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<h2 id="Block本质"><a href="#Block本质" class="headerlink" title="Block本质"></a>Block本质</h2><blockquote>
<p>1、Block本质是一个对象，内部有一个isa指针<br>2、封装了函数调用以及函数调用环境的OC对象</p>
</blockquote>
<h3 id="探索本质"><a href="#探索本质" class="headerlink" title="探索本质"></a>探索本质</h3><figure class="highlight objc"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">int</span> main(<span class="keyword">int</span> argc, <span class="keyword">const</span> <span class="keyword">char</span> * argv[]) {</span><br><span class="line"> <span class="keyword">@autoreleasepool</span> {</span><br><span class="line"> <span class="keyword">int</span> age = <span class="number">10</span>; <span class="comment">// 默认是 auto 类型变量</span></span><br><span class="line"> <span class="keyword">void</span>(^block)(<span class="keyword">int</span>, <span class="keyword">int</span>) = ^(<span class="keyword">int</span> a, <span class="keyword">int</span> b){</span><br><span class="line"> <span class="built_in">NSLog</span>(<span class="string">@"this is block a = %d, b = %d"</span>, a, b);</span><br><span class="line"> <span class="built_in">NSLog</span>(<span class="string">@"this is block, age = %d"</span>,age);</span><br><span class="line"> };</span><br><span class="line"> block(<span class="number">2</span>,<span class="number">1</span>);</span><br><span class="line"> }</span><br><span class="line"> <span class="keyword">return</span> <span class="number">0</span>;</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>将示例代码转换为C++查看内部结构，与OC代码进行比较，转换代码如下:<br><code>xcrun -sdk iphoneos clang -arch arm64 -rewrite-objc main.m</code></p>
<p>编译之后的代码如下</p>
<figure class="highlight objc"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">int</span> main(<span class="keyword">int</span> argc, <span class="keyword">const</span> <span class="keyword">char</span> * argv[]) {</span><br><span class="line"> <span class="comment">/* @autoreleasepool */</span> { __AtAutoreleasePool __autoreleasepool; </span><br><span class="line"> <span class="keyword">int</span> age = <span class="number">10</span>;</span><br><span class="line"> <span class="keyword">void</span>(*block)(<span class="keyword">int</span>, <span class="keyword">int</span>) = ((<span class="keyword">void</span> (*)(<span class="keyword">int</span>, <span class="keyword">int</span>))&__main_block_impl_0((<span class="keyword">void</span> *)__main_block_func_0, &__main_block_desc_0_DATA, age));</span><br><span class="line"></span><br><span class="line"> ((<span class="keyword">void</span> (*)(__block_impl *, <span class="keyword">int</span>, <span class="keyword">int</span>))((__block_impl *)block)->FuncPtr)((__block_impl *)block, <span class="number">2</span>, <span class="number">1</span>);</span><br><span class="line"> }</span><br><span class="line"> <span class="keyword">return</span> <span class="number">0</span>;</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<h4 id="定义block变量"><a href="#定义block变量" class="headerlink" title="定义block变量"></a>定义block变量</h4><figure class="highlight c++"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="built_in"><span class="keyword">void</span></span>(*block)(<span class="keyword">int</span>, <span class="keyword">int</span>) = ((<span class="built_in"><span class="keyword">void</span></span> (*)(<span class="keyword">int</span>, <span class="keyword">int</span>))&__main_block_impl_0((<span class="keyword">void</span> *)__main_block_func_0, &__main_block_desc_0_DATA, age));</span><br></pre></td></tr></table></figure>
<p>上述代码中，可以发现block的定义调用了 <code>__main_block_impl_0</code> 函数，并将 <code>__main_block_impl_0</code> 的地址赋值给了block。下面我们分析下 <code>__main_block_impl_0</code> 的内部结构。</p>
<h5 id="main-block-impl-0-结构体"><a href="#main-block-impl-0-结构体" class="headerlink" title="__main_block_impl_0 结构体"></a>__main_block_impl_0 结构体</h5><figure class="highlight c++"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br></pre></td><td class="code"><pre><span class="line"><span class="class"><span class="keyword">struct</span> __<span class="title">main_block_impl_0</span> {</span></span><br><span class="line"> <span class="class"><span class="keyword">struct</span> __<span class="title">block_impl</span> <span class="title">impl</span>;</span></span><br><span class="line"> <span class="class"><span class="keyword">struct</span> __<span class="title">main_block_desc_0</span>* <span class="title">Desc</span>;</span></span><br><span class="line"> <span class="keyword">int</span> age;</span><br><span class="line"> __main_block_impl_0(<span class="keyword">void</span> *fp, struct __main_block_desc_0 *desc, <span class="keyword">int</span> _age, <span class="keyword">int</span> flags=<span class="number">0</span>) : <span class="built_in">age</span>(_age) {</span><br><span class="line"> impl.isa = &_NSConcreteStackBlock;</span><br><span class="line"> impl.Flags = flags;</span><br><span class="line"> impl.FuncPtr = fp;</span><br><span class="line"> Desc = desc;</span><br><span class="line"> }</span><br><span class="line">};</span><br></pre></td></tr></table></figure>
<p>在 <code>__main_block_impl_0</code> 结构体内部有一个同名构造函数 <code>__main_block_impl_0</code>，构造函数对变量进行了赋值，最终返回一个结构体。<br>构造函数中对一些变量进行了赋值（或者其他操作）最终会返回一个结构体。<br><code>__main_block_impl_0</code> 内部的构造函数传入了四个参数，<code>(void *)__main_block_func_0</code>、<code>&__main_block_desc_0_DATA</code>、<code>age</code>、<code>flags</code>。其中 <code>flags</code> 有默认参数，在函数调用的时候可以省略不传。最后的 <code>age(_age)</code> 表示传入的 <code>_age</code> 会自动赋值给结构体内部的 <code>age</code> 成员，相当于 <code>age = _age</code>。</p>
<p>接下来分析一下前面三个参数的具体内容</p>
<h5 id="void-main-block-func-0"><a href="#void-main-block-func-0" class="headerlink" title="(void *)__main_block_func_0"></a>(void *)__main_block_func_0</h5><figure class="highlight c++"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">static</span> <span class="keyword">void</span> __main_block_func_0(struct __main_block_impl_0 *__cself, <span class="keyword">int</span> a, <span class="keyword">int</span> b) {</span><br><span class="line"> <span class="keyword">int</span> age = __cself->age; <span class="comment">// bound by copy</span></span><br><span class="line"> <span class="built_in">NSLog</span>((NSString *)&__NSConstantStringImpl__var_folders_nf_7714tb2x50vctp5b0v2xmpt4nrww4l_T_main_6c06d8_mi_0, a, b);</span><br><span class="line"> <span class="built_in">NSLog</span>((NSString *)&__NSConstantStringImpl__var_folders_nf_7714tb2x50vctp5b0v2xmpt4nrww4l_T_main_6c06d8_mi_1,age);</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>在 <code>__main_block_func_0</code> 函数中首先取出block中 <code>age</code> 的值，然后是两个NSLog的打印信息。在 <code>__main_block_func_0</code> 内部存储了block引用的外部的变量的值。而 <code>__main_block_impl_0</code> 函数中传入的第二个参数是 <code>(void *)__main_block_func_0</code>,也就是说block块内部的代码封装成了 <code>__main_block_func_0</code> 函数，并将 <code>__main_block_func_0</code> 函数的地址传入了 <code>__main_block_impl_0</code> 的构造函数中保存在了结构体内。</p>
<h5 id="amp-main-block-desc-0-DATA"><a href="#amp-main-block-desc-0-DATA" class="headerlink" title="&__main_block_desc_0_DATA"></a>&__main_block_desc_0_DATA</h5><figure class="highlight c++"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">static</span> <span class="class"><span class="keyword">struct</span> __<span class="title">main_block_desc_0</span> {</span></span><br><span class="line"> <span class="keyword">size_t</span> reserved;</span><br><span class="line"> <span class="keyword">size_t</span> Block_size;</span><br><span class="line">} __main_block_desc_0_DATA = { <span class="number">0</span>, <span class="built_in"><span class="keyword">sizeof</span></span>(struct __main_block_impl_0)};</span><br></pre></td></tr></table></figure>
<p>在 <code>__main_block_desc_0_DATA</code> 内部存储了两个参数，<code>reserved</code> 和 <code>Block_size</code>, <code>reserved</code> 直接赋值为0，<code>Block_size</code> 存储的是 <code>__main_block_impl_0</code> 所占用空间大小。最终将 <code>__main_block_desc_0</code> 结构体的地址传入 <code>__main_block_impl_0</code> 并赋值给 <code>Desc</code></p>
<h5 id="age"><a href="#age" class="headerlink" title="age"></a>age</h5><p><code>age</code> 是我们定义的局部变量，因为在block内部使用到了局部变量，所以block声明的时候，这里会将 <code>age</code> 作为参数传入，也就是说block会捕获 <code>age</code>，如果没有在block中使用 <code>age</code> ，这里将只会传入 <code>(void *)__main_block_func_0</code> 和 <code>&__main_block_desc_0_DATA</code> 这两个参数，如果有多个参数，那么传入的时候就会变成多个</p>
<figure class="highlight c++"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// 没有捕获的情况</span></span><br><span class="line"><span class="built_in"><span class="keyword">void</span></span>(*block)(<span class="keyword">int</span>, <span class="keyword">int</span>) = ((<span class="built_in"><span class="keyword">void</span></span> (*)(<span class="keyword">int</span>, <span class="keyword">int</span>))&__main_block_impl_0((<span class="keyword">void</span> *)__main_block_func_0, &__main_block_desc_0_DATA));</span><br><span class="line"><span class="comment">// 捕获多个变量</span></span><br><span class="line"><span class="built_in"><span class="keyword">void</span></span>(*block)(<span class="keyword">int</span>, <span class="keyword">int</span>) = ((<span class="built_in"><span class="keyword">void</span></span> (*)(<span class="keyword">int</span>, <span class="keyword">int</span>))&__main_block_impl_0((<span class="keyword">void</span> *)__main_block_func_0, &__main_block_desc_0_DATA, age, name, numer));</span><br></pre></td></tr></table></figure>
<h4 id="再次查看-main-block-impl-0"><a href="#再次查看-main-block-impl-0" class="headerlink" title="再次查看 __main_block_impl_0"></a>再次查看 __main_block_impl_0</h4><figure class="highlight c++"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br></pre></td><td class="code"><pre><span class="line"><span class="class"><span class="keyword">struct</span> __<span class="title">main_block_impl_0</span> {</span></span><br><span class="line"> <span class="class"><span class="keyword">struct</span> __<span class="title">block_impl</span> <span class="title">impl</span>;</span></span><br><span class="line"> <span class="class"><span class="keyword">struct</span> __<span class="title">main_block_desc_0</span>* <span class="title">Desc</span>;</span></span><br><span class="line"> <span class="keyword">int</span> age;</span><br><span class="line"> __main_block_impl_0(<span class="keyword">void</span> *fp, struct __main_block_desc_0 *desc, <span class="keyword">int</span> _age, <span class="keyword">int</span> flags=<span class="number">0</span>) : <span class="built_in">age</span>(_age) {</span><br><span class="line"> impl.isa = &_NSConcreteStackBlock;</span><br><span class="line"> impl.Flags = flags;</span><br><span class="line"> impl.FuncPtr = fp; <span class="comment">// block内部代码块地址</span></span><br><span class="line"> Desc = desc; <span class="comment">// block描述信息(占用内存大小)</span></span><br><span class="line"> }</span><br><span class="line">};</span><br></pre></td></tr></table></figure>
<p><code>__main_block_impl_0</code> 的第一个变量就是 <code>__block_impl</code> 结构体，内部代码如下：</p>
<figure class="highlight c++"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line"><span class="class"><span class="keyword">struct</span> __<span class="title">block_impl</span> {</span></span><br><span class="line"> <span class="keyword">void</span> *isa;</span><br><span class="line"> <span class="keyword">int</span> Flags;</span><br><span class="line"> <span class="keyword">int</span> Reserved;</span><br><span class="line"> <span class="keyword">void</span> *FuncPtr;</span><br><span class="line">};</span><br></pre></td></tr></table></figure>
<p><code>__block_impl</code> 内部有一个 <code>isa</code> 指针。因此可以证明block本质上就是一个OC对象。而在构造函数中将函数传入的值分别存储在 <code>__main_block_impl_0</code> 结构体实例中，最终将结构体的地址赋值给block。</p>
<p>通过上面对 <code>__main_block_impl_0</code> 结构体构造函数三个参数的问题，可以得出结论：<br>1.<code>__block_impl</code>结构体中 <code>isa</code> 指针存储着 <code>&_NSConcreteStackBlock</code> 地址，可以暂时理解为其类对象地址，block 就是 <code>_NSConcreteStackBlock</code> 类型的。<br>2.block代码块中的代码呗封装成 <code>__main_block_func_0</code> 函数，<code>FuncPtr</code> 存储着 <code>__main_block_func_0</code> 函数的地址。<br>3.<code>Desc</code> 指向 <code>__main_block_desc_0</code>结构体对象，其中存储 <code>__main_block_impl_0</code> 结构体所占用的内存大小。</p>
<h4 id="调用block执行内部代码"><a href="#调用block执行内部代码" class="headerlink" title="调用block执行内部代码"></a>调用block执行内部代码</h4><figure class="highlight c++"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// 执行block代码</span></span><br><span class="line">((<span class="built_in"><span class="keyword">void</span></span> (*)(__block_impl *, <span class="keyword">int</span>, <span class="keyword">int</span>))((__block_impl *)block)->FuncPtr)((__block_impl *)block, <span class="number">2</span>, <span class="number">1</span>);</span><br></pre></td></tr></table></figure>
<p>通过上述代码可以发现，block 调用是通过 <code>block</code> 找到 <code>FuncPtr</code> 直接调用，通上面分析可以明确 block 执行的是 __main_block_impl_0 类型的结构体，但是在 <code>__main_block_impl_0</code> 内部并不能直接找到 <code>FuncPtr</code>，<code>FuncPtr</code> 是存在 <code>__block_impl</code> 中的。</p>
<p><code>block</code> 可以直接调用 <code>__block_impl</code> 中的 <code>FuncPtr</code>原因，通过查看上述源码可以发现，<code>(__block_impl *)</code> 将 block 强制转换为了 <code>__block_impl</code> 类型，因为 <code>__block_impl</code> 是 <code>__main_block_impl_0</code> 结构体的第一个成员，相当于将 <code>__block_impl</code> 结构体的成员拿出来直接放在 <code>__main_block_impl_0</code> 中，也就说明 <code>__block_impl</code> 的内存地址就是 <code>__main_block_impl_0</code> 结构体内存地址的开头。所以可以转换成功，并找到 <code>FuncPtr</code> 成员。</p>
<p>通过上述可以知道，<code>FuncPtr</code> 存储着代码块的函数地址，调用此函数就会执行代码块中的代码，回头查看 <code>__main_block_func_0</code> ，可以发现第一个参数就是 <code>__main_block_impl_0</code> 类型的指针。也就是说将 <code>block</code> 传入 <code>__main_block_func_0</code> 函数中，便于从中捕获 <code>block</code> 的值。</p>
<h4 id="验证block的本质确实是-main-block-impl-0-结构体类型"><a href="#验证block的本质确实是-main-block-impl-0-结构体类型" class="headerlink" title="验证block的本质确实是 __main_block_impl_0 结构体类型"></a>验证block的本质确实是 __main_block_impl_0 结构体类型</h4><p>通过代码证明一下上述内容：通用使用之前的方法，我们按照上面分析block内部结构自定义结构体，并将block内部的结构体强制转换为自定义的结构体，转换成功说明底层结构体确实如之前分析的一样。</p>
<figure class="highlight objc"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">struct</span> __block_impl {</span><br><span class="line"> <span class="keyword">void</span> *isa;</span><br><span class="line"> <span class="keyword">int</span> Flags;</span><br><span class="line"> <span class="keyword">int</span> Reserved;</span><br><span class="line"> <span class="keyword">void</span> *FuncPtr;</span><br><span class="line">};</span><br><span class="line"></span><br><span class="line"><span class="keyword">struct</span> __main_block_desc_0 {</span><br><span class="line"> size_t reserved;</span><br><span class="line"> size_t Block_size;</span><br><span class="line">};</span><br><span class="line"></span><br><span class="line"><span class="keyword">struct</span> __main_block_impl_0 {</span><br><span class="line"> <span class="keyword">struct</span> __block_impl impl;</span><br><span class="line"> <span class="keyword">struct</span> __main_block_desc_0* Desc;</span><br><span class="line"> <span class="keyword">int</span> age;</span><br><span class="line">};</span><br><span class="line"></span><br><span class="line"><span class="keyword">int</span> main(<span class="keyword">int</span> argc, <span class="keyword">const</span> <span class="keyword">char</span> * argv[]) {</span><br><span class="line"> <span class="keyword">@autoreleasepool</span> {</span><br><span class="line"> <span class="keyword">int</span> age = <span class="number">10</span>; <span class="comment">// 默认是 auto 类型变量</span></span><br><span class="line"> <span class="keyword">void</span>(^block)(<span class="keyword">int</span>, <span class="keyword">int</span>) = ^(<span class="keyword">int</span> a, <span class="keyword">int</span> b){</span><br><span class="line"> <span class="built_in">NSLog</span>(<span class="string">@"this is block a = %d, b = %d"</span>, a, b);</span><br><span class="line"> <span class="built_in">NSLog</span>(<span class="string">@"this is block, age = %d"</span>,age);</span><br><span class="line"> };</span><br><span class="line"> <span class="comment">// 进行类型转换</span></span><br><span class="line"> <span class="keyword">struct</span> __main_block_impl_0 *blockStruct = (__bridge <span class="keyword">struct</span> __main_block_impl_0 *)block;</span><br><span class="line"> block(<span class="number">2</span>,<span class="number">1</span>);</span><br><span class="line"> }</span><br><span class="line"> <span class="keyword">return</span> <span class="number">0</span>;</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>通过xcode断点可以看出自定义结构体可以被成功赋值。</p>
<p> <img src="https://xhp281-blog.oss-cn-beijing.aliyuncs.com/ios_objc/block_%E8%87%AA%E5%AE%9A%E4%B9%89%E6%9E%84%E4%BD%93%E9%AA%8C%E8%AF%81.jpeg" alt="block_自定义构体验证"></p>
<p>接下来断点来到block代码块中，看下对战信息中心的函数调用地址。<code>Debug -> Debug workflow -> always show Disassembly</code> 。</p>
<p> <img src="https://xhp281-blog.oss-cn-beijing.aliyuncs.com/ios_objc/block_%E8%87%AA%E5%AE%9A%E4%B9%89%E6%9E%84%E4%BD%93%E9%AA%8C%E8%AF%81_%E6%B1%87%E7%BC%96.jpeg" alt="block_自定义构体验证_汇编"></p>
<p>通过上图可以看到 <code>block</code> 的地址确实和 <code>FuncPtr</code> 的地址是一样的。</p>
<h3 id="总结"><a href="#总结" class="headerlink" title="总结"></a>总结</h3><p>通过上述分析已经对block底层有了一个基本认识，将上述代码转换为一张图片看下具体的关系。</p>
<p><img src="https://xhp281-blog.oss-cn-beijing.aliyuncs.com/ios_objc/block_%E7%BB%93%E6%9E%84%E4%BD%93%E5%85%B3%E7%B3%BB%E5%9B%BE.png"></p>
<p>block的底层数据可以通过一张图来展示</p>
<p><img src="https://xhp281-blog.oss-cn-beijing.aliyuncs.com/ios_objc/block_%E5%BA%95%E5%B1%82%E6%95%B0%E6%8D%AE%E7%BB%93%E6%9E%84.png" alt="block_底层数据结构"></p>
<h3 id="block变量的捕获"><a href="#block变量的捕获" class="headerlink" title="block变量的捕获"></a>block变量的捕获</h3><p>为了保证block内部能正常访问外部的变量，block有一个变量捕获机制。</p>
<h4 id="局部变量"><a href="#局部变量" class="headerlink" title="局部变量"></a>局部变量</h4><h5 id="auto变量"><a href="#auto变量" class="headerlink" title="auto变量"></a>auto变量</h5><p>上述代码中已经了解过block对age变量的捕获。auto自动变量，离开作用域就销毁，通常局部变量前面自动添加 <code>auto</code> 关键字。自动变量会捕获到block内部，也就是说block会专门新增一个参数来存储变量的值。<code>auto</code> 只存在于局部变量中，访问方式为值传递，通过上述对age参数额解释，我们也可以确定确实是值传递。</p>
<h5 id="static变量"><a href="#static变量" class="headerlink" title="static变量"></a>static变量</h5><p>static修饰的变量为指针传递，同样会被block捕获。</p>
<p>接下来分别添加auto修饰的局部变量和static修饰的局部变量，通过源码查看下他们的区别。</p>
<figure class="highlight objc"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">int</span> main(<span class="keyword">int</span> argc, <span class="keyword">const</span> <span class="keyword">char</span> * argv[]) {</span><br><span class="line"> <span class="keyword">@autoreleasepool</span> {</span><br><span class="line"> auto <span class="keyword">int</span> a = <span class="number">66</span>;</span><br><span class="line"> <span class="keyword">static</span> <span class="keyword">int</span> b = <span class="number">99</span>;</span><br><span class="line"> <span class="keyword">void</span>(^block)(<span class="keyword">void</span>) = ^{</span><br><span class="line"> <span class="built_in">NSLog</span>(<span class="string">@"this is block a = %d, b = %d"</span>, a, b);</span><br><span class="line"> };</span><br><span class="line"> a = <span class="number">88</span>;</span><br><span class="line"> b = <span class="number">77</span>;</span><br><span class="line"> block();</span><br><span class="line"> }</span><br><span class="line"> <span class="keyword">return</span> <span class="number">0</span>;</span><br><span class="line">}</span><br><span class="line"><span class="comment">// log：this is block a = 66, b = 77</span></span><br><span class="line"><span class="comment">// block中a的值并没有被改变，b的值被改变了</span></span><br></pre></td></tr></table></figure>
<p>将源码转换成c++代码之后的两个参数的区别如下图：</p>
<p><img src="https://xhp281-blog.oss-cn-beijing.aliyuncs.com/ios_objc/block_%E4%B8%8D%E5%90%8C%E5%8F%98%E9%87%8F%E6%8D%95%E8%8E%B7%E7%9A%84%E5%8C%BA%E5%88%AB.jpeg"></p>
<p>从上述源码中可以看出，a，b两个变量都有捕获到block内部。但是a传入的是值，b传入的是地址。</p>
<p>这两种变量产生差异的原因是，自动变量可能会销毁，block执行的时候自动变量有可能已经被销毁了，那么此时再去访问被销毁的地址肯定会发生坏的内存访问，因此对于自动变量一定是值传递而不是指针传递。而静态变量不会被销毁，所以完全可以传递地址。因为传递的是值的地址，所以block调用之前修改地址中保存的值，block中的地址是不会变的，所以值会随之改变。</p>
<h4 id="全局变量"><a href="#全局变量" class="headerlink" title="全局变量"></a>全局变量</h4><p>示例代码：</p>
<figure class="highlight objc"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">int</span> a = <span class="number">66</span>;</span><br><span class="line"><span class="keyword">static</span> <span class="keyword">int</span> b = <span class="number">99</span>;</span><br><span class="line"><span class="keyword">int</span> main(<span class="keyword">int</span> argc, <span class="keyword">const</span> <span class="keyword">char</span> * argv[]) {</span><br><span class="line"> <span class="keyword">@autoreleasepool</span> {</span><br><span class="line"> <span class="keyword">void</span>(^block)(<span class="keyword">void</span>) = ^{</span><br><span class="line"> <span class="built_in">NSLog</span>(<span class="string">@"this is block a = %d, b = %d"</span>, a, b);</span><br><span class="line"> };</span><br><span class="line"> a = <span class="number">88</span>;</span><br><span class="line"> b = <span class="number">77</span>;</span><br><span class="line"> block();</span><br><span class="line"> }</span><br><span class="line"> <span class="keyword">return</span> <span class="number">0</span>;</span><br><span class="line">}</span><br><span class="line"><span class="comment">// log：this is block a = 88, b = 77</span></span><br></pre></td></tr></table></figure>
<p>生成的c++代码看下全局调用方式，可以发现结构体中没有a，b的成员变量，传递的时候是直接调用。</p>
<p><img src="https://xhp281-blog.oss-cn-beijing.aliyuncs.com/ios_objc/block_%E5%85%A8%E5%B1%80%E5%8F%98%E9%87%8F_c%2B%2B%E6%BA%90%E7%A0%81.jpg" alt="block_全局变量_c++源码"></p>
<p>通过上述代码可以发现，<code>__main_block_impl_0</code> 没有添加任何变量，因此block不需要捕获全局变量，因为全局变量在哪都可以访问。</p>
<p><strong>局部变量因为跨函数访问所以需要捕获，全局变量哪里都可以访问，因此不需要捕获。</strong></p>
<table>
<thead>
<tr>
<th align="center">变量类型</th>
<th align="center">捕获到block内部</th>
<th align="center">访问方式</th>
</tr>
</thead>
<tbody><tr>
<td align="center">局部变量（auto）</td>
<td align="center">✅</td>
<td align="center">值传递</td>
</tr>
<tr>
<td align="center">局部变量（static）</td>
<td align="center">✅</td>
<td align="center">指针传递</td>
</tr>
<tr>
<td align="center">全局变量</td>
<td align="center">❌</td>
<td align="center">直接访问</td>
</tr>
</tbody></table>
<p><strong>总结：局部变量都会被block捕获，自动变量是值捕获，静态变量是地址捕获，全局变量则不会被捕获。</strong></p>
<h5 id="疑问：以下代码中block是否会捕获变量呢？"><a href="#疑问：以下代码中block是否会捕获变量呢？" class="headerlink" title="疑问：以下代码中block是否会捕获变量呢？"></a>疑问：以下代码中block是否会捕获变量呢？</h5><p>1、在block中使用self会不会被捕获？</p>
<figure class="highlight objc"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta">#import <span class="meta-string">"Person.h"</span></span></span><br><span class="line"><span class="class"><span class="keyword">@implementation</span> <span class="title">Person</span></span></span><br><span class="line">- (<span class="keyword">void</span>)test {</span><br><span class="line"> <span class="keyword">void</span>(^block)(<span class="keyword">void</span>) = ^{</span><br><span class="line"> <span class="built_in">NSLog</span>(<span class="string">@"%@"</span>,<span class="keyword">self</span>);</span><br><span class="line"> };</span><br><span class="line"> block();</span><br><span class="line">}</span><br><span class="line">- (<span class="keyword">instancetype</span>)initWithName:(<span class="built_in">NSString</span> *)name {</span><br><span class="line"> <span class="keyword">if</span> (<span class="keyword">self</span> = [<span class="keyword">super</span> init]) {</span><br><span class="line"> <span class="keyword">self</span>.name = name;</span><br><span class="line"> }</span><br><span class="line"> <span class="keyword">return</span> <span class="keyword">self</span>;</span><br><span class="line">}</span><br><span class="line">+ (<span class="keyword">void</span>) test2 {</span><br><span class="line"> <span class="built_in">NSLog</span>(<span class="string">@"类方法test2"</span>);</span><br><span class="line">}</span><br><span class="line"><span class="keyword">@end</span></span><br></pre></td></tr></table></figure>
<p>同样转化为c++代码查看其内部结构</p>
<p><img src="https://xhp281-blog.oss-cn-beijing.aliyuncs.com/ios_objc/block_%E6%8D%95%E8%8E%B7_%E7%A4%BA%E4%BE%8B%E4%BB%A3%E7%A0%81.jpg" alt="block_捕获_示例代码"></p>
<p>上图中可以发现，self同样被block捕获，通过test方法发可以发现，test方法默认传递了两个参数self和_cmd。类方法test也是同样的。</p>
<p>不论对象方法还是类方法都会默认将self作为参数传递给方法内部，既然是作为参数传入，那么self肯定是局部变量。局部变量肯定会被block捕获。</p>
<p>2、在block中使用成员变量或者调用实例的属性会有什么不同的结果？</p>
<figure class="highlight objc"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br></pre></td><td class="code"><pre><span class="line">- (<span class="keyword">void</span>)test {</span><br><span class="line"> <span class="keyword">void</span>(^block)(<span class="keyword">void</span>) = ^{</span><br><span class="line"> <span class="built_in">NSLog</span>(<span class="string">@"%@"</span>,<span class="keyword">self</span>.name);</span><br><span class="line"> <span class="built_in">NSLog</span>(<span class="string">@"%@"</span>,_name);</span><br><span class="line"> };</span><br><span class="line"> block();</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p><img src="https://xhp281-blog.oss-cn-beijing.aliyuncs.com/ios_objc/block_%E8%87%AA%E5%AE%9A%E4%B9%89%E5%AF%B9%E8%B1%A1%E5%B1%9E%E6%80%A7%E5%92%8C%E6%88%90%E5%91%98%E5%8F%98%E9%87%8F.jpg" alt="block_自定义对象属性和成员变量"></p>
<p>通过上面图片可以发现，即使block中使用的是实例对象的属性，block中捕获的仍然是实例对象，并通过实例对象通过不同的方式去获取使用到的属性。</p>
<h3 id="block的类型"><a href="#block的类型" class="headerlink" title="block的类型"></a>block的类型</h3><p>block对象是什么类型的，之前的代码中提到过，通过源码可以知道block中的isa指针指向的是 <code>_NSConcreteStackBlock</code> 类对象地址。那么block的类型是否就是 <code>_NSConcreteStackBlock</code>类型吗？</p>
<p>通过代码调用class方法或者isa指针查看具体类型。</p>
<figure class="highlight objc"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">int</span> main(<span class="keyword">int</span> argc, <span class="keyword">const</span> <span class="keyword">char</span> * argv[]) {</span><br><span class="line"> <span class="keyword">@autoreleasepool</span> {</span><br><span class="line"> <span class="keyword">void</span> (^block)(<span class="keyword">void</span>) = ^{</span><br><span class="line"> <span class="built_in">NSLog</span>(<span class="string">@"Hello"</span>);</span><br><span class="line"> };</span><br><span class="line"> </span><br><span class="line"> <span class="built_in">NSLog</span>(<span class="string">@"%@"</span>, [block <span class="keyword">class</span>]);</span><br><span class="line"> <span class="built_in">NSLog</span>(<span class="string">@"%@"</span>, [[block <span class="keyword">class</span>] superclass]);</span><br><span class="line"> <span class="built_in">NSLog</span>(<span class="string">@"%@"</span>, [[[block <span class="keyword">class</span>] superclass] superclass]);</span><br><span class="line"> <span class="built_in">NSLog</span>(<span class="string">@"%@"</span>, [[[[block <span class="keyword">class</span>] superclass] superclass] superclass]);</span><br><span class="line"> }</span><br><span class="line"> <span class="keyword">return</span> <span class="number">0</span>;</span><br><span class="line">}</span><br><span class="line"><span class="comment">// log： __NSGlobalBlock__ : __NSGlobalBlock : NSBlock : NSObject</span></span><br></pre></td></tr></table></figure>
<p>从上述打印内容可以看出block最终都是继承自NSBlock类型，而NSBlock继承于NSObjcet。那么block其中的isa指针其实是来自NSObject中的。证明了block的本质其实就是OC对象。</p>
<h4 id="block的3种类型"><a href="#block的3种类型" class="headerlink" title="block的3种类型"></a>block的3种类型</h4><p>block有3中类型</p>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line">__NSGlobalBlock__ （ _NSConcreteGlobalBlock ）</span><br><span class="line">__NSStackBlock__ （ _NSConcreteStackBlock ）</span><br><span class="line">__NSMallocBlock__ （ _NSConcreteMallocBlock ）</span><br></pre></td></tr></table></figure>
<p>通过代码查看一下block在什么情况下其类型会各不相同</p>
<figure class="highlight objc"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">int</span> main(<span class="keyword">int</span> argc, <span class="keyword">const</span> <span class="keyword">char</span> * argv[]) {</span><br><span class="line"> <span class="keyword">@autoreleasepool</span> {</span><br><span class="line"> <span class="comment">// 1. 内部没有调用外部变量的block</span></span><br><span class="line"> <span class="keyword">void</span> (^block1)(<span class="keyword">void</span>) = ^{</span><br><span class="line"> <span class="built_in">NSLog</span>(<span class="string">@"Hello"</span>);</span><br><span class="line"> };</span><br><span class="line"> <span class="comment">// 2. 内部调用外部变量的block</span></span><br><span class="line"> <span class="keyword">int</span> a = <span class="number">10</span>;</span><br><span class="line"> <span class="keyword">void</span> (^block2)(<span class="keyword">void</span>) = ^{</span><br><span class="line"> <span class="built_in">NSLog</span>(<span class="string">@"Hello - %d"</span>,a);</span><br><span class="line"> };</span><br><span class="line"> <span class="comment">// 3. 直接调用的block的class</span></span><br><span class="line"> <span class="built_in">NSLog</span>(<span class="string">@"%@ %@ %@"</span>, [block1 <span class="keyword">class</span>], [block2 <span class="keyword">class</span>], [^{</span><br><span class="line"> <span class="built_in">NSLog</span>(<span class="string">@"%d"</span>,a);</span><br><span class="line"> } <span class="keyword">class</span>]);</span><br><span class="line"> }</span><br><span class="line"> <span class="keyword">return</span> <span class="number">0</span>;</span><br><span class="line">} </span><br></pre></td></tr></table></figure>
<p>通过打印内容确实发现block的三种类型</p>
<figure class="highlight shell"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">__NSGlobalBlock__ __NSMallocBlock__ __NSStackBlock__</span><br></pre></td></tr></table></figure>
<p>上述代码转化为c++代码查看源码时却发现block的类型与打印出来的类型不一样，c++源码中三个block的isa指针全部都指向_NSConcreteStackBlock类型地址。</p>
<p>可以猜测runtime运行时过程中也许对类型进行了转变。最终类型当然以runtime运行时类型也就是我们打印出的类型为准。</p>
<h4 id="block的内存中的存储"><a href="#block的内存中的存储" class="headerlink" title="block的内存中的存储"></a>block的内存中的存储</h4><p><img src="/Users/xhp281/Documents/%E7%AC%94%E8%AE%B0/iOS/blog_imags/block/block_%E4%B8%8D%E5%90%8Cblock%E7%9A%84%E5%AD%98%E6%94%BE%E5%8C%BA%E5%9F%9F.png" alt="block_不同block的存放区域"></p>
<p>上图中可以发现，根据block的类型不同，block存放在不同的区域中。 数据段中的 <code>__NSGlobalBlock__</code> 直到程序结束才会被回收，不过我们很少使用到 <code>__NSGlobalBlock__</code> 类型的block，因为这样使用block并没有什么意义。</p>
<p><code>__NSStackBlock__</code> 类型的block存放在栈中，我们知道栈中的内存由系统自动分配和释放，作用域执行完毕之后就会被立即释放，而在相同的作用域中定义block并且调用block似乎也多此一举。</p>
<p><code>__NSMallocBlock__</code> 是在平时编码过程中最常使用到的。存放在堆中需要我们自己进行内存管理。</p>
<h4 id="block的如何定义其类型"><a href="#block的如何定义其类型" class="headerlink" title="block的如何定义其类型"></a>block的如何定义其类型</h4><p>block是如何定义其类型，依据什么来为block定义不同类型并分配不同的空间呢？首先看下面内容</p>
<table>
<thead>
<tr>
<th align="center">block类型</th>
<th align="center">环境</th>
<th align="center">内存区域</th>
</tr>
</thead>
<tbody><tr>
<td align="center"><em>NSGlobalBlock</em></td>
<td align="center">没有访问auto变量</td>
<td align="center">数据段</td>
</tr>
<tr>
<td align="center"><em>NSStackBlock</em></td>
<td align="center">访问了auto变量</td>
<td align="center">栈</td>
</tr>
<tr>
<td align="center"><em>NSMallocBlock</em></td>
<td align="center"><em>NSStackBlock</em> 调用了copy</td>
<td align="center">堆</td>
</tr>
</tbody></table>
<p>接着我们使用代码验证上述问题，首先关闭ARC回到MRC环境下，因为ARC会帮助我们做很多事情，可能会影响我们的观察。</p>
<figure class="highlight objc"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// MRC环境！！！</span></span><br><span class="line"><span class="keyword">int</span> main(<span class="keyword">int</span> argc, <span class="keyword">const</span> <span class="keyword">char</span> * argv[]) {</span><br><span class="line"> <span class="keyword">@autoreleasepool</span> {</span><br><span class="line"> <span class="comment">// Global：没有访问auto变量：__NSGlobalBlock__</span></span><br><span class="line"> <span class="keyword">void</span> (^block1)(<span class="keyword">void</span>) = ^{</span><br><span class="line"> <span class="built_in">NSLog</span>(<span class="string">@"block1---------"</span>);</span><br><span class="line"> };</span><br><span class="line"> </span><br><span class="line"> <span class="comment">// Stack：访问了auto变量： __NSStackBlock__</span></span><br><span class="line"> <span class="keyword">int</span> a = <span class="number">10</span>;</span><br><span class="line"> <span class="keyword">void</span> (^block2)(<span class="keyword">void</span>) = ^{</span><br><span class="line"> <span class="built_in">NSLog</span>(<span class="string">@"block2---------%d"</span>, a);</span><br><span class="line"> };</span><br><span class="line"> </span><br><span class="line"> <span class="built_in">NSLog</span>(<span class="string">@"%@ %@"</span>, [block1 <span class="keyword">class</span>], [block2 <span class="keyword">class</span>]);</span><br><span class="line"> </span><br><span class="line"> <span class="comment">// __NSStackBlock__调用copy ： __NSMallocBlock__</span></span><br><span class="line"> <span class="built_in">NSLog</span>(<span class="string">@"%@"</span>, [[block2 <span class="keyword">copy</span>] <span class="keyword">class</span>]);</span><br><span class="line"> }</span><br><span class="line"> <span class="keyword">return</span> <span class="number">0</span>;</span><br><span class="line">}</span><br><span class="line"><span class="comment">// log: __NSGlobalBlock__ __NSStackBlock__ __NSMallocBlock__</span></span><br></pre></td></tr></table></figure>
<p>通过打印的内容可以发现：</p>
<p>1.<strong>没有访问auto变量的block是 <code>__NSGlobalBlock__</code> 类型的，存放在数据段中。</strong></p>
<p>2.<strong>访问了auto变量的block是 <code>__NSStackBlock__</code> 类型的，存放在栈中。</strong></p>
<p>3.<strong><code>__NSStackBlock__</code> 类型的block调用copy成为 <code>__NSMallocBlock__</code> 类型并被复制存放在堆中。</strong></p>
<p>上面提到过 <code>__NSGlobalBlock__</code> 类型的我们很少使用到，因为如果不需要访问外界的变量，直接通过函数实现就可以了，不需要使用block。</p>
<p>但是 <code>__NSStackBlock__</code> 访问了auto变量，并且是存放在栈中的，上面提到过，栈中的代码在作用域结束之后内存就会被销毁，那么我们很有可能block内存销毁之后才去调用他，那样就会发生问题，通过下面代码可以证实这个问题。</p>
<figure class="highlight objc"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">void</span> (^block)(<span class="keyword">void</span>);</span><br><span class="line"><span class="keyword">void</span> test() {</span><br><span class="line"> <span class="comment">// __NSStackBlock__</span></span><br><span class="line"> <span class="keyword">int</span> a = <span class="number">10</span>;</span><br><span class="line"> block = ^{</span><br><span class="line"> <span class="built_in">NSLog</span>(<span class="string">@"block---------%d"</span>, a);</span><br><span class="line"> };</span><br><span class="line">}</span><br><span class="line"><span class="keyword">int</span> main(<span class="keyword">int</span> argc, <span class="keyword">const</span> <span class="keyword">char</span> * argv[]) {</span><br><span class="line"> <span class="keyword">@autoreleasepool</span> {</span><br><span class="line"> test();</span><br><span class="line"> block();</span><br><span class="line"> }</span><br><span class="line"> <span class="keyword">return</span> <span class="number">0</span>;</span><br><span class="line">}</span><br><span class="line"><span class="comment">// log: block---------68334376</span></span><br></pre></td></tr></table></figure>
<p>可以发现a的值变为了不可控的一个数字。因为上述代码中创建的block是 <code>__NSStackBlock__</code> 类型的，因此block是存储在栈中的，那么当test函数执行完毕之后，栈内存中block所占用的内存已经被系统回收，因此就有可能出现乱得数据。查看其c++代码可以更清楚的理解。</p>
<p><img src="https://xhp281-blog.oss-cn-beijing.aliyuncs.com/ios_objc/block_%E6%A0%88%E5%86%85%E5%AD%98%E8%A2%AB%E5%9B%9E%E6%94%B6.png" alt="block_栈内存被回收"></p>
<p>为了避免这种情况发生，可以通过copy将 <strong>NSStackBlock</strong> 类型的block转化为 <strong>NSMallocBlock</strong> 类型的block，将block存储在堆中，以下是修改后的代码。</p>
<figure class="highlight objc"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">void</span> (^block)(<span class="keyword">void</span>);</span><br><span class="line"><span class="keyword">void</span> test()</span><br><span class="line">{</span><br><span class="line"> <span class="comment">// __NSStackBlock__ 调用copy 转化为__NSMallocBlock__</span></span><br><span class="line"> <span class="keyword">int</span> age = <span class="number">10</span>;</span><br><span class="line"> block = [^{</span><br><span class="line"> <span class="built_in">NSLog</span>(<span class="string">@"block---------%d"</span>, age);</span><br><span class="line"> } <span class="keyword">copy</span>];</span><br><span class="line"> [block release];</span><br><span class="line">}</span><br><span class="line"><span class="comment">// log: block---------10</span></span><br></pre></td></tr></table></figure>
<p>那么其他类型的block调用copy会改变block类型吗？下面表格已经展示的很清晰了。</p>
<table>
<thead>
<tr>
<th align="center">block类型</th>
<th align="center">内存区域</th>
<th align="center">copy效果</th>
</tr>
</thead>
<tbody><tr>
<td align="center"><em>NSGlobalBlock</em></td>
<td align="center">数据段</td>
<td align="center">什么都不做，不改变类型</td>
</tr>
<tr>
<td align="center"><em>NSStackBlock</em></td>
<td align="center">栈</td>
<td align="center">从栈赋值到堆，改变类型为 <em>NSMallocBlock</em></td>
</tr>
<tr>
<td align="center"><em>NSMallocBlock</em></td>
<td align="center">堆</td>
<td align="center">引用计数增加，不改变类型</td>
</tr>
</tbody></table>
<p>所以在平时开发过程中MRC环境下经常需要使用copy来保存block，将栈上的block拷贝到堆中，即使栈上的block被销毁，堆上的block也不会被销毁，需要我们自己调用release操作来销毁。而在ARC环境下系统会自动调用copy操作，使block不会被销毁。</p>
<h3 id="ARC帮我们做了什么"><a href="#ARC帮我们做了什么" class="headerlink" title="ARC帮我们做了什么"></a>ARC帮我们做了什么</h3><p>在ARC环境下，编译器会根据情况自动将栈上的block进行一次copy操作，将block复制到堆上。</p>
<p><strong>什么情况下ARC会自动将block进行一次copy操作？</strong> （以下代码都在ARC环境下执行）</p>
<h4 id="1-block作为函数返回值时"><a href="#1-block作为函数返回值时" class="headerlink" title="1. block作为函数返回值时"></a>1. block作为函数返回值时</h4><figure class="highlight objc"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">typedef</span> <span class="keyword">void</span>(^Block)(<span class="keyword">void</span>);</span><br><span class="line">Block myblock() {</span><br><span class="line"> <span class="keyword">int</span> a = <span class="number">10</span>;</span><br><span class="line"> Block block = ^{</span><br><span class="line"> <span class="built_in">NSLog</span>(<span class="string">@"----------%d"</span>,a);</span><br><span class="line"> };</span><br><span class="line"> <span class="keyword">return</span> block;</span><br><span class="line">}</span><br><span class="line"><span class="keyword">int</span> main(<span class="keyword">int</span> argc, <span class="keyword">const</span> <span class="keyword">char</span> * argv[]) {</span><br><span class="line"> <span class="keyword">@autoreleasepool</span> {</span><br><span class="line"> Block block = myblock();</span><br><span class="line"> block();</span><br><span class="line"> <span class="comment">// 打印block类型为 __NSMallocBlock__</span></span><br><span class="line"> <span class="built_in">NSLog</span>(<span class="string">@"%@"</span>,[block <span class="keyword">class</span>]);</span><br><span class="line"> }</span><br><span class="line"> <span class="keyword">return</span> <span class="number">0</span>;</span><br><span class="line">}</span><br><span class="line"><span class="comment">// log: ----------10 </span></span><br><span class="line"><span class="comment">// log: __NSMallocBlock__</span></span><br></pre></td></tr></table></figure>
<p><strong>上文提到过，如果在block中访问了auto变量时，block的类型为<code>__NSStackBlock__</code>，上面打印内容发现blcok为 <code>__NSMallocBlock__</code> 类型的，并且可以正常打印出a的值，说明block内存并没有被销毁。</strong></p>
<p><strong>block进行copy操作会转化为 <code>__NSMallocBlock__</code> 类型，来将block复制到堆中，那么说明RAC在 block作为函数返回值时会自动帮助我们对block进行copy操作，以保存block，并在适当的地方进行release操作。</strong></p>
<h4 id="2-将block赋值给-strong指针时"><a href="#2-将block赋值给-strong指针时" class="headerlink" title="2. 将block赋值给__strong指针时"></a>2. 将block赋值给__strong指针时</h4><p>block被强指针引用时，RAC也会自动对block进行一次copy操作。</p>
<figure class="highlight objc"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">int</span> main(<span class="keyword">int</span> argc, <span class="keyword">const</span> <span class="keyword">char</span> * argv[]) {</span><br><span class="line"> <span class="keyword">@autoreleasepool</span> {</span><br><span class="line"> <span class="comment">// block内没有访问auto变量</span></span><br><span class="line"> Block block = ^{</span><br><span class="line"> <span class="built_in">NSLog</span>(<span class="string">@"block---------"</span>);</span><br><span class="line"> };</span><br><span class="line"> <span class="built_in">NSLog</span>(<span class="string">@"%@"</span>,[block <span class="keyword">class</span>]);</span><br><span class="line"> </span><br><span class="line"> <span class="comment">// block内访问了auto变量，但没有赋值给__strong指针</span></span><br><span class="line"> <span class="keyword">int</span> a = <span class="number">10</span>;</span><br><span class="line"> <span class="built_in">NSLog</span>(<span class="string">@"%@"</span>,[^{</span><br><span class="line"> <span class="built_in">NSLog</span>(<span class="string">@"block1---------%d"</span>, a);</span><br><span class="line"> } <span class="keyword">class</span>]);</span><br><span class="line"> </span><br><span class="line"> <span class="comment">// block赋值给__strong指针</span></span><br><span class="line"> Block block2 = ^{</span><br><span class="line"> <span class="built_in">NSLog</span>(<span class="string">@"block2---------%d"</span>, a);</span><br><span class="line"> };</span><br><span class="line"> <span class="built_in">NSLog</span>(<span class="string">@"%@"</span>,[block2 <span class="keyword">class</span>]);</span><br><span class="line"> }</span><br><span class="line"> <span class="keyword">return</span> <span class="number">0</span>;</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>查看打印内容可以看出，当block被赋值给__strong指针时，RAC会自动进行一次copy操作。</p>
<figure class="highlight objc"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">__NSGlobalBlock__ __NSStackBlock__ __NSMallocBlock__</span><br></pre></td></tr></table></figure>
<h4 id="3-block作为Cocoa-API中方法名含有usingBlock的方法参数时"><a href="#3-block作为Cocoa-API中方法名含有usingBlock的方法参数时" class="headerlink" title="3. block作为Cocoa API中方法名含有usingBlock的方法参数时"></a>3. block作为Cocoa API中方法名含有usingBlock的方法参数时</h4><p>例如：遍历数组的block方法，将block作为参数的时候。</p>
<figure class="highlight objc"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line"><span class="built_in">NSArray</span> *array = @[];</span><br><span class="line">[array enumerateObjectsUsingBlock:^(<span class="keyword">id</span> _Nonnull obj, <span class="built_in">NSUInteger</span> idx, <span class="built_in">BOOL</span> * _Nonnull stop) {</span><br><span class="line">}];</span><br></pre></td></tr></table></figure>
<h4 id="4-block作为GCD-API的方法参数时"><a href="#4-block作为GCD-API的方法参数时" class="headerlink" title="4. block作为GCD API的方法参数时"></a>4. block作为GCD API的方法参数时</h4><p>例如：GDC的一次性函数或延迟执行的函数，执行完block操作之后系统才会对block进行release操作。</p>
<figure class="highlight objc"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">static</span> <span class="built_in">dispatch_once_t</span> onceToken;</span><br><span class="line"><span class="built_in">dispatch_once</span>(&onceToken, ^{</span><br><span class="line"> </span><br><span class="line">}); </span><br><span class="line">dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(<span class="number">1.0</span> * <span class="built_in">NSEC_PER_SEC</span>)), dispatch_get_main_queue(), ^{</span><br><span class="line"> </span><br><span class="line">});</span><br></pre></td></tr></table></figure>
<h3 id="block声明写法"><a href="#block声明写法" class="headerlink" title="block声明写法"></a>block声明写法</h3><p>通过上面对MRC及ARC环境下block的不同类型的分析，总结出不同环境下block属性建议写法。</p>
<p>MRC下block属性的建议写法</p>
<p><code>@property (copy, nonatomic) void (^block)(void);</code></p>
<p>ARC下block属性的建议写法</p>
<p><code>@property (strong, nonatomic) void (^block)(void);</code> </p>
<p><code>@property (copy, nonatomic) void (^block)(void);</code></p>
<h3 id="block对对象变量的捕获"><a href="#block对对象变量的捕获" class="headerlink" title="block对对象变量的捕获"></a>block对对象变量的捕获</h3><p>block一般使用过程中都是对对象变量的捕获，那么对象变量的捕获同基本数据类型变量相同吗？</p>
<p>查看一下代码思考：当在block中访问的为对象类型时，对象什么时候会销毁？</p>
<figure class="highlight objc"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">typedef</span> <span class="keyword">void</span> (^Block)(<span class="keyword">void</span>);</span><br><span class="line"><span class="keyword">int</span> main(<span class="keyword">int</span> argc, <span class="keyword">const</span> <span class="keyword">char</span> * argv[]) {</span><br><span class="line"> <span class="keyword">@autoreleasepool</span> {</span><br><span class="line"> Block block;</span><br><span class="line"> {</span><br><span class="line"> Person *person = [[Person alloc] init];</span><br><span class="line"> person.name = <span class="string">@"cook"</span>;</span><br><span class="line"> </span><br><span class="line"> block = ^{</span><br><span class="line"> <span class="built_in">NSLog</span>(<span class="string">@"------block内部%@"</span>,person.name);</span><br><span class="line"> };</span><br><span class="line"> } <span class="comment">// 执行完毕，person没有被释放</span></span><br><span class="line"> <span class="built_in">NSLog</span>(<span class="string">@"--------"</span>);</span><br><span class="line"> } <span class="comment">// person 释放</span></span><br><span class="line"> <span class="keyword">return</span> <span class="number">0</span>;</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>大括号执行完毕之后，<code>person</code> 依然不会被释放。<code>person</code> 为 <code>aotu</code> 变量，传入的 <code>block</code> 的变量同样为 <code>person</code>，即 <code>block</code> 有一个强引用引用 <code>person</code>，所以 <code>block </code>不被销毁的话，<code>peroson</code> 也不会销毁。 查看源代码确实如此</p>
<p><img src="https://xhp281-blog.oss-cn-beijing.aliyuncs.com/ios_objc/block_block%E5%AF%B9%E5%AF%B9%E8%B1%A1%E5%8F%98%E9%87%8F%E7%9A%84%E6%8D%95%E8%8E%B7.png" alt="block_block对对象变量的捕获"></p>
<p>将上述代码转移到MRC环境下，在MRC环境下即使block还在，<code>person</code> 却被释放掉了。因为MRC环境下block在栈空间，栈空间对外面的 <code>person</code> 不会进行强引用。</p>
<figure class="highlight objc"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">typedef</span> <span class="keyword">void</span> (^Block)(<span class="keyword">void</span>);</span><br><span class="line"><span class="keyword">int</span> main(<span class="keyword">int</span> argc, <span class="keyword">const</span> <span class="keyword">char</span> * argv[]) {</span><br><span class="line"> <span class="keyword">@autoreleasepool</span> {</span><br><span class="line"> Block block;</span><br><span class="line"> {</span><br><span class="line"> Person *person = [[Person alloc] init];</span><br><span class="line"> person.name = <span class="string">@"cook"</span>;</span><br><span class="line"> </span><br><span class="line"> block = ^{</span><br><span class="line"> <span class="built_in">NSLog</span>(<span class="string">@"------block内部%@"</span>,person.name);</span><br><span class="line"> };</span><br><span class="line"> [person release];</span><br><span class="line"> } <span class="comment">// 执行完毕，person没有被释放</span></span><br><span class="line"> <span class="built_in">NSLog</span>(<span class="string">@"--------"</span>);</span><br><span class="line"> } <span class="comment">// person 释放</span></span><br><span class="line"> <span class="keyword">return</span> <span class="number">0</span>;</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>block调用copy操作之后，person不会被释放。</p>
<figure class="highlight objc"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line">block = [^{</span><br><span class="line"> <span class="built_in">NSLog</span>(<span class="string">@"------block内部%@"</span>,person.name);</span><br><span class="line">} <span class="keyword">copy</span>];</span><br></pre></td></tr></table></figure>
<p>上面提到只需要对栈空间的 <code>block</code> 进行一次 <code>copy</code> 操作，将栈空间的 <code>block</code> 拷贝到堆中，<code>person</code> 就不会被释放，说明堆空间的 <code>block</code> 可能会对 <code>person</code> 进行一次 <code>retain</code> 操作，保证 <code>person</code> 不会被销毁。堆空间的 <code>block</code> 自己销毁之后也会对持有的对象进行 <code>release</code> 操作。</p>
<p>也就是说栈空间上的 <code>block</code> 不会对对象强引用，堆空间的 <code>block</code> 有能力持有外部调用的对象，即对对象进行强引用或去除强引用的操作。</p>
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