Explicit Animations - iOS Core Animation - page 181

- ( void )viewDidLoad
{
[ super viewDidLoad ];
//add the ship
CALayer *shipLayer = [ CALayer layer ];
shipLayer. frame = CGRectMake ( 0 , 0 , 128 , 128 );
shipLayer. position = CGPointMake ( 150 , 150 );
shipLayer. contents = ( __bridge id )[ UIImage imageNamed :
@"Ship.png" ]. CGImage ;
[ self .containerView. layer addSublayer :shipLayer];
//animate the ship rotation
CABasicAnimation *animation = [ CABasicAnimation animation ];
animation. keyPath = @"transform" ;
animation. duration = 2.0 ;
animation. toValue = [ NSValue valueWithCATransform3D :
CATransform3DMakeRotation ( M_PI , 0 , 0 , 1 )];
[shipLayer addAnimation :animation forKey : nil ];
}
@end
This works, but it turns out that this is more by luck than design. If we were to change the
rotation value from M_PI (180 degrees) to 2 * M_PI (360 degrees) and run the animation,
we'd find that the ship doesn't move at all. That's because the matrix representation for a
rotation of 360 degrees is the same as for 0 degrees, so as far as the animation is concerned,
the value hasn't changed.
Now try using M_PI again, but set it as the byValue property instead of toValue , to indicate
that the rotation should be relative to the current value. You might expect that to have the
same effect as setting toValue , because 0 + 90 degrees == 90 degrees, but in fact the ship
image expands instead of rotating because transform matrices cannot be added together like
angular values can.
What if we want to animate the translation or scale of the ship independently of its angle?
Because both of those require us to modify the transform property, we would need to
recalculate the combined effect of each of those animations at each point in time and create
a complex keyframe animation from the combined transform values, even though all we
really want to do is animate a few conceptually discrete attributes of our layer
independently.
Fortunately, there is a solution: To rotate the layer, we can apply our animation to the
transform.rotation key path instead of animating the transform property itself (see
Listing 8.9).