blocks（闭包）学习笔记

小咪咪 2022-06-08 06:17 130阅读 0赞

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1、block 可以访问相同作用域范围内定义的变量

2、Blocks  的最大一个特色就是可以修改相同作用域的变量。你可以使用\_\_block 存 储类型修饰符来给出信号要修改一个变量。

在函数级别是\_\_block变量。这些在block里面是可变的(和封闭范围),并任何引 用 block 的都被保存一份副本到堆里面。

3、 一个 block  的大小是无法在编译的时候计算的。

4、 默认情况下当你在  block  里面引用一个  Objective-C  对象的时 候,该对象会被  retain。 但是被\_\_block 存储类型修饰符标记的对象变量不会被 retain.

如果你在实现方法的时候使用了 block,对象的内存管理规则更微妙:

如果你通过引用来访问一个实例变量,self 会被 retain。  
 如果你通过值来访问一个实例变量,那么变量会被 retain。

下面举例说明两个方式的不同:

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                   <pre style="display:inline!important"><span style="font-size:8pt; font-family:Courier; color:rgb(101,101,101)">dispatch_async(queue, ^{
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         <pre style="display:inline!important"><span style="font-size:8pt; font-family:Courier; color:rgb(101,101,101)">// instanceVariable is used by reference, self is retained</span></pre> 
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         <pre style="display:inline!important"><span style="font-size:8pt; font-family:Courier; color:rgb(101,101,101)">doSomethingWithObject(instanceVariable);</span></pre> 
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         <p style="display:inline!important"><span style="font-size:8pt; font-family:Courier; color:rgb(101,101,101)">});</span></p> 
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         <pre style="display:inline!important"><span style="font-size:8pt; font-family:Courier; color:rgb(101,101,101)">id localVariable = instanceVariable;</span></pre> 
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         <pre style="display:inline!important"><span style="font-size:8pt; font-family:Courier; color:rgb(101,101,101)">dispatch_async(queue, ^{
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         <p style="display:inline!important"><span style="font-size:8pt; font-family:Courier; color:rgb(101,101,101)">// localVariable is used by value, localVariable is retained (not self)</span></p> 
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         <pre style="display:inline!important"><span style="font-size:8pt; font-family:Courier; color:rgb(101,101,101)">doSomethingWithObject(localVariable);</span></pre> 
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             <p style="display:inline!important"><span style="font-size:8pt; font-family:Courier; color:rgb(101,101,101)">});</span><span style=""></span> </p> 
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5、 当你拷贝一个基于栈的  block  时,你会获得一个新的  block。但是如果你拷贝一个  基于堆的  block,你只是简单的递增了该 block  的引用数,并把原始的  block  作为函数 或方法的返回值

6、 通常,你不需要  copy(或 retain)一个 block.在你希望 block  在它被声明的作用域 被销毁后继续使用的话,你子需要做一份拷贝。拷贝会把 block 移到堆里面（block默认是在栈中）。

你可以使用 C 函数来  copy  和  release  一个  block:

如果你使用  Objective-C,你可以给一个 block  发送  copy、retain  和  release(或 autorelease)消息。

7、 调试  
你可以在 blocks 里面设置断点并单步调试。你可以在一个 GDB 的对话里面使用invoke-block 来调用一个 block。如下面的例子所示:

$ invoke-block myBlock 10 20

如果你想传递一个 C 字符串,你必须用引号括这它。例如,为了传递  this string  给  doSomethingWithString  的  block,你可以类似下面这样写:

$ invoke-block doSomethingWithString "\\"this string\\""

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