1、在使用block时，如果block内部需要访问self的方法、属性、或者实例变量应当使用weakSelf

    __weak __typeof__(self) weakSelf = self;

    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{

        weakSelf.titleLabel.text = @"111"; 

         weakSelf.nameLabel.text = @"222"; 

    });
`

2、如果在block内需要多次访问self，则需要使用strongSelf

    __weak __typeof__(self) weakSelf = self;

    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{

        __strong __typeof(self) strongSelf = weakSelf;

        [strongSelf doSomething];

        [strongSelf doOtherThing];

    });

3、系统的block内部不需要使用weakSelf和strongSelf。

4、为了防止weakSelf 有可能被释放，通常和strongSelf一块上场。

__weak typeof(self) weakSelf = self;
[self doSomeBackgroundJob:^{
    __strong typeof(weakSelf) strongSelf = weakSelf;
    if (strongSelf) {
        ...
    }
}];

5、多层嵌套block。

    __weak typeof(self) weakSelf = self;
    [self.device connect:10 success:^{
        __strong typeof(self) strongSelf = weakSelf;
        __weak typeof(self) weakSelf2 = strongSelf;
        [strongSelf.device setBoMoDeviceWifi:jsonString timeout:10 callback:^(CMDAckType ack) {
            __strong typeof(self) strongSelf2 = weakSelf2;
            [strongSelf2.device disconnect];
            [strongSelf2 removeLoadingView];

            if (ack == CMDAckTypeSuccess) {
                [strongSelf2 showToastWithText:LOCALIZATION(@"setDeviceWifi Success!")];
            }else{
                [strongSelf2 showToastWithText:LOCALIZATION(@"setDeviceWifi Failed!")];
            }
        }];
        NSLog(@"-----");//因为是strongSelf强指针指着第二个block，所以即使走到这里，第一个代码块也不会释放。只有等第二个结束才会一起释放。

    } failed:^(CMDAckType ack) {
        __strong typeof(weakSelf) strongSelf = weakSelf;
        [strongSelf removeLoadingView];
        [strongSelf showToastWithText:LOCALIZATION(@"Connect Failed!")];
    }];
//weakSelf是为了避免循环引用，strongSelf是为了防止weakSelf被提前释放。如此类推，嵌套的第二个block也应该这样做。不管多少个都这样。