iOS中block的weakSelf、strongSelf怎么配

如果在 block 中必须多次使用到 weakSelf 会有危险，因为在多事件执行时，weakSelf 有可能在 block 跑到一半的时候被设成 nil

__weak __typeof__(self) weakSelf = self;
dispatch_async(queue, ^{
    //执行到这里不会被释放
    [weakSelf doSomething];

    //执行到这里有可能会被释放
    [weakSelf doOtherThing];
});

因此必须在 block 内使用 strongSelf，确保 reference 不会执行到一半变成 nil (注意在建立 strongSelf 以后还会再判断其是否为 nil，因为有可能在指定 strongSelf 的时间点 weakSelf = self 就已经为 nil 了)

__weak typeof(self) weakSelf = self;
myObj.myBlock =  ^{
    __strong typeof(self) strongSelf = weakSelf;
    if (strongSelf) {
      [strongSelf doSomething]; // strongSelf != nil
      // preemption, strongSelf still not nil
      [strongSelf doSomethingElse]; // strongSelf != nil
    }
    else {
        // Probably nothing...
        return;
    }
};

使用strongSelf之后，指针连带关系self的引用计数还会增加。但是strongSelf是在Block里面，生命周期也只在当前Block的作用域。所以，当这个Block结束，strongSelf随之也就被释放了。同时也不会影响Block外部的self的生命周期

• SVProgressHUD源码

    __block void (^animationsBlock)(void) = ^{
        __strong SVProgressHUD *strongSelf = weakSelf;
        if(strongSelf) {
            // Shrink HUD to finish pop up animation
            strongSelf.hudView.transform = CGAffineTransformScale(strongSelf.hudView.transform, 1/1.3f, 1/1.3f);
            strongSelf.alpha = 1.0f;
            strongSelf.hudView.alpha = 1.0f;
        }
    };

• AFNetworking源码

__weak __typeof(self)weakSelf = self;
    AFNetworkReachabilityStatusBlock callback = ^(AFNetworkReachabilityStatus status) {
        __strong __typeof(weakSelf)strongSelf = weakSelf;

        strongSelf.networkReachabilityStatus = status;
        if (strongSelf.networkReachabilityStatusBlock) {
            strongSelf.networkReachabilityStatusBlock(status);
        }

    };

总结：当 block 内会多次使用 weakSelf，且有用到多次执行时，需使用 strongSelf

如果出现双层block嵌套甚至更多怎么办😰

- (void)setUpModel{
    XYModel *model = [XYModel new];
   
    __weak typeof(self) weakSelf = self;
    model.dataChanged = ^(NSString *title) {
        __strong typeof(self) strongSelf = weakSelf;
        strongSelf.titleLabel.text = title;
        
        __weak typeof(self) weakSelf2 = strongSelf;
        strongSelf.model.dataChanged = ^(NSString *title2) {
            __strong typeof(self) strongSelf2 = weakSelf2;
            strongSelf2.titleLabel.text = title2;
        };
    };
    
    self.model = model;
}

参考文章
iOS的weakSelf与strongSelf
为什么 weakSelf 需要配合 strong self 使用