Animate a layer property which simply changes other properties?

Question

Imagine a CAGradientLayer.

It's very easy to animate .startPoint and .endPoint.

Now imagine a float spinLike which simply sets both of them at once.

{So, instead of having two different animations, you could simply animate spinLike.}

So something like ..

class CustomGradientLayer: CAGradientLayer {
    
    @objc var spinLike: CGFloat = 0 {
        
        didSet {
            
            startPoint = CGPoint( ... )
            endPoint = CGPoint( ... )
            setNeedsDisplay()
        }
    }
}

To animate spinLike...

class Test: UIView {

     ...
     g = CustomGradientLayer()
     a = CABasicAnimation(keyPath: "spinLike")
     ...
     g.add(a, forKey: nil)
     ...

But.

It doesn't work, startPoint and endPoint are not moved at all.

What is wrong?

---

Note - tragically it seems you can not @NSManaged a property which has a didSet...

---

Note - it's easy enough to make your own custom animation by just overriding the draw loop.

There are many examples of this around. This is how you do it:

class CircleProgressLayer: CALayer {
     
    @NSManaged var progress: CGFloat

    override class func needsDisplayForKey(key: String) -> Bool {
         
        if key == "progress" {
            return true
        }
        return super.needsDisplayForKey(key)
    }
    
    override func draw(in ctx: CGContext) {
        
        path.fill() etc etc... your usual drawing code
    }
}

Unfortunately my question here is

Unrelated to actual drawing:

By animating the property spinLike,

I simply want to change each frame existing ordinary animatable properties (in the example, .startPoint and .endPoint )

How do you do this?

Note! You can't change .startPoint and .endPoint in drawInContext - you'd be attempting to modify read-only layer

Answer 1

To animate custom properties, you should mark them with @NSManaged. You should not force redrawing when you assign a new value. Instead, you should overwrite needsDisplay(forKey:).

class CustomedGradLayer: CAGradientLayer {
    @NSManaged var spinLike: CGFloat

    class func needsDisplay(forKey key: String) -> Bool {
        return key == "spinLike" || super.needsDisplay(forKey: key)
    }

    class func defaultValue(forKey key: String) -> Any? {
        return key == "spinLike" ? CGFloat(0) : super.defaultValue(forKey: key)
    }
}

Finally, you should implement the drawing of the layer according to the Apple documentation.

I wrote a small project in Swift some months ago. It demonstrates custom layer animations with the depth of a Koch curve.

This is the code of the layer class:

class KochLayer: CALayer {
    fileprivate let kPI = CGFloat(Double.pi)
    @NSManaged var depth : CGFloat
    var midPoint: CGPoint {
        get {
            let theBounds = self.bounds

            return CGPoint(x: theBounds.midX, y: theBounds.midY)
        }
    }
    var color: CGColor!

    override class func defaultValue(forKey inKey: String) -> Any? {
        return inKey == kDepthKey ? 0.0 : super.defaultValue(forKey: inKey)
    }

    override class func needsDisplay(forKey inKey: String) -> Bool {
        if inKey == kDepthKey {
            return true
        }
        else {
            return super.needsDisplay(forKey: inKey)
        }
    }

    override init() {
        super.init()
    }

    override init(layer inLayer: Any) {
        super.init(layer: inLayer)
        if let theLayer = inLayer as? KochLayer {
            depth = theLayer.depth
            color = theLayer.color
        }
    }

    required init(coder inCoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }

    func pointWithRadius(_ inRadius: CGFloat, angle inAngle: CGFloat) -> CGPoint {
        let theCenter = midPoint

        return CGPoint(x: theCenter.x + inRadius * sin(inAngle),
            y: theCenter.y - inRadius * cos(inAngle));
    }

    override func draw(in inContext: CGContext) {
        let theBounds = self.bounds
        let theRadius = fmin(theBounds.width, theBounds.height) / 2.0
        let thePoints: [CGPoint] = [
            pointWithRadius(theRadius, angle:0.0),
            pointWithRadius(theRadius, angle:2 * kPI / 3.0),
            pointWithRadius(theRadius, angle:4 * kPI / 3.0)
        ]
        let thePath = CGMutablePath()

        inContext.setLineWidth(0.5)
        inContext.setLineCap(.round)
        inContext.setLineJoin(.round)
        inContext.setFillColor(color)
        thePath.move(to: thePoints[0])
        for i in 0..<3 {
            addPointsToPath(thePath, fromPoint:thePoints[i], toPoint:thePoints[(i + 1) % 3], withDepth:self.depth)
        }
        inContext.addPath(thePath)
        inContext.fillPath()
    }

    func addPointsToPath(_ inoutPath: CGMutablePath, fromPoint inFromPoint: CGPoint, toPoint inToPoint: CGPoint, withDepth inDepth: CGFloat) {
        var thePoints = Array<CGPoint>(repeating: inFromPoint, count: 5)

        thePoints[4] = inToPoint;
        if inDepth <= 1.0 {
            curveWithWeight(inDepth, points:&thePoints)
            for i in 1..<5 {
                inoutPath.addLine(to: thePoints[i])
            }
        }
        else {
            let theDepth = inDepth - 1;

            curveWithWeight(1.0, points:&thePoints)
            for i in 0..<4  {
                addPointsToPath(inoutPath, fromPoint:thePoints[i], toPoint:thePoints[i + 1], withDepth:theDepth)
            }
        }
    }

    func curveWithWeight(_ inWeight: CGFloat, points inoutPoints: inout [CGPoint]) {
        let theFromPoint = inoutPoints[0]
        let theToPoint = inoutPoints[4]
        let theFactor = inWeight / (2 * sqrt(3))
        let theDelta = CGSize(width: theToPoint.x - theFromPoint.x, height: theToPoint.y - theFromPoint.y);

        inoutPoints[1] = CGPoint(x: theFromPoint.x + theDelta.width / 3,
            y: theFromPoint.y + theDelta.height / 3)
        inoutPoints[2] = CGPoint(x: theFromPoint.x + theDelta.width / 2 + theFactor * theDelta.height,
            y: theFromPoint.y + theDelta.height / 2 - theFactor * theDelta.width);
        inoutPoints[3] = CGPoint(x: theToPoint.x - theDelta.width / 3,
            y: theToPoint.y - theDelta.height / 3)
    }
}