iOS上的后台任务的UI更新

Question

我有一个同步过程，有时（甚至很少）需要20秒。我曾经在后台调用选择器。有其停止对同步完成的通知动画进度指示器，并与它下面的代码马上改变：

[self performSelectorInBackground:@selector(updateFunction) withObject:nil]; 

但为了让进度继续我决定使用后台任务：

UIApplication *application = [UIApplication sharedApplication]; //Get the shared application instance 

    __block UIBackgroundTaskIdentifier background_task; //Create a task object 

    background_task = [application beginBackgroundTaskWithExpirationHandler:^{ 
     [application endBackgroundTask: background_task]; //Tell the system that we are done with the tasks 
     background_task = UIBackgroundTaskInvalid; //Set the task to be invalid 

     //System will be shutting down the app at any point in time now 
    }]; 

    //Background tasks require you to use asyncrous tasks 

    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{ 
     //Perform your tasks that your application requires 

     NSLog(@"\n\nRunning in the background!\n\n"); 
     [self updateFunction]; 

     [application endBackgroundTask: background_task]; //End the task so the system knows that you are done with what you need to perform 
     background_task = UIBackgroundTaskInvalid; //Invalidate the background_task 
     NSLog(@"Finished in the background!"); 
    }); 

但是现在，当任务完成后通知被发送（和执行），但在表中的单元格不更新，直到（有时）的时间显著量。所以在我看来，'需求显示'在后台任务等等时不会被触发，而只是在常规的刷新间隔内进行更新。

有没有办法让表格单元更新使用后台任务，或者我错过了什么？

编辑： 通知监听器是：

[[NSNotificationCenter defaultCenter] addObserver:self 
             selector:@selector(refreshBttn) 
              name:@"finishedUpdate" 
              object:nil]; 

的代码是：

- (void) refreshBttn { 
    NSLog(@"Refresh Buttun action"); 
    iDomsAppDelegate *delegate = (iDomsAppDelegate *)[[UIApplication sharedApplication] delegate]; 
    if([delegate updating]){ 
     [_updateBttn setHidden:TRUE]; 
     [_activityIndicator startAnimating];   
    } else { 
     [_updateBttn setHidden:FALSE]; 
     [_activityIndicator stopAnimating];   
    } 
    [self setNeedsDisplay]; 
    [self setNeedsLayout]; 
} 

Answer 1

您需要将背景和前景线程之间的逻辑分开。您发送的通知应该从主线程发送，以便您的UI对象在收听时可以安全地进行自我更新。所以，把updateFunction分成两块。也许是这样的：

NSLog(@"\n\nRunning in the background!\n\n"); 
[self updateFunctionBackground]; 
dispatch_async(dispatch_get_main_queue(), ^{ 
    // send your notification here instead of in updateFunction 
    [[NSNotificationCenter defaultCenter] post...]; 
}); 

Answer 2

为了使UI更新，你必须从你的后台调用主线程功能像这样的线程：

[self performSelectorOnMainThread:@selector(updateFunction) withObject:nil waitUntilDone:NO]; 

如果你需要通过任何变量你可以去一个并将其添加到-withObject中

Answer 3

可以确保通知从你想与任何队列......收到

[[NSNotificationCenter defaultCenter] addObserverForName:SomeNotification 
                object:sender 
                queue:operationQueue 
               usingBlock:^(NSNotification *){ 
    // Your notification will be processed in this block, and it will be 
    // received on the notification queue you specify, regardless of what 
    // thread the sender was running when the notification was posted. 
}]; 

如果你想确保它接收到主线程，你可以使用

[NSOperationQueue mainQueue]