__weak __typeof(self)weakSelf = self和__strong __typeof(weakSelf)strongSelf = weakSelf

1、默认strong，可选weak。strong下不管成员变量还是property，每次使用指针指向一个对象，等于自动调用retain(), 并对旧对象调用release(),所以设为nil等于release。

2、只要某个对象被任一strong指针指向，那么它将不会被销毁，否则立即释放，不用等runloop结束。所有strong指针变量不需要在dealloc中手动设为nil，ios会自动处理，debug可以看到全部被置为nil，最先声明的变量最后调用dealloc释放。

3、官方建议IBOutlet加上__weak，实际上不用加也会自动释放；

4、优先使用私有成员变量，除非需要公开属性才用property。

5、避免循环引用，否则手动设置nil释放。

6、block方法常用声明：@property (copy) void(^MyBlock)(void);  如果超出当前作用域之后仍然继续使用block，那么最好使用copy关键字，拷贝到堆区，防止栈区变量销毁。

7、创建block匿名函数之前一般需要对self进行weak化，否则造成循环引用无法释放controller:

     __weak MyController *weakSelf = self 或者 __weak __typeof(self) weakSelf = self;

    执行block方法体的时候也可以转换为强引用之后再使用：MyController* strongSelf = weakSelf; if (!strongSelf) { return; }

在学习AFNetWorking的过程中，经常看到类似：

__weak __typeof(self)weakSelf = self; 

然后在block中，看到：

__strong __typeof(weakSelf)strongSelf = weakSelf;

如下代码：

- (void)setShouldExecuteAsBackgroundTaskWithExpirationHandler:(void (^)(void))handler {

    [self.lock lock];

    if (!self.backgroundTaskIdentifier) {

        UIApplication *application = [UIApplication sharedApplication];

        __weak __typeof(self)weakSelf = self;

        self.backgroundTaskIdentifier = [application beginBackgroundTaskWithExpirationHandler:^{

            __strong __typeof(weakSelf)strongSelf = weakSelf;

            

            if (handler) {

                handler();

            }

            

            if (strongSelf) {

                [strongSelf cancel];

                

                [application endBackgroundTask:strongSelf.backgroundTaskIdentifier];

                strongSelf.backgroundTaskIdentifier = UIBackgroundTaskInvalid;

            }

        }];

    }

    [self.lock unlock];

}

这是因为：没有添加__strong 引用的话，编译器会有警告，为什么会警告呢，因为弱引用的weakself会在某个时间被释放，有可能是在执行之后的block之前就会被释放，这样在后续的操作操作就有可能出错，所以最好是添加一个对weakSelf的__strong引用。

在不久前看AFNetworking的源码时候发现了这么一句:

<span class="line-number">1</span><span class="line-number">2</span><span class="line-number">3</span><span class="line-number">4</span><span class="line-number">5</span><span class="line-number">6</span><span class="line-number">7</span><span class="line-number">8</span><span class="line-number">9</span><span class="line-number">10</span>

<code class="objc" style="padding: 0.3em; font-family: Monaco, Menlo, Consolas, 'Courier New', monospace; border: 0px; display: block; overflow-y: auto;"><span class="line"><span class="c1"><span class="comment" style="color: rgb(153, 153, 136); font-style: italic;">// 不知道这行代码的使用场景的同学你该去自习看看ARC的注意事项和Block的使用了</span></span></span><span class="line"><span class="c1"><span class="comment" style="color: rgb(153, 153, 136); font-style: italic;">// AFNetworking的写法</span></span></span><span class="line"><span class="n">__weak</span> <span class="n">__typeof</span><span class="p">(</span><span class="o">&*</span><span class="n"><span class="keyword" style="font-weight: bold;">self</span></span><span class="p">)</span><span class="n">weakSelf</span> <span class="o">=</span> <span class="n"><span class="keyword" style="font-weight: bold;">self</span></span><span class="p">;</span></span><span class="line"></span><span class="line"><span class="c1"><span class="comment" style="color: rgb(153, 153, 136); font-style: italic;">// 我之前一直这么写的</span></span></span><span class="line"><span class="n">__weak</span> <span class="n">__typeof</span><span class="p">(</span><span class="n"><span class="keyword" style="font-weight: bold;">self</span></span><span class="p">)</span> <span class="n">weakSelf</span> <span class="o">=</span> <span class="n"><span class="keyword" style="font-weight: bold;">self</span></span><span class="p">;</span></span><span class="line"><span class="c1"><span class="comment" style="color: rgb(153, 153, 136); font-style: italic;">// 或者这么写</span></span></span><span class="line"><span class="n">__weak</span> <span class="n">XxxViewController</span> <span class="o">*</span><span class="n">weakSelf</span> <span class="o">=</span> <span class="n"><span class="keyword" style="font-weight: bold;">self</span></span><span class="p">;</span></span><span class="line"><span class="c1"><span class="comment" style="color: rgb(153, 153, 136); font-style: italic;">// 或者这么写</span></span></span><span class="line"><span class="n">__weak</span> <span class="kt">id</span> <span class="n">weakSelf</span> <span class="o">=</span> <span class="n"><span class="keyword" style="font-weight: bold;">self</span></span><span class="p">;</span></span></code>