我们在研发的过程中，为了避免循环引用常常会用weak若饮用来打破循环链
__weak typeof(self) weakSelf = self;
但是有时候，在异步多任务的时候，为了避免weakself提前被释放，需要配合
__strong typeof(self) strongSelf = weakSelf;
__weak __typeof__(self) weakSelf = self;
dispatch_group_async(_operationsGroup, _operationsQueue, ^
{
__typeof__(self) strongSelf = weakSelf;
[strongSelf doSomething];
[strongSelf doSomethingElse];
} );
