replacing CALayer and CABasicAnimation with SKScene and SKActions

Question

I am trying to restructure this Github Swift project on Metaballs so that the circles are represented by SKShapeNodes that are moved around by SKActions instead of CAB

Answer 1

You can easily achieve this kind of animation using moveToX and the timingMode parameter.

New Swift 3 translation below at the end of this answer.

To make an example I use the Xcode Sprite-Kit "Hello, World!" official project demo:

class GameScene: SKScene {
    override func didMoveToView(view: SKView) {
        /* Setup your scene here */
        let myLabel = SKLabelNode(fontNamed:"Chalkduster")
        myLabel.text = "Hello, World!"
        myLabel.fontSize = 15
        myLabel.position = CGPoint(x:CGRectGetMidX(self.frame), y:CGRectGetMidY(self.frame))

        self.addChild(myLabel)
        mediaTimingFunctionEaseInEaseOutEmulate(myLabel)
    }
    func mediaTimingFunctionEaseInEaseOutEmulate(node:SKLabelNode) {
        let actionMoveLeft = SKAction.moveToX(CGRectGetMidX(self.frame)-100, duration:1.5)
        actionMoveLeft.timingMode = SKActionTimingMode.EaseInEaseOut

        let actionMoveRight = SKAction.moveToX(CGRectGetMidX(self.frame)+100, duration:1.5)
        actionMoveRight.timingMode = SKActionTimingMode.EaseInEaseOut

        node.runAction(SKAction.repeatActionForever(SKAction.sequence([actionMoveLeft,actionMoveRight])))
    }
}

Output:

Update (This part start to emulate the static ball and the dynamic ball moving left and right but without metaball animations)

class GameScene: SKScene {
    var dBCircle : SKShapeNode!
    let radiusDBCircle: CGFloat = 10
    let radiusBall: CGFloat = 15
    private let SCALE_RATE: CGFloat = 0.3
    override func didMoveToView(view: SKView) {
        // Some parameters
        let strokeColor = SKColor.orangeColor()
        let dBHeight = CGRectGetMaxY(self.frame)-84 // 64 navigationController height + 20 reasonable distance
        let dBStartX = CGRectGetMidX(self.frame)-160 // extreme left
        let dBStopX = CGRectGetMidX(self.frame)+160 // extreme right
        let dBWidth = dBStopX - dBStartX
        let totalBalls = 7 // first and last will be hidden
        let ballArea = dBWidth / CGFloat(totalBalls-1)
        let distanceBtwBalls = ((ballArea-(radiusBall*2))+radiusBall*2)

        // Create dbCircle
        dBCircle = SKShapeNode.init(circleOfRadius: radiusDBCircle)
        dBCircle.position = CGPointMake(CGRectGetMidX(self.frame), dBHeight)
        dBCircle.strokeColor = strokeColor
        dBCircle.name = "dBCircle"
        dBCircle.fillColor = UIColor.clearColor()
        addChild(dBCircle)
        // Make static balls
        for i in 0..

New update with metaball animation:

Finally I've realize this result, my goal is to make it very similar to the original :

is it possible to make some variations to times (for example zoomIn or zoomOut time values or actionMoveLeft, actionMoveRight time values), this is the code:

import SpriteKit
class GameScene: SKScene {
    var dBCircle : SKShapeNode!
    let radiusDBCircle: CGFloat = 10
    let radiusBall: CGFloat = 15
    private let SCALE_RATE: CGFloat = 0.3
    override func didMoveToView(view: SKView) {
        // Some parameters
        let strokeColor = SKColor.orangeColor()
        let dBHeight = CGRectGetMaxY(self.frame)-84 // 64 navigationController height + 20 reasonable distance
        let dBStartX = CGRectGetMidX(self.frame)-160 // extreme left
        let dBStopX = CGRectGetMidX(self.frame)+160 // extreme right
        let dBWidth = dBStopX - dBStartX
        let totalBalls = 7 // first and last will be hidden
        let ballArea = dBWidth / CGFloat(totalBalls-1)
        let distanceBtwBalls = ((ballArea-(radiusBall*2))+radiusBall*2)

        // Create dbCircle
        dBCircle = SKShapeNode.init(circleOfRadius: radiusDBCircle)
        dBCircle.position = CGPointMake(CGRectGetMidX(self.frame), dBHeight)
        dBCircle.strokeColor = strokeColor
        dBCircle.name = "dBCircle"
        dBCircle.fillColor = UIColor.clearColor()
        addChild(dBCircle)
        // Make static balls
        for i in 0.. maxDistance || d <= abs(radius1 - radius2)) {
            return
        } else if (d < radius1 + radius2) {
            u1 = acos((radius1 * radius1 + d * d - radius2 * radius2) / (2 * radius1 * d))
            u2 = acos((radius2 * radius2 + d * d - radius1 * radius1) / (2 * radius2 * d))
        } else {
            u1 = 0.0
            u2 = 0.0
        }
        let angle1 = center1.angleBetween(center2)
        let angle2 = acos((radius1 - radius2) / d)
        let angle1a = angle1 + u1 + (angle2 - u1) * v
        let angle1b = angle1 - u1 - (angle2 - u1) * v
        let angle2a = angle1 + CGFloat(M_PI) - u2 - (CGFloat(M_PI) - u2 - angle2) * v
        let angle2b = angle1 - CGFloat(M_PI) + u2 + (CGFloat(M_PI) - u2 - angle2) * v
        let p1a = center1.point(radians: angle1a, withLength: radius1)
        let p1b = center1.point(radians: angle1b, withLength: radius1)
        let p2a = center2.point(radians: angle2a, withLength: radius2)
        let p2b = center2.point(radians: angle2b, withLength: radius2)
        let totalRadius = radius1 + radius2
        var d2 = min(v * handeLenRate, p1a.minus(p2a).length() / totalRadius)
        d2 *= min(1, d * 2 / totalRadius)
        radius1 *= d2
        radius2 *= d2
        let cp1a = p1a.point(radians: angle1a - CGFloat(M_PI_2), withLength: radius1)
        let cp2a = p2a.point(radians: angle2a + CGFloat(M_PI_2), withLength: radius2)
        let cp2b = p2b.point(radians: angle2b - CGFloat(M_PI_2), withLength: radius2)
        let cp1b = p1b.point(radians: angle1b + CGFloat(M_PI_2), withLength: radius1)
        let pathJoinedCircles = UIBezierPath()
        pathJoinedCircles.moveToPoint(p1a)
        pathJoinedCircles.addCurveToPoint(p2a, controlPoint1: cp1a, controlPoint2: cp2a)
        pathJoinedCircles.addLineToPoint(p2b)
        pathJoinedCircles.addCurveToPoint(p1b, controlPoint1: cp2b, controlPoint2: cp1b)
        pathJoinedCircles.addLineToPoint(p1a)
        pathJoinedCircles.closePath()
        let shapeNode = SKShapeNode(path: pathJoinedCircles.CGPath)
        shapeNode.strokeColor = SKColor.orangeColor()
        shapeNode.fillColor = UIColor.clearColor()
        addChild(shapeNode)
        let wait = SKAction.waitForDuration(vanishingTime)
        self.runAction(wait,completion: {
            shapeNode.removeFromParent()
        })
    }
    override func update(currentTime: NSTimeInterval) {
        var i = 0
        self.enumerateChildNodesWithName("ball") {
            node, stop in
            let ball = node as! SKShapeNode
            let enlargeFrame = CGRectMake(ball.frame.origin.x-self.radiusBall*3,ball.frame.origin.y,ball.frame.width+(self.radiusBall*6),ball.frame.height)
            if CGRectContainsRect(enlargeFrame, self.dBCircle.frame) {
                if (ball.actionForKey("zoom") == nil) {
                    let zoomIn = SKAction.scaleTo(1.5, duration: 0.25)
                    zoomIn.timingMode = SKActionTimingMode.EaseInEaseOut
                    let zoomOut = SKAction.scaleTo(1.0, duration: 0.25)
                    let wait = SKAction.waitForDuration(0.8)
                    let seq = SKAction.sequence([zoomIn,zoomOut,wait])
                    ball.runAction(seq,withKey: "zoom")
                }
            }
            self._metaball(ball, circle1: self.dBCircle, v: 0.6, handeLenRate: 2.0, maxDistance: 4 * self.radiusBall)
            i += 1
        }
    }
}
//MARK: - Extensions
extension CGPoint {
    func distance(point: CGPoint) -> CGFloat {
        let dx = point.x - self.x
        let dy = point.y - self.y
        return sqrt(dx * dx + dy * dy)
    }
    func angleBetween(point: CGPoint) -> CGFloat {
        return atan2(point.y - self.y, point.x - self.x)
    }
    func point(radians radians: CGFloat, withLength length: CGFloat) -> CGPoint   {
        return CGPoint(x: self.x + length * cos(radians), y: self.y + length * sin(radians))
    }
    func minus(point: CGPoint) -> CGPoint {
        return CGPoint(x: self.x - point.x, y: self.y - point.y)
    }
    func length() -> CGFloat {
        return sqrt(self.x * self.x + self.y + self.y)
    }
}

Swift 3:

(I've made a little change to maxDistance: 4 * self.radiusBall with maxDistance: 5 * self.radiusBall to become more similar to the original but you can change it as you wish)

import SpriteKit
class GameScene: SKScene {
    var dBCircle : SKShapeNode!
    let radiusDBCircle: CGFloat = 10
    let radiusBall: CGFloat = 15
    private let SCALE_RATE: CGFloat = 0.3
    override func didMove(to view: SKView) {
        let label = self.childNode(withName: "//helloLabel") as? SKLabelNode
        label?.removeFromParent()
        self.anchorPoint = CGPoint.zero
        // Some parameters
        let strokeColor = SKColor.orange
        let dBHeight = self.frame.midY
        let dBStartX = self.frame.midX-260 // extreme left
        let dBStopX = self.frame.midX+260 // extreme right
        let dBWidth = dBStopX - dBStartX
        let totalBalls = 7 // first and last will be hidden
        let ballArea = dBWidth / CGFloat(totalBalls-1)
        let distanceBtwBalls = ((ballArea-(radiusBall*2))+radiusBall*2)

        // Create dbCircle
        dBCircle = SKShapeNode.init(circleOfRadius: radiusDBCircle)
        dBCircle.position = CGPoint(x:self.frame.midX, y:dBHeight)
        dBCircle.strokeColor = strokeColor
        dBCircle.name = "dBCircle"
        dBCircle.fillColor = UIColor.clear
        addChild(dBCircle)
        // Make static balls
        for i in 0.. maxDistance || d <= abs(radius1 - radius2)) {
            return
        } else if (d < radius1 + radius2) {
            u1 = acos((radius1 * radius1 + d * d - radius2 * radius2) / (2 * radius1 * d))
            u2 = acos((radius2 * radius2 + d * d - radius1 * radius1) / (2 * radius2 * d))
        } else {
            u1 = 0.0
            u2 = 0.0
        }
        let angle1 = center1.angleBetween(point: center2)
        let angle2 = acos((radius1 - radius2) / d)
        let angle1a = angle1 + u1 + (angle2 - u1) * v
        let angle1b = angle1 - u1 - (angle2 - u1) * v
        let angle2a = angle1 + CGFloat(M_PI) - u2 - (CGFloat(M_PI) - u2 - angle2) * v
        let angle2b = angle1 - CGFloat(M_PI) + u2 + (CGFloat(M_PI) - u2 - angle2) * v
        let p1a = center1.point(radians: angle1a, withLength: radius1)
        let p1b = center1.point(radians: angle1b, withLength: radius1)
        let p2a = center2.point(radians: angle2a, withLength: radius2)
        let p2b = center2.point(radians: angle2b, withLength: radius2)
        let totalRadius = radius1 + radius2
        var d2 = min(v * handeLenRate, p1a.minus(point: p2a).length() / totalRadius)
        d2 *= min(1, d * 2 / totalRadius)
        radius1 *= d2
        radius2 *= d2
        let cp1a = p1a.point(radians: angle1a - CGFloat(M_PI_2), withLength: radius1)
        let cp2a = p2a.point(radians: angle2a + CGFloat(M_PI_2), withLength: radius2)
        let cp2b = p2b.point(radians: angle2b - CGFloat(M_PI_2), withLength: radius2)
        let cp1b = p1b.point(radians: angle1b + CGFloat(M_PI_2), withLength: radius1)
        let pathJoinedCircles = UIBezierPath()
        pathJoinedCircles.move(to: p1a)
        pathJoinedCircles.addCurve(to: p2a, controlPoint1: cp1a, controlPoint2: cp2a)
        pathJoinedCircles.addLine(to: p2b)
        pathJoinedCircles.addCurve(to: p1b, controlPoint1: cp2b, controlPoint2: cp1b)
        pathJoinedCircles.addLine(to: p1a)
        pathJoinedCircles.close()
        let shapeNode = SKShapeNode(path: pathJoinedCircles.cgPath)
        shapeNode.strokeColor = SKColor.orange
        shapeNode.fillColor = UIColor.clear
        addChild(shapeNode)
        let wait = SKAction.wait(forDuration: vanishingTime)
        self.run(wait,completion: {
            shapeNode.removeFromParent()
        })
    }
    override func update(_ currentTime: TimeInterval) {
        var i = 0
        self.enumerateChildNodes(withName: "ball") {
            node, stop in
            let ball = node as! SKShapeNode
            let enlargeFrame = CGRect(x:ball.frame.origin.x-self.radiusBall*3,y:ball.frame.origin.y,width:ball.frame.width+(self.radiusBall*6),height:ball.frame.height)
            if enlargeFrame.contains(self.dBCircle.frame) {
                if (ball.action(forKey: "zoom") == nil) {
                    let zoomIn = SKAction.scale(to: 1.5, duration: 0.25)
                    zoomIn.timingMode = SKActionTimingMode.easeInEaseOut
                    let zoomOut = SKAction.scale(to: 1.0, duration: 0.25)
                    let wait = SKAction.wait(forDuration: 0.7)
                    let seq = SKAction.sequence([zoomIn,zoomOut,wait])
                    ball.run(seq,withKey: "zoom")
                }
            }
            self._metaball(circle2: ball, circle1: self.dBCircle, v: 0.6, handeLenRate: 2.0, maxDistance: 5 * self.radiusBall)
            i += 1
        }
    }
}
//MARK: - Extensions
extension CGPoint {
    func distance(point: CGPoint) -> CGFloat {
        let dx = point.x - self.x
        let dy = point.y - self.y
        return sqrt(dx * dx + dy * dy)
    }
    func angleBetween(point: CGPoint) -> CGFloat {
        return atan2(point.y - self.y, point.x - self.x)
    }
    func point(radians: CGFloat, withLength length: CGFloat) -> CGPoint   {
        return CGPoint(x: self.x + length * cos(radians), y: self.y + length * sin(radians))
    }
    func minus(point: CGPoint) -> CGPoint {
        return CGPoint(x: self.x - point.x, y: self.y - point.y)
    }
    func length() -> CGFloat {
        return sqrt(self.x * self.x + self.y + self.y)
    }
}