[SpriteKit] Custom SKAction to transition the fill/stroke color of an SKShapeNode.

SKActionTransitionColor.swift

import CoreGraphics
import SpriteKit

extension SKAction {

    static func transitionStrokeColor(from: SKColor, to: SKColor, duration: TimeInterval) -> SKAction {
        return transitionColor(from: from, to: to, duration: duration, fill: false)
    }

    static func transitionFillColor(from: SKColor, to: SKColor, duration: TimeInterval) -> SKAction {
        return transitionColor(from: from, to: to, duration: duration, fill: true)
    }

    static func transitionColor(from: SKColor, to: SKColor, duration: TimeInterval, fill: Bool) -> SKAction {
        var fromRed: CGFloat = 0
        var fromGreen: CGFloat = 0
        var fromBlue: CGFloat = 0
        var fromAlpha: CGFloat = 0

        from.getRed(&fromRed, green: &fromGreen, blue: &fromBlue, alpha: &fromAlpha)

        var toRed: CGFloat = 0
        var toGreen: CGFloat = 0
        var toBlue: CGFloat = 0
        var toAlpha: CGFloat = 0

        to.getRed(&toRed, green: &toGreen, blue: &toBlue, alpha: &toAlpha)

        return SKAction.customAction(withDuration: duration, actionBlock: { (node, elapsed) in
            guard let shapeNode = node as? SKShapeNode else { fatalError("Can only use SKAction.transitionColor(from:to:duration:fill) on an SKShapeNode.") }

            let step: CGFloat = elapsed / CGFloat(duration)

            let r = CGFloat.lerp(from: fromRed, to: toRed, step: step)
            let g = CGFloat.lerp(from: fromGreen, to: toGreen, step: step)
            let b = CGFloat.lerp(from: fromBlue, to: toBlue, step: step)
            let a = CGFloat.lerp(from: fromAlpha, to: toAlpha, step: step)

            let color = UIColor(red: r, green: g, blue: b, alpha: a)

            if fill {
                shapeNode.fillColor = color
            } else {
                shapeNode.strokeColor = color
            }
        })
    }
}

extension CGFloat {

    static func lerp(from: CGFloat, to: CGFloat, step: CGFloat) -> CGFloat {
        return (1.0 - step) * from + step * to
    }
}