How to apply different easing effects to sprite action?

Question

I use a lot of CCEase* functionalities in Cocos2D described here. iOS 7 Sprite Kit also have SKActionTimingMode.

Answer 1

Sprite Kit left easing (or tweening) intentionally limited with the expectation that the developer would take control of the specifics of the motion of the sprites. Basically, what you need to do is make a custom action and apply an easing curve to the parameter before changing the property (rotation, position, scale, etc) of the sprite. Here's an example.

CGFloat initialScale = mySprite.xScale;
SKAction *scaleAction = [SKAction customActionWithDuration:duration actionBlock:^(SKNode *node, CGFloat elapsedTime) {
  CGFloat t = elapsedTime/duration;
  CGFloat p = t*t;
  CGFloat s = initialScale*(1-p) + scale * p;
  [node setScale:s];
}];
[mySprite runAction:scaleAction];

The part of this that determines the easing is p = t*t. So, p is a function of t such that :

• when t is 0, p is 0
• when t is 1, p is 1

That means that you will start at the beginning and end at the end but the shape of the curve in between will determine how you get there. Easing functions can be simple, like the one shown here, which is basically an ease-in, or quite complex such as elastic or bounce. To generate your own, try this : http://www.timotheegroleau.com/Flash/experiments/easing_function_generator.htm Or take a more detailed look at Robert Penner's equations: http://www.robertpenner.com/easing/

Answer 2

For arbitrary easing, Kardasis' answer says it all.

If you're looking for an easy way to add a bouncing effect to your animations, that is consistent with the way things are done in UIKit, I have something for you.

Apple introduced spring animations in UIKit a couple years ago, by letting you set a spring damping and initial velocity when performing a UIView animation. Unfortunately they didn't implement that in SpriteKit, so I made my own library that does just that.

It's a set of extensions on SKAction that replicate most factory methods, adding the damping and velocity parameters.

The code is on GitHub, feel free to use it: https://github.com/ataugeron/SpriteKit-Spring