Site updated: 2017-05-31 15:19:51

2017/05/31/20170532/index.html

@@ -91,9 +91,10 @@
 <meta property="og:site_name" content="三味猪屋">
 <meta property="og:description" content="使用block时利用”weak-strong dance”解决循环引用大家都知道，但这里有三点值得注意：1、当self指向的对象已经被废弃的情况下，持有block成员变量也不存在了，这个时候block对象应该已经没有被变量所持有了，它的引用计数应该已经为0了，它应该被废弃了啊，为什么它还能继续存在并执行。如以下代码，在 block 执行前退出这个页面的话，该 Controller 实例会被废弃">
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 <meta name="twitter:title" content="深入理解"weak-strong dance"">
 <meta name="twitter:description" content="使用block时利用”weak-strong dance”解决循环引用大家都知道，但这里有三点值得注意：1、当self指向的对象已经被废弃的情况下，持有block成员变量也不存在了，这个时候block对象应该已经没有被变量所持有了，它的引用计数应该已经为0了，它应该被废弃了啊，为什么它还能继续存在并执行。如以下代码，在 block 执行前退出这个页面的话，该 Controller 实例会被废弃">
@@ -355,7 +356,7 @@ <h1 class="post-title" itemprop="name headline">
 
         <script src="/assets/js/DPlayer.min.js"> </script><script src="/assets/js/APlayer.min.js"> </script><p>使用block时利用”weak-strong dance”解决循环引用大家都知道，但这里有三点值得注意：<br>1、当self指向的对象已经被废弃的情况下，持有block成员变量也不存在了，这个时候block对象应该已经没有被变量所持有了，它的引用计数应该已经为0了，它应该被废弃了啊，为什么它还能继续存在并执行。<br>如以下代码，在 block 执行前退出这个页面的话，该 Controller 实例会被废弃，但 Block 还是会执行，会打印“self is (null)”。<br><figure class="highlight plain"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div><div class="line">10</div><div class="line">11</div><div class="line">12</div></pre></td><td class="code"><pre><div class="line">- (void)viewDidLoad &#123;</div><div class="line">    [super viewDidLoad];</div><div class="line"></div><div class="line">    __weak typeof(self) weakSelf = self;</div><div class="line">    self.handler = ^&#123;</div><div class="line">        __strong typeof(weakSelf) strongSelf = weakSelf;</div><div class="line">        NSLog(@&quot;self is %@&quot;, strongSelf);</div><div class="line">    &#125;;</div><div class="line"></div><div class="line">    NSTimeInterval interval = 6.0;</div><div class="line">    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(interval * NSEC_PER_SEC)), dispatch_get_main_queue(), weakSelf.handler);</div><div class="line">&#125;</div></pre></td></tr></table></figure></p>
 <p>理解这个问题需要了解block原理，简单一点的说，block有三种类型：<br>NSConcreteStackBlock（栈）<br>NSConcreteGlobalBlock（全局）<br>NSConcreteMallocBlock（堆）</p>
-<p>如何实现这三种类型的block：<br><img src="/images/20170531/block_objc.png" alt=""><br>使用<code>clang -rewrite-objc $filePath</code>将OC代码转化为C++代码实现<br><img src="/images/20170531/block_clang.png" alt=""><br>如果 block 在记述全局变量的地方被设置或者 block 没有捕获外部变量，那就生成一个 NSConcreteGlobalBlock 实例。其它情况都会生成一个 NSConcreteStackBlock 实例，也就是说，它是在栈上的，所以一旦它所属的变量超出了变量作用域，该 block 就被废弃了。而当发生以下任一情况时：<br>a、手动调用 block 的实例方法copy。<br>b、block 作为函数返回值返回。<br>c、将 block 赋值给附有__strong修饰符的成员变量。<br>d、在方法名中含有usingBlock的 Cocoa 框架方法或 GCD 的 API 中传递 block。<br>所以NSConcreteMallocBlock 类型的 block 通常不会在源码中直接出现。<br><a href="http://www.galloway.me.uk/2013/05/a-look-inside-blocks-episode-3-block-copy/" target="_blank" rel="external">看一下Block_copy()实现</a>：<br><figure class="highlight plain"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div><div class="line">10</div><div class="line">11</div><div class="line">12</div><div class="line">13</div><div class="line">14</div><div class="line">15</div><div class="line">16</div><div class="line">17</div><div class="line">18</div><div class="line">19</div><div class="line">20</div><div class="line">21</div><div class="line">22</div><div class="line">23</div><div class="line">24</div><div class="line">25</div><div class="line">26</div><div class="line">27</div><div class="line">28</div><div class="line">29</div><div class="line">30</div><div class="line">31</div><div class="line">32</div><div class="line">33</div><div class="line">34</div><div class="line">35</div><div class="line">36</div><div class="line">37</div><div class="line">38</div><div class="line">39</div><div class="line">40</div><div class="line">41</div><div class="line">42</div><div class="line">43</div></pre></td><td class="code"><pre><div class="line">static void *_Block_copy_internal(const void *arg, const int flags) &#123;</div><div class="line">    struct Block_layout *aBlock;</div><div class="line">    const bool wantsOne = (WANTS_ONE &amp; flags) == WANTS_ONE;</div><div class="line"></div><div class="line">    // 1</div><div class="line">    if (!arg) return NULL;</div><div class="line"></div><div class="line">    // 2</div><div class="line">    aBlock = (struct Block_layout *)arg;</div><div class="line"></div><div class="line">    // 3</div><div class="line">    if (aBlock-&gt;flags &amp; BLOCK_NEEDS_FREE) &#123;</div><div class="line">        // latches on high</div><div class="line">        latching_incr_int(&amp;aBlock-&gt;flags);</div><div class="line">        return aBlock;</div><div class="line">    &#125;</div><div class="line"></div><div class="line">    // 4</div><div class="line">    else if (aBlock-&gt;flags &amp; BLOCK_IS_GLOBAL) &#123;</div><div class="line">        return aBlock;</div><div class="line">    &#125;</div><div class="line"></div><div class="line">    // 5</div><div class="line">    struct Block_layout *result = malloc(aBlock-&gt;descriptor-&gt;size);</div><div class="line">    if (!result) return (void *)0;</div><div class="line"></div><div class="line">    // 6</div><div class="line">    memmove(result, aBlock, aBlock-&gt;descriptor-&gt;size); // bitcopy first</div><div class="line"></div><div class="line">    // 7</div><div class="line">    result-&gt;flags &amp;= ~(BLOCK_REFCOUNT_MASK);    // XXX not needed</div><div class="line">    result-&gt;flags |= BLOCK_NEEDS_FREE | 1;</div><div class="line"></div><div class="line">    // 8</div><div class="line">    result-&gt;isa = _NSConcreteMallocBlock;</div><div class="line"></div><div class="line">    // 9</div><div class="line">    if (result-&gt;flags &amp; BLOCK_HAS_COPY_DISPOSE) &#123;</div><div class="line">        (*aBlock-&gt;descriptor-&gt;copy)(result, aBlock); // do fixup</div><div class="line">    &#125;</div><div class="line"></div><div class="line">    return result;</div><div class="line">&#125;</div></pre></td></tr></table></figure></p>
+<p>如何实现这三种类型的block：<br><img src="/images/20170531/block_objc.png" alt=""><br>使用<code>clang -rewrite-objc $filePath</code>将OC代码转化为C++代码实现<br><img src="/images/20170531/clang_rewrite.png" alt=""><br><img src="/images/20170531/block_clang.png" alt=""><br>如果 block 在记述全局变量的地方被设置或者 block 没有捕获外部变量，那就生成一个 NSConcreteGlobalBlock 实例。其它情况都会生成一个 NSConcreteStackBlock 实例，也就是说，它是在栈上的，所以一旦它所属的变量超出了变量作用域，该 block 就被废弃了。而当发生以下任一情况时：<br>a、手动调用 block 的实例方法copy。<br>b、block 作为函数返回值返回。<br>c、将 block 赋值给附有__strong修饰符的成员变量。<br>d、在方法名中含有usingBlock的 Cocoa 框架方法或 GCD 的 API 中传递 block。<br>所以NSConcreteMallocBlock 类型的 block 通常不会在源码中直接出现。<br><a href="http://www.galloway.me.uk/2013/05/a-look-inside-blocks-episode-3-block-copy/" target="_blank" rel="external">看一下Block_copy()实现</a>：<br><figure class="highlight plain"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div><div class="line">10</div><div class="line">11</div><div class="line">12</div><div class="line">13</div><div class="line">14</div><div class="line">15</div><div class="line">16</div><div class="line">17</div><div class="line">18</div><div class="line">19</div><div class="line">20</div><div class="line">21</div><div class="line">22</div><div class="line">23</div><div class="line">24</div><div class="line">25</div><div class="line">26</div><div class="line">27</div><div class="line">28</div><div class="line">29</div><div class="line">30</div><div class="line">31</div><div class="line">32</div><div class="line">33</div><div class="line">34</div><div class="line">35</div><div class="line">36</div><div class="line">37</div><div class="line">38</div><div class="line">39</div><div class="line">40</div><div class="line">41</div><div class="line">42</div><div class="line">43</div></pre></td><td class="code"><pre><div class="line">static void *_Block_copy_internal(const void *arg, const int flags) &#123;</div><div class="line">    struct Block_layout *aBlock;</div><div class="line">    const bool wantsOne = (WANTS_ONE &amp; flags) == WANTS_ONE;</div><div class="line"></div><div class="line">    // 1</div><div class="line">    if (!arg) return NULL;</div><div class="line"></div><div class="line">    // 2</div><div class="line">    aBlock = (struct Block_layout *)arg;</div><div class="line"></div><div class="line">    // 3</div><div class="line">    if (aBlock-&gt;flags &amp; BLOCK_NEEDS_FREE) &#123;</div><div class="line">        // latches on high</div><div class="line">        latching_incr_int(&amp;aBlock-&gt;flags);</div><div class="line">        return aBlock;</div><div class="line">    &#125;</div><div class="line"></div><div class="line">    // 4</div><div class="line">    else if (aBlock-&gt;flags &amp; BLOCK_IS_GLOBAL) &#123;</div><div class="line">        return aBlock;</div><div class="line">    &#125;</div><div class="line"></div><div class="line">    // 5</div><div class="line">    struct Block_layout *result = malloc(aBlock-&gt;descriptor-&gt;size);</div><div class="line">    if (!result) return (void *)0;</div><div class="line"></div><div class="line">    // 6</div><div class="line">    memmove(result, aBlock, aBlock-&gt;descriptor-&gt;size); // bitcopy first</div><div class="line"></div><div class="line">    // 7</div><div class="line">    result-&gt;flags &amp;= ~(BLOCK_REFCOUNT_MASK);    // XXX not needed</div><div class="line">    result-&gt;flags |= BLOCK_NEEDS_FREE | 1;</div><div class="line"></div><div class="line">    // 8</div><div class="line">    result-&gt;isa = _NSConcreteMallocBlock;</div><div class="line"></div><div class="line">    // 9</div><div class="line">    if (result-&gt;flags &amp; BLOCK_HAS_COPY_DISPOSE) &#123;</div><div class="line">        (*aBlock-&gt;descriptor-&gt;copy)(result, aBlock); // do fixup</div><div class="line">    &#125;</div><div class="line"></div><div class="line">    return result;</div><div class="line">&#125;</div></pre></td></tr></table></figure></p>
 <p>综上所述，已经可以解答问题1。把 block 赋值给self.handler的时候，在栈上生成的 block 被复制了一份放到堆上。而之后如果你把这个 block 当作 GCD 参数使用，GCD 函数内部会把该 block 再 copy 一遍，而此时 block 已经在堆上，则该 block 的引用计数加1。所以此时 block 的引用计数是大于1的，即使self对象被废弃 block 会被 release 一次，但它的引用计数仍然大于0，故而不会被废弃。<br>Tips:<br>其实在 ARC 开启的情况下，将只会有 NSConcreteGlobalBlock 和 NSConcreteMallocBlock 类型的 block。<br><img src="/images/20170531/block_work_in_ARC.png" alt=""><br>更多请查看<a href="https://developer.apple.com/library/content/releasenotes/ObjectiveC/RN-TransitioningToARC/Introduction/Introduction.html" target="_blank" rel="external">苹果官方文档</a></p>
 <p>2、在 block 内部使用weakSelf就是为了让 block 对象不持有self指向的对象，那在 block 内部又把weakSelf赋给strongSelf不就又持有self对象了么？又循环引用了？<br>理解这个问题就需要了解block捕获对象变量。<br>使用<strong>weak修饰的变量时</strong>weak是不会持有对象，它用一张 weak 表来管理对象和变量。赋值的时候它会以赋值对象的地址作为 key，变量的地址为 value，注册到 weak 表中。一旦该对象被废弃，就通过对象地址在 weak 表中找到变量的地址，赋值为 nil，然后将该条记录从 weak 表中删除。<br>那使用 “weak-strong dance” 的时候是怎么个情况呢？<br><figure class="highlight plain"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div><div class="line">10</div><div class="line">11</div><div class="line">12</div><div class="line">13</div><div class="line">14</div><div class="line">15</div><div class="line">16</div><div class="line">17</div></pre></td><td class="code"><pre><div class="line">struct __block_impl &#123;</div><div class="line">    void *isa;</div><div class="line">    int Flags;</div><div class="line">    int Reserved;</div><div class="line">    void *FuncPtr;</div><div class="line">&#125;;</div><div class="line"></div><div class="line">struct __xx_block_impl_y &#123;</div><div class="line">    struct __block_impl impl;</div><div class="line">    __weak OCClass *occlass;</div><div class="line">    // ...</div><div class="line">&#125;;</div><div class="line"></div><div class="line">static void __xx_block_func_y(struct __xx_block_impl_y *__cself) &#123;</div><div class="line">    OCClass *occlass = __cself -&gt; occlass;</div><div class="line">    // ...</div><div class="line">&#125;</div></pre></td></tr></table></figure></p>
 <p>每次使用<strong>weak变量的时候，都会取出该变量指向的对象并 retain，然后将该对象注册到 autoreleasepool 中。通过上述代码我们可以发现，在</strong>xx_block_func_y中，局部变量occlass会持有捕获的对象，然后对象会被注册到 autoreleasepool。这是延长对象生命周期的关键（保证在执行 Block 期间对象不会被废弃），但这不会造成循环引用，当函数执行结束，变量occlass超出作用域，过一会儿（一般一次 RunLoop 之后），对象就被释放了。所以 weak-strong dance 的行为非常符合预期：延长捕获对象的生命周期，一旦 Block 执行完，对象被释放，而 Block 也会被释放（如果被 GCD 之类的 API copy 过一次增加了引用计数，那最终也会被 GCD 释放）。</p>

css/main.css

@@ -1682,7 +1682,7 @@ pre .javascript .function {
   width: 4px;
   height: 4px;
   border-radius: 50%;
-  background: #18a30a;
+  background: #d0ad40;
 }
 .links-of-blogroll {
   font-size: 13px;