iOS-单例创建两种方式

左手的ㄟ右手 2023-10-17 02:00 47阅读 0赞

**创建单例**

有两种方法来创建单例，下面分别介绍

**1、GCD方式创建单例**

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      <code>static&nbsp;id&nbsp;_instance;</code> 
     </div> 
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      <code>+&nbsp;(instancetype)allocWithZone:(struct&nbsp;_NSZone&nbsp;*)zone</code> 
     </div> 
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      <code>{ </code> 
     </div> 
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      <code>static&nbsp;dispatch_once_t&nbsp;onceToken;</code> 
     </div> 
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      <code>dispatch_once(&amp;onceToken,&nbsp;^{ </code> 
     </div> 
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      <code>_instance&nbsp;=&nbsp;[</code> 
      <code>super</code>&nbsp; 
      <code>allocWithZone:zone];</code> 
     </div> 
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      <code>});</code> 
     </div> 
     <div style="margin-left:0px;"> 
      <code>return</code>&nbsp; 
      <code>_instance;</code> 
     </div> 
     <div style="margin-left:0px;"> 
      <code>}</code> 
     </div> 
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      <code>+&nbsp;(instancetype)sharedInstance</code> 
     </div> 
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      <code>{ </code> 
     </div> 
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      <code>static&nbsp;dispatch_once_t&nbsp;onceToken;</code> 
     </div> 
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      <code>dispatch_once(&amp;onceToken,&nbsp;^{ </code> 
     </div> 
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      <code>_instance&nbsp;=&nbsp;[[self&nbsp;alloc]&nbsp;init];</code> 
     </div> 
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      <code>});</code> 
     </div> 
     <div style="margin-left:0px;"> 
      <code>return</code>&nbsp; 
      <code>_instance;</code> 
     </div> 
     <div style="margin-left:0px;"> 
      <code>}</code> 
     </div> 
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      <code>-&nbsp;(id)copyWithZone:(NSZone&nbsp;*)zone</code> 
     </div> 
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      <code>{ </code> 
     </div> 
     <div style="margin-left:0px;"> 
      <code>return</code>&nbsp; 
      <code>_instance;</code> 
     </div> 
     <div style="margin-left:0px;"> 
      <code>}</code> 
     </div> 
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      <code>-&nbsp;(id)mutableCopyWithZone:(NSZone&nbsp;*)zone&nbsp;{ </code> 
     </div> 
     <div style="margin-left:0px;"> 
      <code>return</code>&nbsp; 
      <code>_instance;</code> 
     </div> 
     <div style="margin-left:0px;"> 
      <code>}</code> 
     </div> 
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2、互斥锁方式

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      <code>static&nbsp;id&nbsp;_instance;</code> 
     </div> 
     <div style="margin-left:0px;"> 
      <code>+&nbsp;(instancetype)allocWithZone:(struct&nbsp;_NSZone&nbsp;*)zone</code> 
     </div> 
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      <code>{ </code> 
     </div> 
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      <code>@synchronized(self)&nbsp;{ </code> 
     </div> 
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      <code>if</code>&nbsp; 
      <code>(_instance&nbsp;==&nbsp;nil)&nbsp;{ </code> 
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      <code>_instance&nbsp;=&nbsp;[</code> 
      <code>super</code>&nbsp; 
      <code>allocWithZone:zone];</code> 
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      <code>}</code> 
     </div> 
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      <code>}</code> 
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      <code>return</code>&nbsp; 
      <code>_instance;</code> 
     </div> 
     <div style="margin-left:0px;"> 
      <code>}</code> 
     </div> 
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      <code>+&nbsp;(instancetype)sharedInstance</code> 
     </div> 
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      <code>{ </code> 
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      <code>@synchronized(self)&nbsp;{ </code> 
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      <code>if</code>&nbsp; 
      <code>(_instance&nbsp;==&nbsp;nil)&nbsp;{ </code> 
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      <code>_instance&nbsp;=&nbsp;[[self&nbsp;alloc]&nbsp;init];</code> 
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      <code>}</code> 
     </div> 
     <div style="margin-left:0px;"> 
      <code>}</code> 
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      <code>return</code>&nbsp; 
      <code>_instance;</code> 
     </div> 
     <div style="margin-left:0px;"> 
      <code>}</code> 
     </div> 
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      <code>-&nbsp;(id)copyWithZone:(NSZone&nbsp;*)zone</code> 
     </div> 
     <div style="margin-left:0px;"> 
      <code>{ </code> 
     </div> 
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      <code>return</code>&nbsp; 
      <code>_instance;</code> 
     </div> 
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      <code>}</code> 
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上面两种方式都可以创建单例，而且保证了用户不管是通过shareInstance方法，还是alloc、copy方法得到的实例都是一样的。

上面代码的关键之处就在于如何在多线程情况下保证创建的单例还是同一个。

我们先看看在GCD情况下，如果不使用dispatch\_once和同步锁创建单例会出现什么问题，去掉两者后创建单例的代码如下

假设此时有两条线程：线程1和线程2，都在调用shareInstance方法来创建单例，那么线程1运行到if (\_instance == nil)出发现\_instance = nil,那么就会初始化一个\_instance，假设此时线程2也运行到if的判断处了，此时线程1还没有创建完成实例\_instance，所以此时\_instance = nil还是成立的，那么线程2又会创建一个\_instace。

此时就创建了两个实例对象，导致问题。

**解决办法1、使用dispatch\_once**

dispatch\_once保证程序在运行过程中只会被运行一次，那么假设此时线程1先执行shareInstance方法，创建了一个实例对象，线程2就不会再去执行dispatch\_once的代码了。从而保证了只会创建一个实例对象。

**解决办法2、使用互斥锁**

假设此时线程1在执行shareInstance方法，那么synchronize大括号内创建单例的代码，如下所示：

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      <code>if</code>&nbsp; 
      <code>(_instance&nbsp;==&nbsp;nil)&nbsp;{ </code> 
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      <code>_instance&nbsp;=&nbsp;[[self&nbsp;alloc]&nbsp;init];</code> 
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      <code>}</code> 
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就会被当做一个任务被加上了一把锁。此时假设线程2也想执行shareInstance方法创建单例，但是看到了线程1加的互斥锁，就会进入睡眠模式。等到线程1执行完毕，才会被唤醒，然后去执行上面所示的创建单例的代码，但是此时\_instance !=nil,所以不会再创建新的实例对象了。从而保证只会创建一个实例对象。

但是互斥锁会影响性能，所以最好还是使用GCD方式创建单例。