Animate transition between normal and highlighted state of a UIButton

Question

I want to animate the transition between normal and highlighted state of a UIButton.

I put 2 images as background of the two states in IB and wrote the following code for TouchUpInside event:

- (void) animate {
    [UIView transitionWithView:self.myButton
                      duration:2
                       options:UIViewAnimationOptionTransitionCrossDissolve
                    animations:^{ self.myButton.highlighted = YES; }
                    completion:^(BOOL finished) {[UIView transitionWithView:self.myButton
                                                                   duration:2
                                                                    options:UIViewAnimationOptionTransitionCrossDissolve
                                                                 animations:^{ self.myButton.highlighted = NO; }
                                                                 completion:nil];}
     ];
}

In this way the animation is not started regularly and in particular if I tap quickly on the button.

I found that by preceding the above method with the following (linked to the UIButton in place of the previous one) the animation works without any problem ... but I don't understand the reason !

Could someone explain what exactly happens when I add this last method ?

- (void) firstThis {

    [self performSelector: @selector(animate) withObject: nil afterDelay: 0];
}

Thanks, Corrado

Answer 1

performSelector always schedules your method to be performed later (in this case on main), even if you set delay = 0. So, in your case the system gets a chance to properly start/stop animations and/or finish other stuff, like for example: the automatic highlighting of the button that happens because of touch-down events. The system starts the animation after having finished its own highlight animation of the button. The direct call interferes with this.