Explicit Animations - iOS Core Animation - page 187

[ super viewDidLoad ];
//set up images
self .images = @[ [ UIImage imageNamed : @"Anchor.png" ],
[ UIImage imageNamed : @"Cone.png" ],
[ UIImage imageNamed : @"Igloo.png" ],
[ UIImage imageNamed : @"Spaceship.png" ] ] ;
}
- ( IBAction )switchImage
{
//set up crossfade transition
CATransition *transition = [ CATransition animation ];
transition. type = kCATransitionFade ;
//apply transition to imageview backing layer
[ self .imageView. layer addAnimation :transition forKey : nil ];
//cycle to next image
UIImage *currentImage = self .imageView. image ;
NSUInteger index = [ self .images indexOfObject :currentImage];
index = (index + 1 ) % [ self .images count ];
self .imageView. image = self .images[index];
}
@end
As you can see from the code, transitions are added to a layer in the same way as property
or group animations, using the -addAnimation:forKey: method. Unlike property
animations, though, only one CATransition can operate on a given layer at a time. For this
reason, regardless of what value you specify for the key, the transition will actually be
attached with a key of "transition" , which is represented by the constant
kCATransition .