SKBounceAnimation is a
CAKeyframeAnimation subclass that creates an animation for you based on start and end values and a number of bounces. It’s based on the math and technology in this blogpost: khanlou.com/2012/01/cakeyframeanimation-make-it-bounce/ which in turn was based partially on Matt Gallagher’s work here: cocoawithlove.com/2008/09/parametric-acceleration-curves-in-core.html.
Basic code is simple:
NSString *keyPath = @"position.y"; id finalValue = [NSNumber numberWithFloat:300]; [view.layer setValue:finalValue forKeyPath:keyPath]; SKBounceAnimation *bounceAnimation = [SKBounceAnimation animationWithKeyPath:keyPath]; bounceAnimation.fromValue = [NSNumber numberWithFloat:view.center.x]; bounceAnimation.toValue = finalValue; bounceAnimation.duration = 0.5f; bounceAnimation.numberOfBounces = 2; [view.layer addAnimation:bounceAnimation forKey:@"someKey"];
We set the value of our keypath to the final value, and then perform the animation. When the animation finishes, it is automatically removed from the layer, and the
finalValue takes over. If you do not use
-(void)setValue:forKeyPath:, the original value for the keyPath will take over and the animation will snap back to original location after the animation is over.
The math is simple. Check out the blogpost and the informational post preceding it for exact details, but essentially the system behaves with oscillating exponential decay in the form of the equation:
x = Ae^(-αt)•cos(ωt) + B.
A is the difference between start and end values, B is the end value, α is determined by the number of frames required to get the exponential decay portion to close enough to 0, and ω is determined by the number of periods required to get the desired number of bounces.
shouldOvershoot is a property that you can change. It defaults to
YES; if you set it to
NO, the animation will bounce as if it were hitting a wall, instead of overshooting the target value and bouncing back. It looks a lot like the Anvil effect in Keynote.
shake is a property that controls the oscilliation function. Setting it to
YES lets you shake the element instead of moving it. To use it, set the
fromValue to the maximum amount you want it to go to and
toValue to its current location. It uses a sine wave for the oscillation intead of cosine, since it starts at 0 (i.e., the current location.)
The demo app contains demos for several different animations that are supported by
- One-axis animation: Using a keypath like
position.x, we can animate along one axis.
- Two-axis animation: Using a keypath like
SKBounceAnimationwill generate a path, and your layer will follow it.
- Size: Using the
boundskeypath, we can make the size increase. The center of the size increase is determined by
anchorPoint, which can be moved. It defaults to the center of the layer
- Color: I have no idea why anyone would want to bounce a color animation, but I was feeling whimsical, so I added support for this as well.
- Scale: Using a
CATransform3Dstruct and the
transformkeypath, we can scale objects. This is very useful to create an effect like UIAlerts bouncing in. The
anchorPointalso judges how this effect happens.
- Scale & Rotate: Using multiple CATransform3Ds on top of each other, we can do super weird effects like scale and rotating. They look really cool.
- Rect: The last demo creates two
SKBounceAnimationswith two different
bounds) but attaches them to the same layer. The effect looks like a
SKBounceAnimation doesn’t support the
byValue property yet.