Question

In example code below UIAlertView is show after delay, but i need display it immediately

//metoda zapisuje komentrz na serwerze
-(void) saveAction {

    UIAlertView *progressAlert = [[UIAlertView alloc] initWithTitle:NSLocalizedString(@"imageGalleries.sendAction", @"") message:@" " delegate:self cancelButtonTitle:NSLocalizedString(@"alert.cancel", @"") otherButtonTitles:nil];

    [progressAlert addSubview:progressView];
    [progressAlert show];

    // some long performance instructions
}

- (void)loadView {
    [super loadView];
    self.navigationItem.rightBarButtonItem =  [NavButton buttonWithTitle:NSLocalizedString(@"sendImage.saveButtonTitle", @"") target:self action:@selector(saveAction)];
    progressView = [[UIProgressView alloc] initWithFrame: CGRectMake(30.0f, 80.0f - 26, 225.0f, 10.0f)];
}

Why UIAlertView do not show immediately when I call saveAction?

Answer 1

If the “long performance instructions” following the alert code run on the main thread, they will block the alert from appearing. Read something about Cocoa run loops, that should make things more clear. (Essentially it could be said that all the UI instructions in your method are not performed immediately – they have to wait for the method to end, and then the main run loop picks them up and runs them.)

The code could better look like this:

- (void) startSomeLongOperation {
   [self createAndDisplayProgressSpinner];
   dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_NORMAL, 0), ^{
       // …do something that takes long…
       dispatch_async(dispatch_get_main_queue(), ^{
            [self dismissProgressSpinner];
       });
   });
}

This moves the long operation into background, so that the main thread can continue executing immediately.