animation delegate

1. initial

// if want to change something, such as move a UIImageView,

// then pass this UIImageView as context argument

[UIView beginAnimations:@"test" context:nil];

[UIView setAnimationDelegate:self];

[UIView setAnimationDidStopSelector:

@selector(animationDidStop:finished:context:)];

[UIView commitAnimations];

2. when animation finish, animationDidStop is called

- (void)animationDidStop:(NSString *)animationID finished:(NSNumber *)finished

context:(void *)context