推荐的异步块设计模式？

Question

我正在开发一个具有高度异步设计的iOS应用程序.在某些情况下,单个概念性"操作"可能会排队许多子块,这些子块将异步执行并异步接收它们的响应(调用远程服务器).这些子块中的任何一个都可以在错误状态下完成执行.如果在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级,并且应执行父级的错误处理块.

我想知道在这样的环境中可能建议使用哪些设计模式和其他技巧？

我知道GCD的dispatch_group_async和dispatch_group_wait功能.这可能是这个应用程序设计中的一个缺陷,但我对dispatch_group_async没有好运,因为该组似乎对子块没有"粘性".

提前致谢!

Answer 1

有一个WWDC视频(2012)可能会帮助你.它使用自定义NSOperationQueue并将异步块放在内部,NSOperations这样您就可以对块保持句柄并取消剩余的排队块.

一个想法是让子块的错误处理调用处理该类的类中的主线程上的方法NSOperationQueue.然后,班级可以适当地取消其余的.这样子块只需要知道自己的线程和主线程.这是视频的链接

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

该视频称为"在iOS上构建并发用户界面".相关部分主要是在下半部分,但你可能想要观察整个事情,因为它很好地将它放在上下文中.

编辑:

如果可能的话,我建议在嵌入式块中处理响应,它将它很好地包装在一起,这就是我认为你想要的...

//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];