Core Animation - modify animation property

Question

I have animation

 func startRotate360() {
    let rotation : CABasicAnimation = CABasicAnimation(keyPath: \"transform.rotation.z\")
    rotation.fromValue =          


        

Answer 1

2019 typical modern syntax

Setup the arc and the layer like this:

import Foundation
import UIKit

class RoundChaser: UIView {
    
    private let lineThick: CGFloat = 10.0
    private let beginFraction: CGFloat = 0.15
    // where does the arc drawing begin?
    // 0==top, .25==right, .5==bottom, .75==left
    
    private lazy var arcPath: CGPath = {
        let b = beginFraction * .pi * 2.0
        return UIBezierPath(
            arcCenter: bounds.centerOfCGRect(),
            radius: bounds.width / 2.0 - lineThick / 2.0,
            startAngle: .pi * -0.5 + b,
            // recall that .pi * -0.5 is the "top"
            endAngle: .pi * 1.5 + b,
            clockwise: true
        ).cgPath
    }()
    
    private lazy var arcLayer: CAShapeLayer = {
        let l = CAShapeLayer()
        l.path = arcPath
        l.fillColor = UIColor.clear.cgColor
        l.strokeColor = UIColor.purple.cgColor
        l.lineWidth = lineThick
        l.lineCap = CAShapeLayerLineCap.round
        l.strokeStart = 0
        l.strokeEnd = 0
        // if both are same, it is hidden. initially hidden
        layer.addSublayer(l)
        return l
    }()
    

then initialization is this easy

    open override func layoutSubviews() {
        super.layoutSubviews()
        arcLayer.frame = bounds
    }
    

finally animation is easy

    public func begin() {
        CATransaction.begin()
        
        let e : CABasicAnimation = CABasicAnimation(keyPath: "strokeEnd")
        e.duration = 2.0
        e.fromValue = 0
        e.toValue = 1.0
        // recall 0 will be our beginFraction, see above
        e.repeatCount = .greatestFiniteMagnitude
        
        self.arcLayer.add(e, forKey: nil)
        CATransaction.commit()
    }
}