Implementation of Flinto for Mac's spring.
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To reproduce spring values created in Flinto for Mac, use RK4SpringAnimation in RK4Spring.swift.
public func RK4SpringAnimation(tension tension:Double, friction:Double, initialVelocity:Double, delta:Double = 1/60.0) -> [Double]
tensionshould be between 5.0 to 1000.0.frictionshould be between 5.0 to 100.0.initialVelocityshould be between -100.0 to 100.0.deltais optional and only 1/60 is supported at this moment.
initialVelocity value is usually 0 in Flinto.app. You can supply velocity values into spring by using UIPanGestureRecognizer's velocityInView: method. It's a unit coordinate system, where 1 is defined as travelling the total animation distance in a second. So if you're changing an object's position by 200px in this animation, and velocityInView returns 100 (means 100px/s), you'd pass 0.5.
The return values are sample values between 0.0 to 1.0, and one sample duration is 1/60 second. Therefore, if you want to move a layer from Y=0 to Y=100, you should multiply the values by 100 and set duration to Double(values.count) * 1 / 60.0. The code with CAKeyframeAnimation would look like the following:
let values = RK4SpringAnimation(tension: tension, friction: friction, initialVelocity: velocity)
let positions = values.map {$0 * 100}
let anim = CAKeyframeAnimation(keyPath: "position.y")
anim.values = positions
anim.duration = Double(values.count) * 1 / 60.0
layer.add(anim, forKey: "spring")
layer.setValue(NSNumber(value: 100.0 as Double), forKey: "position.y")