iOS __weak __strong

在block中常常会用到self，可是会造成循环引用。这时候就需要这样来解决这个问题：

#define WeakSelf __weak __typeof(self) weakSelf = self
#define StrongSelf __strong __typeof(weakSelf) self = weakSelf

- (void)aFunc:(id)sender {

    WeakSelf;//1
    [UIView animateWithDuration:1 animations:^{
        StrongSelf;//2
        [self.view doSomething];//use self
    }];
}