ios - 为什么 NSOperationQueue 不执行使用 addOperationWithBlock 提交的 block ？

Question

事情是这样的，我得到了一些不执行的代码(编译器运行代码但不做任何事情)...这是代码...使用 NSURLSessionDelegate 和 [NSOperationQueue mainQueue] addOperationWithBlock

@interface tablaPosViewController : UIViewController  <NSURLSessionDelegate>

@end

@implementation tablaPosViewController ()

- (void)viewDidLoad
{
//some code to set the view, labels and stuff

[self downloadTheHTMLdata] // this code download some data from WWW

}

- (void)downloadTheHMTLdata
{
//some code to set the session using an object

[object.downloadTask resume]; //Begins the download

}

- (void)toFixTablaPosiciones
{
//some code to do with the downloaded data from WWW (and HTML sheet)
//here, parse the HTML Sheet, and put some data into an Arrays, and another public vars

//call another method
[self PutTheDataInLabel];

}

- (void)PutTheDataInLabel
{
//some code to put all the data in every label
//take the public vars that was set in the previous method, and do some code with it
//and put the info into the labels

//call another method
[self MoreStuff];

}

- (void)MoreStuff
{

//some code..

}


-(void)URLSession:(NSURLSession *)session downloadTask:(NSURLSessionDownloadTask *)downloadTask didFinishDownloadingToURL:(NSURL *)location
{
    //When the download finish this method fire up!

    //this is to copy file from tmp folder to Docs folder
    if ([fileManager fileExistsAtPath:[destinationURL path]])
    {
        [fileManager removeItemAtURL:destinationURL error:nil];
    }
    BOOL success = [fileManager copyItemAtURL:location       //TMP download folder
                                        toURL:destinationURL //Documents folder 
                                        error:&error];
     //HERES COMES THE TROUBLE!!!
        [[NSOperationQueue mainQueue] addOperationWithBlock:^{
            [self toFixTablaPosiciones]; //Call this method, that has other method calls!
         }];

}
@end

更新

这另一个代码将方法放入队列中...

-(void)URLSessionDidFinishEventsForBackgroundURLSession:(NSURLSession *)session{
    AppDelegate *appDelegate = [UIApplication sharedApplication].delegate;
    [self.session getTasksWithCompletionHandler:^(NSArray *dataTasks, NSArray *uploadTasks, NSArray *downloadTasks) {
        if ([downloadTasks count] == 0) {
            if (appDelegate.backgroundTransferCompletionHandler != nil) {
                void(^completionHandler)() = appDelegate.backgroundTransferCompletionHandler;
                appDelegate.backgroundTransferCompletionHandler = nil;
                [[NSOperationQueue mainQueue] addOperationWithBlock:^{
                    completionHandler();
                }];
            }
        }
    }];
}

问题是当下载文件结束时，调用 -(void)URLSession:(NSURLSession *)session downloadTask:... 方法，我等待 [[NSOperationQueue mainQueue] addOperationWithBlock :^{... 运行所有内容...但它不执行 [self toFixTablaPosiciones] 的任何内容!!。

我一步一步地运行代码，我看到编译器如何运行所有代码，一个方法一个方法...但是 View 从不更新，运行但不执行，只是做任何事情，我有一个事件指示器，并想要停止它，但标签仍然带有虚拟数据，并且事件指示器永远不会停止，也永远不会消失。在之前的 View 中，我使用类似的类下载另一个文件，并且下载速度非常快。进入这个 View /类尝试执行下载，这就是事情......

希望任何人都可以帮助我并向我发送任何建议。谢谢!

Answer 1

与大多数有经验的开发人员一样，我使用的一种技术是使用断言和 NSLog(您可以注释掉它)来验证您对代码所做的假设实际上是正确的。尝试剪切并粘贴以下代码，看看会发生什么 - 它应该有所帮助。将来，不要用头撞墙——开始添加断言和日志。在某些时候，您会发现某些假设是不正确的。

-(void)URLSessionDidFinishEventsForBackgroundURLSession:(NSURLSession *)session {
    NSLog(@"Received URLSessionDidFinishEventsForBackgroundURLSession: application state is %d", [UIApplication sharedApplication] applicationState];
    AppDelegate *appDelegate = [UIApplication sharedApplication].delegate;

    [self.session getTasksWithCompletionHandler:^(NSArray *dataTasks, NSArray *uploadTasks, NSArray *downloadTasks) {
        NSLog(@"Download task count %d", [downloadTasks count]);
#warning "This looks like incorrect logic, but I don't know this background code. Wouldn't you see "1", or more? Won't you get an array of the tasks you submitted???
        if ([downloadTasks count] == 0) {
            assert(appDelegate.backgroundTransferCompletionHandler); // if this is nil, your app will be stuck in some odd state forever, so treat this like a fatal error
            void(^completionHandler)() = appDelegate.backgroundTransferCompletionHandler;
            appDelegate.backgroundTransferCompletionHandler = nil;

            assert([NSOperationQueue mainQueue]);   // why not?
            assert(![NSOperationQueue mainQueue].isSuspended);  // I looked at the class description, the queue **could** be suspended
            [[NSOperationQueue mainQueue] addOperationWithBlock:^{
                NSLog(@"Completion Operation called!");
                assert(completionHandler);  // perhaps an ARC bug, unlikely, but then you cannot get this code to work, right?
                completionHandler();
            }];
        }
    }];
}