Animate an expanding drawing in iOS UIView Swift

Question

I would like to create a custom dynamic drawing inside a UIView using a CGContext so that when I execute a UIView animation on it the drawing will be rendered during animation/interpolation. I tried using a custom CALayer and override its draw method but that fails. I tried subclassing a UIView draw(_ in:CGRect) but that gets drawn only first time. This is what I want to do:

class RadioWaveView : UIView {
    override func draw(_ in:CGRect) {
        // draw something, for example an ellipse based on frame size, something like this:
        // let context = CGGraphicsCurrentContext()
        // context.addEllipse(self.frame)
        // Unfortunately this method is called only once, not during animation
    }
}

in viewDidAppear():

UIView.animate(withDuration: 5, delay: 1, options: [.repeat], animations: {
        self.myRadioWaveView.frame = CGRect(x:100,y:100,width:200,heigh:300)
    }, completion: { flag in
    })

The animation works because I set the background color of myRadioWaveView and see it expand on screen. Placing a breakpoint in draw shows that it is executed only once.

EDIT: In essence, I am asking this question: How can we create a custom UIView and use UIView.animate() to animate that view's rendering? Let's keep the UIView.animate() method, and how can we animate an expanding circle (or any other custom drawing within a UIView) ?

EDIT: Here is a custom class I created to draw just a triangle. When I animate its bounds, the triangle scales with the underlying image of the view but the draw method is not called during the animation. I don't want that; I want the draw() method to redraw the drawing every frame of the animation.

open class TriangleView : UIView {

override open func draw(_ rect: CGRect) {
    let context = UIGraphicsGetCurrentContext()!
    let path = CGMutablePath()
    path.move(to: CGPoint.zero)
    path.addLine(to: CGPoint(x:self.frame.width,y:0))
    path.addLine(to: CGPoint(x:self.frame.width/2,y:self.frame.height))
    path.addLine(to: CGPoint.zero)
    path.closeSubpath()
    context.beginPath()
    context.setStrokeColor(UIColor.white.cgColor)
    context.setLineWidth(3)
    context.addPath(path)
    context.closePath()
    context.strokePath()
}

}

Answer 1

I had been working in your question, this approach is using Using CABasicAnimation and CAShapeLayer I had defined a custom UIView Class to make the work, I will add further improvements @IBDesignable and @IBInspectables

import UIKit

class RadioWaveAnimationView: UIView {

    var animatableLayer : CAShapeLayer?

    override func awakeFromNib() {
        super.awakeFromNib()
        self.layer.cornerRadius = self.bounds.height/2

        self.animatableLayer = CAShapeLayer()
        self.animatableLayer?.fillColor = self.backgroundColor?.cgColor
        self.animatableLayer?.path = UIBezierPath(roundedRect: self.bounds, cornerRadius: self.layer.cornerRadius).cgPath
        self.animatableLayer?.frame = self.bounds
        self.animatableLayer?.cornerRadius = self.bounds.height/2
        self.animatableLayer?.masksToBounds = true
        self.layer.addSublayer(self.animatableLayer!)
        self.startAnimation()
    }


    func startAnimation()
    {
        let layerAnimation = CABasicAnimation(keyPath: "transform.scale")
        layerAnimation.fromValue = 1
        layerAnimation.toValue = 3
        layerAnimation.isAdditive = false

        let layerAnimation2 = CABasicAnimation(keyPath: "opacity")
        layerAnimation2.fromValue = 1
        layerAnimation2.toValue = 0
        layerAnimation2.isAdditive = false

        let groupAnimation = CAAnimationGroup()
        groupAnimation.animations = [layerAnimation,layerAnimation2]
        groupAnimation.duration = CFTimeInterval(2)
        groupAnimation.fillMode = kCAFillModeForwards
        groupAnimation.isRemovedOnCompletion = true
        groupAnimation.repeatCount = .infinity

        self.animatableLayer?.add(groupAnimation, forKey: "growingAnimation")
    }
    /*
    // Only override draw() if you perform custom drawing.
    // An empty implementation adversely affects performance during animation.
    override func draw(_ rect: CGRect) {
        // Drawing code
    }
    */

}

Results

Hope this helps