UIView animation completion block not called

Question

When I first call following code from dispatch queue, completion block not called.

- (void)viewDidLoad
{
    [super viewDidLoad];
    dispatch_queue_t initialize_queue;
    initialize_queue = dispatch_queue_create("init", NULL);
    dispatch_async(initialize_queue, ^{
        _onInit = YES;
        _isRunning = NO;
        [self startProgress];
        _onInit = NO;
    });
} 

 - (void)startProgress
 {
  if (!_isRunning) {
    _isRunning = YES;
    NSLog(@"Starting");
    [UIView animateWithDuration:0.5 animations:^{
        self.progressStatusButton.hidden = NO;
        self.activityLeftConstraint.constant = _activityLeftSpace;
        self.activityWidthConstraint.constant = _activityWidth;
        self.buttonWidthConstraint.constant = _progressStatusButtonWidth;
        self.buttonLeftConstraint.constant = _progressStatusButtonLeftSpace;
        self.activityView.alpha = 1.0;
    }completion:^(BOOL finished){
        NSLog(@"Start Animating");
        [self.activityView startAnimating];
    }];
  }
 }

When I delete dispatch_aync method and execute startProgress method in viewDidLoad,
completion block called. How should I correct my code?
I tried to change dispatch_async to use dispatch_async(dispatch_get_main_queue(),..),
then completion block called, but I would like to execute asynchronously startProgress method.

Please Let me know.

Answer 1

Everything connected with GUI should be called from main thread/ pushed to main queue. In other case expected behavior is not guaranteed.

So you should use

dispatch_get_main_queue()

Also animation ALWAYS executed asynchronously so I do not think you really need separate queue for it.