block的实现思想是通过建立局部变量的快照，让block中的修改不影响原始变量。在实现方式上为了能够延迟执行某个block，需要hold住相关的变量，内部会retain变量。比方说含有copy属性的block在使用_instanceObj时不会retain变量，但是会retain self，如果使用不当就会导致循环引用。

无需解决循环引用的闭包

可以直接使用self或者_instanceObj

不持有block

[UIView animateWithDuration:0.2 animations:^{
    self.alpha = 0.0f;
} completion:^(BOOL finished) {
    [self stopLoading];
}];

同步执行

[_messageList enumerateObjectsUsingBlock:^(TimelineMessageData *nodeData, NSUInteger index, BOOL *stop) {
    if (nodeData.svrID == msgData.svrID) {
        [_tableView reloadRow:index inSection:kSectionOffset rowAnimation:UITableViewRowAnimationNone];
        *stop = YES;
    }
}];

需要解决循环引用的闭包

不可以直接使用self或者_instanceObj，需要做些额外的工作，常用的处理方式有两种

使用weakSelf+strongSelf

直接调用一个实例变量，会被编译器处理成 ‘self->theVar’，’self’ 是一个 strong 类型的变量，引用计数会加 1

__weak __typeof__(self) weakSelf = self;
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
    [weakSelf doSomething];
});

在 doSomething 中，weakSelf 不会变成 nil，不过在 doSomething 执行完成，调用第二个方法 doOtherThing 的时候，weakSelf 有可能被释放

__weak __typeof__(self) weakSelf = self;
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
    [weakSelf doSomething];
    [weakSelf doOtherThing];
});

__strong 确保在 Block 内，strongSelf 不会被释放。

__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];
});

主动断开循环引用

在如下两个block中可直接用self。

- (void)startWithCompletionBlockWithSuccess:(void (^)(YTKBatchRequest *batchRequest))success
                                    failure:(void (^)(YTKBatchRequest *batchRequest))failure;
- (void)setCompletionBlockWithSuccess:(void (^)(YTKBatchRequest *batchRequest))success
                              failure:(void (^)(YTKBatchRequest *batchRequest))failure;

因为在回调函数中主动断开了循环引用。

- (void)handleRequestResult:(AFHTTPRequestOperation *)operation {
    NSString *key = [self requestHashKey:operation];
    YTKBaseRequest *request = _requestsRecord[key];
    YTKLog(@"Finished Request: %@", NSStringFromClass([request class]));
    if (request) {
        BOOL succeed = [self checkResult:request];
        if (succeed) {
            [request toggleAccessoriesWillStopCallBack];
            [request requestCompleteFilter];
            if (request.delegate != nil) {
                [request.delegate requestFinished:request];
            }
            if (request.successCompletionBlock) {
                request.successCompletionBlock(request);
            }
            [request toggleAccessoriesDidStopCallBack];
        } else {
            YTKLog(@"Request %@ failed, status code = %ld",
                     NSStringFromClass([request class]), (long)request.responseStatusCode);
            [request toggleAccessoriesWillStopCallBack];
            [request requestFailedFilter];
            if (request.delegate != nil) {
                [request.delegate requestFailed:request];
            }
            if (request.failureCompletionBlock) {
                request.failureCompletionBlock(request);
            }
            [request toggleAccessoriesDidStopCallBack];
        }
    }
    [self removeOperation:operation];
    [request clearCompletionBlock];
}

- (void)cancelRequest:(YTKBaseRequest *)request {
    [request.requestOperation cancel];
    [self removeOperation:request.requestOperation];
    [request clearCompletionBlock];
}

- (void)clearCompletionBlock {
    self.successCompletionBlock = nil;
    self.failureCompletionBlock = nil;
}