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。有一个进度指示器可以停止同步完成通知的动画，并且下面的代码会立即更改：

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

但为了让进度继续下去，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，当任务完成时，会发送（并执行）通知，但表中的单元格不会更新，直到（有时）大量的时间。所以在我看来，需要显示＆＃39;来自后台任务左右时不会触发，并且只在定期刷新间隔左右更新。

有没有办法让表单元格使用后台任务更新，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];

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

为了更新UI，您必须从后台线程调用主线程上的函数，如下所示：

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

如果您需要传递任何变量，可以继续添加-withObject

中的变量

Answer 2

您可以确保从任何队列收到通知......

[[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.
}];

如果要确保在主线程上收到它，可以使用

[NSOperationQueue mainQueue]

Answer 3

您需要在后台和前台线程之间拆分逻辑。您发送的通知应该从主线程发送，以便收听它的UI对象可以安全地更新。所以，将updateFunction分成两部分。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...];
});

在iOS上使用后台任务进行UI更新

3 个答案: