__weak与__block修饰符到底有什么区别

API Reference对__block变量修饰符有如下几处解释：

<code class=" hljs livecodeserver" style="display: block; padding: 0px; background-color: transparent; color: inherit; box-sizing: border-box; font-family: 'Microsoft YaHei';font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background-position: initial initial; background-repeat: initial initial;"><span class="com" style="box-sizing: border-box; padding: 0px; color: rgb(136, 0, 0);">//A powerful feature </span><span class="hljs-operator" style="box-sizing: border-box; padding: 0px;"><span class="com" style="box-sizing: border-box; padding: 0px; color: rgb(136, 0, 0);">of</span></span><span class="com" style="box-sizing: border-box; padding: 0px; color: rgb(136, 0, 0);"> blocks is that they can modify </span><span class="pln" style="box-sizing: border-box; padding: 0px; color: rgb(0, 0, 0);">variables </span><span class="hljs-operator" style="box-sizing: border-box; padding: 0px;"><span class="kwd" style="box-sizing: border-box; padding: 0px; color: rgb(0, 0, 136);">in</span></span><span class="pln" style="box-sizing: border-box; padding: 0px; color: rgb(0, 0, 0);"> </span><span class="hljs-operator" style="box-sizing: border-box; padding: 0px;"><span class="pln" style="box-sizing: border-box; padding: 0px; color: rgb(0, 0, 0);">the</span></span><span class="pln" style="box-sizing: border-box; padding: 0px; color: rgb(0, 0, 0);"> same lexical scope</span><span class="pun" style="box-sizing: border-box; padding: 0px; color: rgb(102, 102, 0);">.</span><span class="pln" style="box-sizing: border-box; padding: 0px; color: rgb(0, 0, 0);"> </span><span class="typ" style="box-sizing: border-box; padding: 0px; color: rgb(102, 0, 102);">You</span><span class="pln" style="box-sizing: border-box; padding: 0px; color: rgb(0, 0, 0);"> signal that </span><span class="hljs-operator" style="box-sizing: border-box; padding: 0px;"><span class="pln" style="box-sizing: border-box; padding: 0px; color: rgb(0, 0, 0);">a</span></span><span class="pln" style="box-sizing: border-box; padding: 0px; color: rgb(0, 0, 0);"> block can modify </span><span class="hljs-operator" style="box-sizing: border-box; padding: 0px;"><span class="pln" style="box-sizing: border-box; padding: 0px; color: rgb(0, 0, 0);">a</span></span><span class="pln" style="box-sizing: border-box; padding: 0px; color: rgb(0, 0, 0);"> </span><span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box; padding: 0px;"><span class="pln" style="box-sizing: border-box; padding: 0px; color: rgb(0, 0, 0);">variable</span></span><span class="pln" style="box-sizing: border-box; padding: 0px; color: rgb(0, 0, 0);"> </span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box; padding: 0px;">using</span><span class="pln" style="box-sizing: border-box; padding: 0px; color: rgb(0, 0, 0);"> </span><span class="hljs-operator" style="box-sizing: border-box; padding: 0px;"><span class="pln" style="box-sizing: border-box; padding: 0px; color: rgb(0, 0, 0);">the</span></span><span class="pln" style="box-sizing: border-box; padding: 0px; color: rgb(0, 0, 0);"> __block storage type modifier</span><span class="pun" style="box-sizing: border-box; padding: 0px; color: rgb(102, 102, 0);">.</span><span class="pln" style="box-sizing: border-box; padding: 0px; color: rgb(0, 0, 0);"> </span><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box; padding: 0px;"><span class="pln" style="color: rgb(0, 0, 0); box-sizing: border-box; padding: 0px;"></span><span class="com" style="box-sizing: border-box; padding: 0px;">//At function level are __block variables. These are mutable</span></span><span class="pln" style="box-sizing: border-box; padding: 0px; color: rgb(0, 0, 0);"> </span><span class="hljs-operator" style="box-sizing: border-box; padding: 0px;"><span class="pln" style="box-sizing: border-box; padding: 0px; color: rgb(0, 0, 0);">within</span></span><span class="pln" style="box-sizing: border-box; padding: 0px; color: rgb(0, 0, 0);"> </span><span class="hljs-operator" style="box-sizing: border-box; padding: 0px;"><span class="pln" style="box-sizing: border-box; padding: 0px; color: rgb(0, 0, 0);">the</span></span><span class="pln" style="box-sizing: border-box; padding: 0px; color: rgb(0, 0, 0);"> block </span><span class="pun" style="box-sizing: border-box; padding: 0px; color: rgb(102, 102, 0);">(</span><span class="hljs-operator" style="box-sizing: border-box; padding: 0px;"><span class="kwd" style="box-sizing: border-box; padding: 0px; color: rgb(0, 0, 136);">and</span></span><span class="pln" style="box-sizing: border-box; padding: 0px; color: rgb(0, 0, 0);"> </span><span class="hljs-operator" style="box-sizing: border-box; padding: 0px;"><span class="pln" style="box-sizing: border-box; padding: 0px; color: rgb(0, 0, 0);">the</span></span><span class="pln" style="box-sizing: border-box; padding: 0px; color: rgb(0, 0, 0);"> enclosing scope</span><span class="pun" style="box-sizing: border-box; padding: 0px; color: rgb(102, 102, 0);">)</span><span class="pln" style="box-sizing: border-box; padding: 0px; color: rgb(0, 0, 0);"> </span><span class="hljs-operator" style="box-sizing: border-box; padding: 0px;"><span class="kwd" style="box-sizing: border-box; padding: 0px; color: rgb(0, 0, 136);">and</span></span><span class="pln" style="box-sizing: border-box; padding: 0px; color: rgb(0, 0, 0);"> are preserved </span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box; padding: 0px;">if</span><span class="pln" style="box-sizing: border-box; padding: 0px; color: rgb(0, 0, 0);"> </span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box; padding: 0px;"><span class="pln" style="box-sizing: border-box; padding: 0px; color: rgb(0, 0, 0);">any</span></span><span class="pln" style="box-sizing: border-box; padding: 0px; color: rgb(0, 0, 0);"> referencing block </span><span class="kwd" style="box-sizing: border-box; padding: 0px; color: rgb(0, 0, 136);">is</span><span class="pln" style="box-sizing: border-box; padding: 0px; color: rgb(0, 0, 0);"> copied </span><span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box; padding: 0px;"><span class="pln" style="box-sizing: border-box; padding: 0px; color: rgb(0, 0, 0);">to</span></span><span class="pln" style="box-sizing: border-box; padding: 0px; color: rgb(0, 0, 0);"> </span><span class="hljs-operator" style="box-sizing: border-box; padding: 0px;"><span class="pln" style="box-sizing: border-box; padding: 0px; color: rgb(0, 0, 0);">the</span></span><span class="pln" style="box-sizing: border-box; padding: 0px; color: rgb(0, 0, 0);"> heap</span><span class="pun" style="box-sizing: border-box; padding: 0px; color: rgb(102, 102, 0);">.</span></code>

大概意思归结出来就是两点： 
1.__block对象在block中是可以被修改、重新赋值的。 
2.__block对象在block中不会被block强引用一次，从而不会出现循环引用问题。

API Reference对__weak变量修饰符有如下几处解释：

<code class=" hljs applescript" style="display: block; padding: 0px; background-color: transparent; color: inherit; box-sizing: border-box; font-family: 'Microsoft YaHei';font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background-position: initial initial; background-repeat: initial initial;"><span class="pln" style="box-sizing: border-box; padding: 0px; color: rgb(0, 0, 0);">__weak specifies </span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box; padding: 0px;"><span class="pln" style="box-sizing: border-box; padding: 0px; color: rgb(0, 0, 0);">a reference</span></span><span class="pln" style="box-sizing: border-box; padding: 0px; color: rgb(0, 0, 0);"> </span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box; padding: 0px;"><span class="pln" style="box-sizing: border-box; padding: 0px; color: rgb(0, 0, 0);">that</span></span><span class="pln" style="box-sizing: border-box; padding: 0px; color: rgb(0, 0, 0);"> </span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box; padding: 0px;"><span class="pln" style="box-sizing: border-box; padding: 0px; color: rgb(0, 0, 0);">does</span></span><span class="pln" style="box-sizing: border-box; padding: 0px; color: rgb(0, 0, 0);"> </span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box; padding: 0px;">not</span><span class="pln" style="box-sizing: border-box; padding: 0px; color: rgb(0, 0, 0);"> keep </span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box; padding: 0px;"><span class="pln" style="box-sizing: border-box; padding: 0px; color: rgb(0, 0, 0);">the</span></span><span class="pln" style="box-sizing: border-box; padding: 0px; color: rgb(0, 0, 0);"> referenced </span><span class="kwd" style="box-sizing: border-box; padding: 0px; color: rgb(0, 0, 136);">object</span><span class="pln" style="box-sizing: border-box; padding: 0px; color: rgb(0, 0, 0);"> alive</span><span class="pun" style="box-sizing: border-box; padding: 0px; color: rgb(102, 102, 0);">.</span><span class="pln" style="box-sizing: border-box; padding: 0px; color: rgb(0, 0, 0);"> A weak </span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box; padding: 0px;"><span class="pln" style="box-sizing: border-box; padding: 0px; color: rgb(0, 0, 0);">reference</span></span><span class="pln" style="box-sizing: border-box; padding: 0px; color: rgb(0, 0, 0);"> </span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box; padding: 0px;">is</span><span class="pln" style="box-sizing: border-box; padding: 0px; color: rgb(0, 0, 0);"> </span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box; padding: 0px;">set</span><span class="pln" style="box-sizing: border-box; padding: 0px; color: rgb(0, 0, 0);"> </span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box; padding: 0px;"><span class="pln" style="box-sizing: border-box; padding: 0px; color: rgb(0, 0, 0);">to</span></span><span class="pln" style="box-sizing: border-box; padding: 0px; color: rgb(0, 0, 0);"> </span><span class="kwd" style="box-sizing: border-box; padding: 0px; color: rgb(0, 0, 136);">nil</span><span class="pln" style="box-sizing: border-box; padding: 0px; color: rgb(0, 0, 0);"> </span><span class="kwd" style="box-sizing: border-box; padding: 0px; color: rgb(0, 0, 136);">when</span><span class="pln" style="box-sizing: border-box; padding: 0px; color: rgb(0, 0, 0);">there are </span><span class="kwd" style="box-sizing: border-box; padding: 0px; color: rgb(0, 0, 136);">no</span><span class="pln" style="box-sizing: border-box; padding: 0px; color: rgb(0, 0, 0);"> strong references </span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box; padding: 0px;"><span class="pln" style="box-sizing: border-box; padding: 0px; color: rgb(0, 0, 0);">to</span></span><span class="pln" style="box-sizing: border-box; padding: 0px; color: rgb(0, 0, 0);"> </span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box; padding: 0px;"><span class="pln" style="box-sizing: border-box; padding: 0px; color: rgb(0, 0, 0);">the</span></span><span class="pln" style="box-sizing: border-box; padding: 0px; color: rgb(0, 0, 0);"> </span><span class="kwd" style="box-sizing: border-box; padding: 0px; color: rgb(0, 0, 136);">object</span><span class="pun" style="box-sizing: border-box; padding: 0px; color: rgb(102, 102, 0);">.</span></code>

使用了__weak修饰符的对象，作用等同于定义为weak的property。自然不会导致循环引用问题，因为苹果文档已经说的很清楚，当原对象没有任何强引用的时候，弱引用指针也会被设置为nil。

因此，__block和__weak修饰符的区别其实是挺明显的： 
1.__block不管是ARC还是MRC模式下都可以使用，可以修饰对象，还可以修饰基本数据类型。 
2.__weak只能在ARC模式下使用，也只能修饰对象（NSString），不能修饰基本数据类型（int）。 
3.__block对象可以在block中被重新赋值，__weak不可以。 
PS：__unsafe_unretained修饰符可以被视为iOS SDK 4.3以前版本的__weak的替代品，不过不会被自动置空为nil。所以尽可能不要使用这个修饰符。