在主线程上做所有工作，渲染，管理触摸反应，回应输入等都需要在它上面完成。

一直使用主线程的风险就是如果你的代码真的block了主线程，你的app会失去反应。这。。。正是在App Store中拿到一颗星的捷径 :\]

大部分阻碍主进程的情形是你的app在做一些牵涉到读写外部资源的I/O操作，比如存储或者网络。

你可以使用\`NSURLConnection\`异步地做网络操作:  
\+ (void)sendAsynchronousRequest:(NSURLRequest \*)request queue:(NSOperationQueue \*)queue completionHandler:(void (^)(NSURLResponse\*, NSData\*, NSError\*))handler

或者使用像 [AFNetworking][]这样的框架来异步地做这些操作。

如果你需要做其它类型的需要耗费巨大资源的操作(比如时间敏感的计算或者存储读写)那就用 Grand Central Dispatch，或者 NSOperation 和 NSOperationQueues.

下面代码是使用GCD的模板

发现代码中有一个嵌套的\`dispatch\_async\`吗？这是因为任何UIKit相关的代码需要在主线程上进行。

如果你对 NSOperation 或者GCD 的细节感兴趣的话，看看Ray Wenderlich的 [Multithreading and Grand Central Dispatch on iOS for Beginners][]， 还有 Soheil Azarpour 的 [How To Use NSOperations and NSOperationQueues][] 教程。

6. 在Image Views中调整图片大小

[![ImageViews][]][ImageViews]

如果要在\`UIImageView\`中显示一个来自bundle的图片，你应保证图片的大小和UIImageView的大小相同。在运行中缩放图片是很耗费资源的，特别是\`UIImageView\`嵌套在\`UIScrollView\`中的情况下。

如果图片是从远端服务加载的你不能控制图片大小，比如在下载前调整到合适大小的话，你可以在下载完成后，最好是用background thread，缩放一次，然后在UIImageView中使用缩放后的图片。

7. 选择正确的Collection

[![Collections][]][Collections]

学会选择对业务场景最合适的类或者对象是写出能效高的代码的基础。当处理collections时这句话尤其正确。

Apple有一个 [Collections Programming Topics][] 的文档详尽介绍了可用的classes间的差别和你该在哪些场景中使用它们。这对于任何使用collections的人来说是一个必读的文档。

呵呵，我就知道你因为太长没看…这是一些常见collection的总结：

*  Arrays: 有序的一组值。使用index来lookup很快，使用value lookup很慢， 插入/删除很慢。
 *  Dictionaries: 存储键值对。 用键来查找比较快。
 *  Sets: 无序的一组值。用值来查找很快，插入/删除很快。

8. 打开gzip压缩

大量app依赖于远端资源和第三方API，你可能会开发一个需要从远端下载XML, JSON, HTML或者其它格式的app。

问题是我们的目标是移动设备，因此你就不能指望网络状况有多好。一个用户现在还在edge网络，下一分钟可能就切换到了3G。不论什么场景，你肯定不想让你的用户等太长时间。

减小文档的一个方式就是在服务端和你的app中打开gzip。这对于文字这种能有更高压缩率的数据来说会有更显著的效用。

好消息是，iOS已经在NSURLConnection中默认支持了gzip压缩，当然AFNetworking这些基于它的框架亦然。像Google App Engine这些云服务提供者也已经支持了压缩输出。

如果你不知道如何利用Apache或者IIS(服务器)来打开gzip，可以读下[这篇文章][Link 2]。

## 中级性能提升 ##

你确信你已经掌握了前述那些基础级的优化方案了吗？但实际情况是，有时一些解决方案并不像那些一样明显，它们往往严重依赖于你如何架构和书写你的app。下面的这些建议就是针对这些场景的。

9. 重用和延迟加载(lazy load) Views

更多的view意味着更多的渲染，也就是更多的CPU和内存消耗，对于那种嵌套了很多view在UIScrollView里边的app更是如此。

这里我们用到的技巧就是模仿\`UITableView\`和\`UICollectionView\`的操作: 不要一次创建所有的subview，而是当需要时才创建，当它们完成了使命，把他们放进一个可重用的队列中。

这样的话你就只需要在滚动发生时创建你的views，避免了不划算的内存分配。

创建views的能效问题也适用于你app的其它方面。想象一下一个用户点击一个按钮的时候需要呈现一个view的场景。有两种实现方法：

*  1. 创建并隐藏这个view当这个screen加载的时候，当需要时显示它；
 *  2. 当需要时才创建并展示。

每个方案都有其优缺点。

用第一种方案的话因为你需要一开始就创建一个view并保持它直到不再使用，这就会更加消耗内存。然而这也会使你的app操作更敏感因为当用户点击按钮的时候它只需要改变一下这个view的可见性。

第二种方案则相反-消耗更少内存，但是会在点击按钮的时候比第一种稍显卡顿。

10. Cache, Cache, 还是Cache!

一个极好的原则就是，缓存所需要的，也就是那些不大可能改变但是需要经常读取的东西。

我们能缓存些什么呢？一些选项是，远端服务器的响应，图片，甚至计算结果，比如UITableView的行高。

NSURLConnection默认会缓存资源在内存或者存储中根据它所加载的HTTP Headers。你甚至可以手动创建一个NSURLRequest然后使它只加载缓存的值。

下面是一个可用的代码段，你可以可以用它去为一个基本不会改变的图片创建一个NSURLRequest并缓存它：

<table style="border-collapse:collapse; border-spacing:0px; border:0px!important; margin:0px!important; padding:0px!important; font-size:12px!important; width:100%!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; min-height:inherit!important"> 
 <tbody style="border:0px!important; margin:0px!important; padding:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important"> 
  <tr style="border:0px!important; margin:0px!important; padding:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important"> 
   <td style="border:0px!important; margin:0px!important; padding:0px!important; vertical-align:baseline!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; width:auto!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; min-height:inherit!important; color:rgb(175,175,175)!important"> 
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      1 
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     <div style="border:0px!important; margin:0px!important; padding:0px 1em!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important; white-space:pre!important"> 
      <code style="border:0px!important; margin:0px!important; padding:0px!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important">&nbsp;&nbsp;&nbsp;&nbsp;</code> 
      <code style="border:0px!important; margin:0px!important; padding:0px!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important">request.cachePolicy = NSURLRequestReturnCacheDataElseLoad; </code> 
      <code style="display:block; clear:both; border:0px!important; margin:0px!important; padding:0px!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; overflow:visible!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important; color:rgb(0,130,0)!important">// this will make sure the request always returns the cached image</code> 
     </div> 
     <div style="border:0px!important; margin:0px!important; padding:0px 1em!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important; white-space:pre!important"> 
      <code style="border:0px!important; margin:0px!important; padding:0px!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important">&nbsp;&nbsp;&nbsp;&nbsp;</code> 
      <code style="border:0px!important; margin:0px!important; padding:0px!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important">request.HTTPShouldHandleCookies = NO;</code> 
     </div> 
     <div style="border:0px!important; margin:0px!important; padding:0px 1em!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important; white-space:pre!important"> 
      <code style="border:0px!important; margin:0px!important; padding:0px!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important">&nbsp;&nbsp;&nbsp;&nbsp;</code> 
      <code style="border:0px!important; margin:0px!important; padding:0px!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important">request.HTTPShouldUsePipelining = YES;</code> 
     </div> 
     <div style="border:0px!important; margin:0px!important; padding:0px 1em!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important; white-space:pre!important"> 
      <code style="border:0px!important; margin:0px!important; padding:0px!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important">&nbsp;&nbsp;&nbsp;&nbsp;</code> 
      <code style="border:0px!important; margin:0px!important; padding:0px!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important">[request addValue:@</code> 
      <code style="border:0px!important; margin:0px!important; padding:0px!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important; color:blue!important">"image/*"</code> 
      <code style="border:0px!important; margin:0px!important; padding:0px!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important"> forHTTPHeaderField:@</code> 
      <code style="border:0px!important; margin:0px!important; padding:0px!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important; color:blue!important">"Accept"</code> 
      <code style="border:0px!important; margin:0px!important; padding:0px!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important">];</code> 
     </div> 
     <div style="border:0px!important; margin:0px!important; padding:0px 1em!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important; white-space:pre!important">
       &nbsp; 
     </div> 
     <div style="border:0px!important; margin:0px!important; padding:0px 1em!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important; white-space:pre!important"> 
      <code style="border:0px!important; margin:0px!important; padding:0px!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important">&nbsp;&nbsp;&nbsp;&nbsp;</code> 
      <code style="border:0px!important; margin:0px!important; padding:0px!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; font-weight:bold!important; min-height:inherit!important; color:rgb(0,102,153)!important">return</code> 
      <code style="border:0px!important; margin:0px!important; padding:0px!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important"> request;</code> 
     </div> 
     <div style="border:0px!important; margin:0px!important; padding:0px 1em!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important; white-space:pre!important"> 
      <code style="border:0px!important; margin:0px!important; padding:0px!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important">}</code> 
     </div> 
    </div> </td> 
  </tr> 
 </tbody> 
</table>

注意你可以通过 NSURLConnection 获取一个URL request， AFNetworking也一样的。这样你就不必为采用这条tip而改变所有的networking代码了。

如果想了解更多关于HTTP caching, NSURLCache, NSURLConnection的相关知识，可以读下这篇[文章][Link 3]()

如果你需要缓存其它不是HTTP Request的东西，你可以用NSCache。

NSCache和NSDictionary类似，不同的是系统回收内存的时候它会自动删掉它的内容。 Mattt Thompson有一篇很棒的关于它的文章：:http://nshipster.com/nscache/

如果你对HTTP感兴趣可以读下Google的这篇 [best-practices document on HTTP caching][]。

11. 权衡渲染方法

在iOS中可以有很多方法做出漂亮的按钮。你可以用整幅的图片，可调大小的图片，uozhe可以用CALayer， CoreGraphics甚至OpenGL来画它们。

当然每个不同的解决方法都有不同的复杂程度和相应的性能。有一篇Apple UIKit team中的一员Andy Matuschak推荐过的很棒的关于graphic性能的[帖子][Link 4]很值得一读。

简单来说，就是用事先渲染好的图片更快一些，因为如此一来iOS就免去了创建一个图片再画东西上去然后显示在屏幕上的程序。问题是你需要把所有你需要用到的图片放到app的bundle里面，这样就增加了体积 – 这就是使用可变大小的图片更好的地方了: 你可以省去一些不必要的空间，也不需要再为不同的元素(比如按钮)来做不同的图。

然而，使用图片也意味着你失去了使用代码调整图片的机动性，你需要一遍又一遍不断地重做他们，这样就很浪费时间了，而且你如果要做一个动画效果，虽然每幅图只是一些细节的变化你就需要很多的图片造成bundle大小的不断增大。

总得来说，你需要权衡一下利弊，到底是要性能能还是要bundle保持合适的大小。

12. 处理内存警告

一旦系统内存过低，iOS会通知所有运行中app。在官方文档中是这样记述:

如果你的app收到了内存警告，它就需要尽可能释放更多的内存。最佳方式是移除对缓存，图片object和其他一些可以重创建的objects的strong references.

幸运的是，UIKit提供了几种收集低内存警告的方法:

*  在app delegate中使用\`applicationDidReceiveMemoryWarning:\` 的方法
 *  在你的自定义UIViewController的子类(subclass)中覆盖\`didReceiveMemoryWarning\`
 *  注册并接收 UIApplicationDidReceiveMemoryWarningNotification 的通知

一旦收到这类通知，你就需要释放任何不必要的内存使用。

例如，UIViewController的默认行为是移除一些不可见的view， 它的一些子类则可以补充这个方法，删掉一些额外的数据结构。一个有图片缓存的app可以移除不在屏幕上显示的图片。

这样对内存警报的处理是很必要的，若不重视，你的app就可能被系统杀掉。

然而，当你一定要确认你所选择的object是可以被重现创建的来释放内存。一定要在开发中用模拟器中的内存提醒模拟去测试一下。

13. 重用大开销对象

[![Reuse Expensive Objects][]][Reuse Expensive Objects]

一些objects的初始化很慢，比如NSDateFormatter和NSCalendar。然而，你又不可避免地需要使用它们，比如从JSON或者XML中解析数据。

想要避免使用这个对象的瓶颈你就需要重用他们，可以通过添加属性到你的class里或者创建静态变量来实现。

注意如果你要选择第二种方法，对象会在你的app运行时一直存在于内存中，和单例(singleton)很相似。

下面的代码说明了使用一个属性来延迟加载一个date formatter. 第一次调用时它会创建一个新的实例，以后的调用则将返回已经创建的实例：

<table style="border-collapse:collapse; border-spacing:0px; border:0px!important; margin:0px!important; padding:0px!important; font-size:12px!important; width:100%!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; min-height:inherit!important"> 
 <tbody style="border:0px!important; margin:0px!important; padding:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important"> 
  <tr style="border:0px!important; margin:0px!important; padding:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important"> 
   <td style="border:0px!important; margin:0px!important; padding:0px!important; vertical-align:baseline!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; width:auto!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; min-height:inherit!important; color:rgb(175,175,175)!important"> 
    <div style="border-width:0px 3px 0px 0px!important; border-right-style:solid!important; border-right-color:rgb(108,226,108)!important; margin:0px!important; padding:0px 0.5em 0px 1em!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; text-align:right!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important; white-space:pre!important">
      1 
    </div> 
    <div style="border-width:0px 3px 0px 0px!important; border-right-style:solid!important; border-right-color:rgb(108,226,108)!important; margin:0px!important; padding:0px 0.5em 0px 1em!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; text-align:right!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important; white-space:pre!important">
      2 
    </div> 
    <div style="border-width:0px 3px 0px 0px!important; border-right-style:solid!important; border-right-color:rgb(108,226,108)!important; margin:0px!important; padding:0px 0.5em 0px 1em!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; text-align:right!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important; white-space:pre!important">
      3 
    </div> 
    <div style="border-width:0px 3px 0px 0px!important; border-right-style:solid!important; border-right-color:rgb(108,226,108)!important; margin:0px!important; padding:0px 0.5em 0px 1em!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; text-align:right!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important; white-space:pre!important">
      4 
    </div> 
    <div style="border-width:0px 3px 0px 0px!important; border-right-style:solid!important; border-right-color:rgb(108,226,108)!important; margin:0px!important; padding:0px 0.5em 0px 1em!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; text-align:right!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important; white-space:pre!important">
      5 
    </div> 
    <div style="border-width:0px 3px 0px 0px!important; border-right-style:solid!important; border-right-color:rgb(108,226,108)!important; margin:0px!important; padding:0px 0.5em 0px 1em!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; text-align:right!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important; white-space:pre!important">
      6 
    </div> 
    <div style="border-width:0px 3px 0px 0px!important; border-right-style:solid!important; border-right-color:rgb(108,226,108)!important; margin:0px!important; padding:0px 0.5em 0px 1em!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; text-align:right!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important; white-space:pre!important">
      7 
    </div> 
    <div style="border-width:0px 3px 0px 0px!important; border-right-style:solid!important; border-right-color:rgb(108,226,108)!important; margin:0px!important; padding:0px 0.5em 0px 1em!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; text-align:right!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important; white-space:pre!important">
      8 
    </div> 
    <div style="border-width:0px 3px 0px 0px!important; border-right-style:solid!important; border-right-color:rgb(108,226,108)!important; margin:0px!important; padding:0px 0.5em 0px 1em!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; text-align:right!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important; white-space:pre!important">
      9 
    </div> 
    <div style="border-width:0px 3px 0px 0px!important; border-right-style:solid!important; border-right-color:rgb(108,226,108)!important; margin:0px!important; padding:0px 0.5em 0px 1em!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; text-align:right!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important; white-space:pre!important">
      10 
    </div> 
    <div style="border-width:0px 3px 0px 0px!important; border-right-style:solid!important; border-right-color:rgb(108,226,108)!important; margin:0px!important; padding:0px 0.5em 0px 1em!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; text-align:right!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important; white-space:pre!important">
      11 
    </div> 
    <div style="border-width:0px 3px 0px 0px!important; border-right-style:solid!important; border-right-color:rgb(108,226,108)!important; margin:0px!important; padding:0px 0.5em 0px 1em!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; text-align:right!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important; white-space:pre!important">
      12 
    </div> </td> 
   <td style="border:0px!important; margin:0px!important; padding:0px!important; vertical-align:baseline!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; width:100%!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; min-height:inherit!important"> 
    <div style="border:0px!important; margin:0px!important; padding:0px!important; width:auto!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:relative!important; right:auto!important; top:auto!important; vertical-align:baseline!important; min-height:inherit!important"> 
     <div style="border:0px!important; margin:0px!important; padding:0px 1em!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important; white-space:pre!important"> 
      <code style="display:block; clear:both; border:0px!important; margin:0px!important; padding:0px!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; overflow:visible!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important; color:rgb(0,130,0)!important">// in your .h or inside a class extension</code> 
     </div> 
     <div style="border:0px!important; margin:0px!important; padding:0px 1em!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important; white-space:pre!important"> 
      <code style="border:0px!important; margin:0px!important; padding:0px!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important">@property (nonatomic, strong) NSDateFormatter *formatter;</code> 
     </div> 
     <div style="border:0px!important; margin:0px!important; padding:0px 1em!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important; white-space:pre!important">
       &nbsp; 
     </div> 
     <div style="border:0px!important; margin:0px!important; padding:0px 1em!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important; white-space:pre!important"> 
      <code style="display:block; clear:both; border:0px!important; margin:0px!important; padding:0px!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; overflow:visible!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important; color:rgb(0,130,0)!important">// inside the implementation (.m)</code> 
     </div> 
     <div style="border:0px!important; margin:0px!important; padding:0px 1em!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important; white-space:pre!important"> 
      <code style="display:block; clear:both; border:0px!important; margin:0px!important; padding:0px!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; overflow:visible!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important; color:rgb(0,130,0)!important">// When you need, just use self.formatter</code> 
     </div> 
     <div style="border:0px!important; margin:0px!important; padding:0px 1em!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important; white-space:pre!important"> 
      <code style="border:0px!important; margin:0px!important; padding:0px!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important">- (NSDateFormatter *)formatter { </code> 
     </div> 
     <div style="border:0px!important; margin:0px!important; padding:0px 1em!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important; white-space:pre!important"> 
      <code style="border:0px!important; margin:0px!important; padding:0px!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important">&nbsp;&nbsp;&nbsp;&nbsp;</code> 
      <code style="border:0px!important; margin:0px!important; padding:0px!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; font-weight:bold!important; min-height:inherit!important; color:rgb(0,102,153)!important">if</code> 
      <code style="border:0px!important; margin:0px!important; padding:0px!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important"> (! _formatter) { </code> 
     </div> 
     <div style="border:0px!important; margin:0px!important; padding:0px 1em!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important; white-space:pre!important"> 
      <code style="border:0px!important; margin:0px!important; padding:0px!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</code> 
      <code style="border:0px!important; margin:0px!important; padding:0px!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important">_formatter = [[NSDateFormatter alloc] init];</code> 
     </div> 
     <div style="border:0px!important; margin:0px!important; padding:0px 1em!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important; white-space:pre!important"> 
      <code style="border:0px!important; margin:0px!important; padding:0px!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</code> 
      <code style="border:0px!important; margin:0px!important; padding:0px!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important">_formatter.dateFormat = @</code> 
      <code style="border:0px!important; margin:0px!important; padding:0px!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important; color:blue!important">"EEE MMM dd HH:mm:ss Z yyyy"</code> 
      <code style="border:0px!important; margin:0px!important; padding:0px!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important">; </code> 
      <code style="display:block; clear:both; border:0px!important; margin:0px!important; padding:0px!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; overflow:visible!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important; color:rgb(0,130,0)!important">// twitter date format</code> 
     </div> 
     <div style="border:0px!important; margin:0px!important; padding:0px 1em!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important; white-space:pre!important"> 
      <code style="border:0px!important; margin:0px!important; padding:0px!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important">&nbsp;&nbsp;&nbsp;&nbsp;</code> 
      <code style="border:0px!important; margin:0px!important; padding:0px!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important">}</code> 
     </div> 
     <div style="border:0px!important; margin:0px!important; padding:0px 1em!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important; white-space:pre!important"> 
      <code style="border:0px!important; margin:0px!important; padding:0px!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important">&nbsp;&nbsp;&nbsp;&nbsp;</code> 
      <code style="border:0px!important; margin:0px!important; padding:0px!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; font-weight:bold!important; min-height:inherit!important; color:rgb(0,102,153)!important">return</code> 
      <code style="border:0px!important; margin:0px!important; padding:0px!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important"> _formatter;</code> 
     </div> 
     <div style="border:0px!important; margin:0px!important; padding:0px 1em!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important; white-space:pre!important"> 
      <code style="border:0px!important; margin:0px!important; padding:0px!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important">}</code> 
     </div> 
    </div> </td> 
  </tr> 
 </tbody> 
</table>

还需要注意的是，其实设置一个NSDateFormatter的速度差不多是和创建新的一样慢的！所以如果你的app需要经常进行日期格式处理的话，你会从这个方法中得到不小的性能提升。

14. 使用Sprite Sheets

你是一个游戏开发者吗，那么Sprite sheets一定是一个你的最好的朋友了。Sprite sheet可以让渲染速度加快，甚至比标准的屏幕渲染方法节省内存。

我们有两个很好的关于Sprite的教程：

1.  [How To Use Animations and Sprite Sheets in Cocos2D][]
2.  [How to Create and Optimize Sprite Sheets in Cocos2D with Texture Packer and Pixel Formats][]

第二个教程涵盖了可能在很大程度上影响你游戏性能的pixel格式的细节。

如果你对于spirte sheet还不是很熟悉，可以看下这两个(youtube)视频SpriteSheets – The Movie, [Part 1][Part 1 1] 和 [Part 2][Part 2 1]。视频的作者是创建Sprite sheet很流行的工具之一Texture Packer的作者Andreas Löw。

除了使用Sprite sheets，其它写在这里的建议当然也可以用于游戏开发中。比如你需要很多的Sprite sheets，像敌人，导弹之类的动作类必备元素，你可以重用这些sprites而不用每次都要重新创建。

15. 避免反复处理数据

许多应用需要从服务器加载功能所需的常为JSON或者XML格式的数据。在服务器端和客户端使用相同的数据结构很重要。在内存中操作数据使它们满足你的数据结构是开销很大的。

比如你需要数据来展示一个table view,最好直接从服务器取array结构的数据以避免额外的中间数据结构改变。

类似的，如果需要从特定key中取数据，那么就使用键值对的dictionary。

16. 选择正确的数据格式

从app和网络服务间传输数据有很多方案，最常见的就是JSON和XML。你需要选择对你的app来说最合适的一个。

解析JSON会比XML更快一些，JSON也通常更小更便于传输。从iOS5起有了官方内建的[JSON deserialization][] 就更加方便使用了。

但是XML也有XML的好处，比如使用[SAX][] 来解析XML就像解析本地文件一样，你不需像解析json一样等到整个文档下载完成才开始解析。当你处理很大的数据的时候就会极大地减低内存消耗和增加性能。

17. 正确设定背景图片

在View里放背景图片就像很多其它iOS编程一样有很多方法:

1.  使用UIColor的 colorWithPatternImage来设置背景色；
2.  在view中添加一个UIImageView作为一个子View。

如果你使用全画幅的背景图，你就必须使用UIImageView因为UIColor的colorWithPatternImage是用来创建小的重复的图片作为背景的。这种情形下使用UIImageView可以节约不少的内存：

如果你用小图平铺来创建背景，你就需要用UIColor的colorWithPatternImage来做了，它会更快地渲染也不会花费很多内存：

18. 减少使用Web特性

UIWebView很有用，用它来展示网页内容或者创建UIKit很难做到的动画效果是很简单的一件事。

但是你可能有注意到UIWebView并不像驱动Safari的那么快。这是由于以[JIT compilation][] 为特色的Webkit的Nitro Engine的限制。

所以想要更高的性能你就要调整下你的HTML了。第一件要做的事就是尽可能移除不必要的javascript，避免使用过大的框架。能只用原生js就更好了。

另外，尽可能异步加载例如用户行为统计script这种不影响页面表达的javascript。

最后，永远要注意你使用的图片，保证图片的符合你使用的大小。使用Sprite sheet提高加载速度和节约内存。

更多相关信息可以看下 [WWDC 2012 session \#601 – Optimizing Web Content in UIWebViews and Websites on iOS][WWDC 2012 session _601 _ Optimizing Web Content in UIWebViews and Websites on iOS]

19. 设定Shadow Path

如何在一个View或者一个layer上加一个shadow呢，QuartzCore框架是很多开发者的选择：

看起来很简单，对吧。

可是，坏消息是使用这个方法也有它的问题… Core Animation不得不先在后台得出你的图形并加好阴影然后才渲染，这开销是很大的。

使用shadowPath的话就避免了这个问题：  
view.layer.shadowPath = \[\[UIBezierPath bezierPathWithRect:view.bounds\] CGPath\];

使用shadow path的话iOS就不必每次都计算如何渲染，它使用一个预先计算好的路径。但问题是自己计算path的话可能在某些View中比较困难，且每当view的frame变化的时候你都需要去update shadow path.

想了解更多可以看看Mark Pospesel的[这篇][Link 5]。

20. 优化Table View

Table view需要有很好的滚动性能，不然用户会在滚动过程中发现动画的瑕疵。

为了保证table view平滑滚动，确保你采取了以下的措施:

*  正确使用\`reuseIdentifier\`来重用cells
 *  尽量使所有的view opaque，包括cell自身
 *  避免渐变，图片缩放，后台选人
 *  缓存行高
 *  如果cell内现实的内容来自web，使用异步加载，缓存请求结果
 *  使用\`shadowPath\`来画阴影
 *  减少subviews的数量
 *  尽量不适用\`cellForRowAtIndexPath:\`，如果你需要用到它，只用一次然后缓存结果
 *  使用正确的数据结构来存储数据
 *  使用\`rowHeight\`, \`sectionFooterHeight\` 和 \`sectionHeaderHeight\`来设定固定的高，不要请求delegate

21. 选择正确的数据存储选项

当存储大块数据时你会怎么做？

你有很多选择，比如：

*  使用\`NSUerDefaults\`
 *  使用XML, JSON, 或者 plist
 *  使用NSCoding存档
 *  使用类似SQLite的本地SQL数据库
 *  使用 Core Data

NSUserDefaults的问题是什么？虽然它很nice也很便捷，但是它只适用于小数据，比如一些简单的布尔型的设置选项，再大点你就要考虑其它方式了

XML这种结构化档案呢？总体来说，你需要读取整个文件到内存里去解析，这样是很不经济的。使用SAX又是一个很麻烦的事情。

NSCoding？不幸的是，它也需要读写文件，所以也有以上问题。

在这种应用场景下，使用SQLite 或者 Core Data比较好。使用这些技术你用特定的查询语句就能只加载你需要的对象。

在性能层面来讲，SQLite和Core Data是很相似的。他们的不同在于具体使用方法。Core Data代表一个对象的graph model，但SQLite就是一个DBMS。Apple在一般情况下建议使用Core Data，但是如果你有理由不使用它，那么就去使用更加底层的SQLite吧。

如果你使用SQLite，你可以用FMDB(https://github.com/ccgus/fmdb)这个库来简化SQLite的操作，这样你就不用花很多经历了解SQLite的C API了。

## 进阶性能提示 ##

想要一些是你成为程序猿忍者的精英级的建议吗？下面这些提示可以帮你把你的app优化到极致！

22. 加速启动时间

快速打开app是很重要的，特别是用户第一次打开它时，对app来讲，第一印象太太太重要了。

你能做的就是使它尽可能做更多的异步任务，比如加载远端或者数据库数据，解析数据。

还是那句话，避免过于庞大的XIB，因为他们是在主线程上加载的。所以尽量使用没有这个问题的Storyboards吧！

注意，用Xcode debug时watchdog并不运行，一定要把设备从Xcode断开来测试启动速度

23. 使用Autorelease Pool

\`NSAutoreleasePool\`负责释放block中的autoreleased objects。一般情况下它会自动被UIKit调用。但是有些状况下你也需要手动去创建它。

假如你创建很多临时对象，你会发现内存一直在减少直到这些对象被release的时候。这是因为只有当UIKit用光了autorelease pool的时候memory才会被释放。

好消息是你可以在你自己的@autoreleasepool里创建临时的对象来避免这个行为：

<table style="border-collapse:collapse; border-spacing:0px; border:0px!important; margin:0px!important; padding:0px!important; font-size:12px!important; width:100%!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; min-height:inherit!important"> 
 <tbody style="border:0px!important; margin:0px!important; padding:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important"> 
  <tr style="border:0px!important; margin:0px!important; padding:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important"> 
   <td style="border:0px!important; margin:0px!important; padding:0px!important; vertical-align:baseline!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; width:auto!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; min-height:inherit!important; color:rgb(175,175,175)!important"> 
    <div style="border-width:0px 3px 0px 0px!important; border-right-style:solid!important; border-right-color:rgb(108,226,108)!important; margin:0px!important; padding:0px 0.5em 0px 1em!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; text-align:right!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important; white-space:pre!important">
      1 
    </div> 
    <div style="border-width:0px 3px 0px 0px!important; border-right-style:solid!important; border-right-color:rgb(108,226,108)!important; margin:0px!important; padding:0px 0.5em 0px 1em!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; text-align:right!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important; white-space:pre!important">
      2 
    </div> 
    <div style="border-width:0px 3px 0px 0px!important; border-right-style:solid!important; border-right-color:rgb(108,226,108)!important; margin:0px!important; padding:0px 0.5em 0px 1em!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; text-align:right!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important; white-space:pre!important">
      3 
    </div> 
    <div style="border-width:0px 3px 0px 0px!important; border-right-style:solid!important; border-right-color:rgb(108,226,108)!important; margin:0px!important; padding:0px 0.5em 0px 1em!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; text-align:right!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important; white-space:pre!important">
      4 
    </div> 
    <div style="border-width:0px 3px 0px 0px!important; border-right-style:solid!important; border-right-color:rgb(108,226,108)!important; margin:0px!important; padding:0px 0.5em 0px 1em!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; text-align:right!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important; white-space:pre!important">
      5 
    </div> 
    <div style="border-width:0px 3px 0px 0px!important; border-right-style:solid!important; border-right-color:rgb(108,226,108)!important; margin:0px!important; padding:0px 0.5em 0px 1em!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; text-align:right!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important; white-space:pre!important">
      6 
    </div> 
    <div style="border-width:0px 3px 0px 0px!important; border-right-style:solid!important; border-right-color:rgb(108,226,108)!important; margin:0px!important; padding:0px 0.5em 0px 1em!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; text-align:right!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important; white-space:pre!important">
      7 
    </div> 
    <div style="border-width:0px 3px 0px 0px!important; border-right-style:solid!important; border-right-color:rgb(108,226,108)!important; margin:0px!important; padding:0px 0.5em 0px 1em!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; text-align:right!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important; white-space:pre!important">
      8 
    </div> 
    <div style="border-width:0px 3px 0px 0px!important; border-right-style:solid!important; border-right-color:rgb(108,226,108)!important; margin:0px!important; padding:0px 0.5em 0px 1em!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; text-align:right!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important; white-space:pre!important">
      9 
    </div> </td> 
   <td style="border:0px!important; margin:0px!important; padding:0px!important; vertical-align:baseline!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; width:100%!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; min-height:inherit!important"> 
    <div style="border:0px!important; margin:0px!important; padding:0px!important; width:auto!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:relative!important; right:auto!important; top:auto!important; vertical-align:baseline!important; min-height:inherit!important"> 
     <div style="border:0px!important; margin:0px!important; padding:0px 1em!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important; white-space:pre!important"> 
      <code style="border:0px!important; margin:0px!important; padding:0px!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important">NSArray *urls = &lt;# An array of file URLs #&gt;;</code> 
     </div> 
     <div style="border:0px!important; margin:0px!important; padding:0px 1em!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important; white-space:pre!important"> 
      <code style="border:0px!important; margin:0px!important; padding:0px!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; font-weight:bold!important; min-height:inherit!important; color:rgb(0,102,153)!important">for</code> 
      <code style="border:0px!important; margin:0px!important; padding:0px!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important"> (NSURL *url in urls) { </code> 
     </div> 
     <div style="border:0px!important; margin:0px!important; padding:0px 1em!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important; white-space:pre!important"> 
      <code style="border:0px!important; margin:0px!important; padding:0px!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important">&nbsp;&nbsp;&nbsp;&nbsp;</code> 
      <code style="border:0px!important; margin:0px!important; padding:0px!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important">@autoreleasepool { </code> 
     </div> 
     <div style="border:0px!important; margin:0px!important; padding:0px 1em!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important; white-space:pre!important"> 
      <code style="border:0px!important; margin:0px!important; padding:0px!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</code> 
      <code style="border:0px!important; margin:0px!important; padding:0px!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important">NSError *error;</code> 
     </div> 
     <div style="border:0px!important; margin:0px!important; padding:0px 1em!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important; white-space:pre!important"> 
      <code style="border:0px!important; margin:0px!important; padding:0px!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</code> 
      <code style="border:0px!important; margin:0px!important; padding:0px!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important">NSString *fileContents = [NSString stringWithContentsOfURL:url</code> 
     </div> 
     <div style="border:0px!important; margin:0px!important; padding:0px 1em!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important; white-space:pre!important"> 
      <code style="border:0px!important; margin:0px!important; padding:0px!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</code> 
      <code style="border:0px!important; margin:0px!important; padding:0px!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important">encoding:NSUTF8StringEncoding error:&amp;error];</code> 
     </div> 
     <div style="border:0px!important; margin:0px!important; padding:0px 1em!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important; white-space:pre!important"> 
      <code style="border:0px!important; margin:0px!important; padding:0px!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</code> 
      <code style="display:block; clear:both; border:0px!important; margin:0px!important; padding:0px!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; overflow:visible!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important; color:rgb(0,130,0)!important">/* Process the string, creating and autoreleasing more objects. */</code> 
     </div> 
     <div style="border:0px!important; margin:0px!important; padding:0px 1em!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important; white-space:pre!important"> 
      <code style="border:0px!important; margin:0px!important; padding:0px!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important">&nbsp;&nbsp;&nbsp;&nbsp;</code> 
      <code style="border:0px!important; margin:0px!important; padding:0px!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important">}</code> 
     </div> 
     <div style="border:0px!important; margin:0px!important; padding:0px 1em!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important; white-space:pre!important"> 
      <code style="border:0px!important; margin:0px!important; padding:0px!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important">}</code> 
     </div> 
    </div> </td> 
  </tr> 
 </tbody> 
</table>

这段代码在每次遍历后释放所有autorelease对象

更多关于NSAutoreleasePool请参考[官方文档][Link 6]。

24. 选择是否缓存图片

常见的从bundle中加载图片的方式有两种，一个是用\`imageNamed\`，二是用\`imageWithContentsOfFile\`，第一种比较常见一点。

既然有两种类似的方法来实现相同的目的，那么他们之间的差别是什么呢？

\`imageNamed\`的优点是当加载时会缓存图片。\`[imageNamed\`的文档][imageNamed]中这么说:  
这个方法用一个指定的名字在系统缓存中查找并返回一个图片对象如果它存在的话。如果缓存中没有找到相应的图片，这个方法从指定的文档中加载然后缓存并返回这个对象。

相反的，\`imageWithContentsOfFile\`仅加载图片。

下面的代码说明了这两种方法的用法:

那么我们应该如何选择呢？

如果你要加载一个大图片而且是一次性使用，那么就没必要缓存这个图片，用\`imageWithContentsOfFile\`足矣，这样不会浪费内存来缓存它。

然而，在图片反复重用的情况下\`imageNamed\`是一个好得多的选择。

25. 避免日期格式转换

如果你要用\`NSDateFormatter\`来处理很多日期格式，应该小心以待。就像先前提到的，任何时候重用\`NSDateFormatters\`都是一个好的实践。

然而，如果你需要更多速度，那么直接用C是一个好的方案。Sam Soffes有一个不错的帖子(http://soff.es/how-to-drastically-improve-your-app-with-an-afternoon-and-instruments)里面有一些可以用来解析ISO-8601日期字符串的代码，简单重写一下就可以拿来用了。

嗯，直接用C来搞，看起来不错了，但是你相信吗，我们还有更好的方案！

如果你可以控制你所处理的日期格式，尽量选择Unix时间戳。你可以方便地从时间戳转换到NSDate:

这样会比用C来解析日期字符串还快！

需要注意的是，许多web API会以微秒的形式返回时间戳，因为这种格式在javascript中更方便使用。记住用\`dateFromUnixTimestamp\`之前除以1000就好了。

[Apple_s official documentation]: https://developer.apple.com/library/ios/#releasenotes/ObjectiveC/RN-TransitioningToARC/Introduction/Introduction.html
[Beginning ARC in iOS Tutorial]: http://www.raywenderlich.com/5677/beginning-arc-in-ios-5-part-1
[How To Enable ARC in a Cocos2D 2.X Project]: http://www.raywenderlich.com/23854/arc-and-cocos2d-v2-x
[eight myths about ARC]: http://www.learn-cocos2d.com/2012/06/mythbusting-8-reasons-arc/
[ReuseIdentifier]: /images/20220731/eedac8afe5914795ab348bac8dd2460c.png
[Opaque]: /images/20220731/5d1dfa32ddb04cf692b116a09aef629d.png
[FatXIB]: /images/20220731/e294b5184eae4bf8af9a4b66e4ec2952.png
[Link 1]: https://developer.apple.com/library/ios/#documentation/Cocoa/Conceptual/LoadingResources/CocoaNibs/CocoaNibs.html
[Part 1]: http://www.raywenderlich.com/5138/beginning-storyboards-in-ios-5-part-1
[Part 2]: http://www.raywenderlich.com/5191/beginning-storyboards-in-ios-5-part-2
[RainbowWheel]: /images/20220731/144a4dc1a19341e5b1ac3885544460f7.png
[AFNetworking]: http://www.raywenderlich.com/30445/afnetworking-crash-course
[Multithreading and Grand Central Dispatch on iOS for Beginners]: http://www.raywenderlich.com/4295/multithreading-and-grand-central-dispatch-on-ios-for-beginners-tutorial
[How To Use NSOperations and NSOperationQueues]: http://www.raywenderlich.com/19788/how-to-use-nsoperations-and-nsoperationqueues
[ImageViews]: /images/20220731/b55db5aa2a554d50a94fd80fe38cf2c4.png
[Collections]: /images/20220731/897ba63c298b4653accaf11b5c70c57a.png
[Collections Programming Topics]: https://developer.apple.com/library/ios/#documentation/cocoa/conceptual/collections/Collections.html
[Link 2]: http://betterexplained.com/articles/how-to-optimize-your-site-with-gzip-compression/
[Link 3]: http://nshipster.com/nsurlcache/
[best-practices document on HTTP caching]: https://developers.google.com/speed/docs/best-practices/caching
[Link 4]: http://robots.thoughtbot.com/post/36591648724/designing-for-ios-graphics-performance
[Reuse Expensive Objects]: /images/20220731/2650d738cfc4458cbc1f2e94029b02ab.png
[How To Use Animations and Sprite Sheets in Cocos2D]: http://www.raywenderlich.com/32045/how-to-use-animations-and-sprite-sheets-in-cocos2d-2-x
[How to Create and Optimize Sprite Sheets in Cocos2D with Texture Packer and Pixel Formats]: http://www.raywenderlich.com/2361/how-to-create-and-optimize-sprite-sheets-in-cocos2d-with-texture-packer-and-pixel-formats
[Part 1 1]: https://www.youtube.com/watch?v=crrFUYabm6E
[Part 2 1]: https://www.youtube.com/watch?v=_KyUqyS5MLA
[JSON deserialization]: http://www.raywenderlich.com/5492/working-with-json-in-ios-5
[SAX]: http://en.wikipedia.org/wiki/Simple_API_for_XML
[JIT compilation]: http://en.wikipedia.org/wiki/Just-in-time_compilation
[WWDC 2012 session _601 _ Optimizing Web Content in UIWebViews and Websites on iOS]: http://developer.apple.com/videos/wwdc/2012
[Link 5]: http://markpospesel.wordpress.com/2012/04/03/on-the-importance-of-setting-shadowpath/
[Link 6]: https://developer.apple.com/library/ios/#documentation/Cocoa/Conceptual/MemoryMgmt/Articles/mmAutoreleasePools.html
[imageNamed]: http://developer.apple.com/library/ios/#documentation/uikit/reference/UIImage_Class/Reference/Reference.html