Moon shadow

Moon.swift

import UIKit
import Foundation
import PlaygroundSupport
import QuartzCore
import CoreGraphics
import Darwin


let view = UIView(frame: .init(x: 0, y: 0, width: 600, height: 600))
view.backgroundColor = .black


func calculateMoonPath(for angle: CGFloat) -> UIBezierPath {
    let center = CGPoint(x: view.bounds.midX, y: view.bounds.midY)
    let radius = view.bounds.height/2
    
    let path = UIBezierPath(arcCenter: center,
                            radius: radius,
                            startAngle: -.pi/2,
                            endAngle: .pi/2,
                            clockwise: true)
    path.addArc(withCenter: .init(x: center.x - radius * tan(angle), y: center.y),
                radius: radius / CGFloat(cosf(Float(angle))),
                startAngle: .pi/2 - angle,
                endAngle: angle - .pi/2,
                clockwise: false
    )
    path.close()
    return path
}

func animatePhase(for layer: CALayer) {
    let animation = CABasicAnimation(keyPath: "path")
    animation.duration = 4
    animation.toValue = calculateMoonPath(for:.pi/2*0.6).cgPath
    animation.timingFunction = .init(name: .easeInEaseOut)
    animation.isRemovedOnCompletion = false
    animation.repeatCount = 100
    
    layer.add(animation, forKey: nil)
}


func addMoonShadow() {
    let shadowLayer = CAShapeLayer()
    shadowLayer.frame = view.bounds
    shadowLayer.path = calculateMoonPath(for: 0).cgPath
    shadowLayer.fillColor = UIColor.black.withAlphaComponent(0.5).cgColor
    shadowLayer.strokeColor = UIColor.red.cgColor
    shadowLayer.lineWidth = 2
    
    view.layer.addSublayer(shadowLayer)
    animatePhase(for: shadowLayer)
}

addMoonShadow()

PlaygroundPage.current.needsIndefiniteExecution = true
PlaygroundPage.current.liveView = view