异步块的推荐设计模式?

有一个WWDC视频(2012)可能会帮助你。它使用自定义 NSOperationQueue 并将异步块放置在内部 NSOperations 这样您就可以保留块的句柄并取消剩余的排队块。

一个想法是让子块的错误处理在处理 操作队列 。然后该课程可以适当地取消其余部分。这样子块只需要知道自己的线程和主线程。这是视频链接

https://developer.apple.com/videos/wwdc/2012/

该视频名为“在iOS上构建并发用户界面”。相关的部分主要在后半部分,但你可能想仔细观察整个事情,因为它很好地将其放在上下文中。

编辑:

如果可能的话,我建议在嵌入式块中处理响应,这样可以很好地将其封装在一起,这就是我 认为 你在追求。。

//Define an NSBlockOperation, and get weak reference to it
NSBlockOperation *blockOp = [[NSBlockOperation alloc]init];
__weak NSBlockOperation *weakBlockOp = blockOp;

//Define the block and add to the NSOperationQueue, when the view controller is popped
//we can call -[NSOperationQueue cancelAllOperations] which will cancel all pending threaded ops
[blockOp addExecutionBlock: ^{

    //Once a block is executing, will need to put manual checks to see if cancel flag has been set otherwise
    //the operation will not be cancelled. The check is rather pointless in this example, but if the
    //block contained multiple lines of long running code it would make sense to do this at safe points
    if (![weakBlockOp isCancelled]) {

        //substitute code in here, possibly use *synchronous* NSURLConnection to get
        //what you need. This code will block the thread until the server response
        //completes. Hence not executing the following block and keeping it on the 
        //queue.  
        __block NSData *temp;
        response = [NSData dataWithContentsOfURL:[NSURL URLWithString:urlString]];

        [operationQueue addOperationWithBlock:^{
            if (error) {
                  dispatch_async(dispatch_get_main_queue(), ^{
                        //Call selector on main thread to handle canceling
                        //Main thread can then use handle on NSOperationQueue
                        //to cancel the rest of the blocks 
                  });
            else {
                 //Continue executing relevant code....      
            }
        }];
    }
}];
[operationQueue addOperation:blockOp];

自从发布这个问题以来,我遇到的一种模式是使用信号量将异步操作更改为同步操作。这非常有用。这篇博客文章更详细地介绍了这个概念。

http://www.g8production.com/post/76942348764/wait-for-blocks-execution-using-a-dispatch-semaphore

有很多方法可以在cocoa中实现异步行为。

GCD、NSOperationQueue、performSelectAfterDelay,创建自己的线程。有适当的时间使用这些机制。这里讨论的时间太长了,但你在帖子中提到的一些事情需要解决。

如果任何子块中发生错误,则应取消任何其他子块,错误状态应渗透到父块,并执行父块的错误处理块。

块不能将错误添加到堆栈中。时期