ButtonWithShapeLayerAnimation.swift

import UIKit
import PlaygroundSupport

class MyViewController: UIViewController {
    
    let button = ButtonWithShapeLayer()
    private var buttonHeightConstraint: NSLayoutConstraint!
    
    override func viewDidLoad() {
        super.viewDidLoad()
        view.backgroundColor = .lightGray
        setupButton()
    }
    
    private func setupButton() {
        button.backgroundColor = .yellow
        button.translatesAutoresizingMaskIntoConstraints = false
        button.addTarget(self, action: #selector(onButtonTap), for: .touchUpInside)
        view.addSubview(button)
        
        buttonHeightConstraint = button.heightAnchor.constraint(equalToConstant: 100.0)
        NSLayoutConstraint.activate([
            button.centerYAnchor.constraint(equalTo: view.centerYAnchor),
            button.centerXAnchor.constraint(equalTo: view.centerXAnchor),
            buttonHeightConstraint,
            button.widthAnchor.constraint(equalTo: button.heightAnchor)
        ])
    }
    
    @objc private func onButtonTap() {
        let currentHeight = buttonHeightConstraint.constant
        buttonHeightConstraint.constant = currentHeight == 100.0 ? 200.0 : 100.0
        UIView.animate(withDuration: 1.0) {
            self.view.layoutIfNeeded()
        }
    }
}

class ButtonWithShapeLayer: UIButton {
    
    private let shapeLayer = CAShapeLayer()
    private var isShapeLayerAdded = false
    private var oldFrame = CGRect.zero
    
    override func layoutSubviews() {
        super.layoutSubviews()
        if !isShapeLayerAdded {
            addShapeLayer()
        } else if oldFrame != frame {
            animateShapeLayer()
        }
        oldFrame = frame
    }
    
    private func animateShapeLayer() {
        let animation = CABasicAnimation(keyPath: "path")
        animation.fromValue = getBezierPath(forFrame: oldFrame)
        animation.toValue = getBezierPath(forFrame: frame)
        animation.duration = 1.0
        animation.fillMode = kCAFillModeForwards
        animation.isRemovedOnCompletion = false
        animation.timingFunction = CAMediaTimingFunction(name: kCAMediaTimingFunctionEaseInEaseOut)
        
        shapeLayer.add(animation, forKey: "path")
    }
    
    private func getBezierPath(forFrame frame: CGRect) -> CGPath {
        let halfBeneathFill = UIBezierPath()
        halfBeneathFill.move(to: CGPoint(x: 0, y: frame.height / 2))
        halfBeneathFill.addCurve(
            to: CGPoint(x: frame.width, y: frame.height / 2),
            controlPoint1: CGPoint(x: frame.width / 10, y: frame.height / 2 + frame.height / 10),
            controlPoint2: CGPoint(x: frame.width, y: frame.height / 2 + frame.height / 10)
        )
        halfBeneathFill.addLine(to: CGPoint(x: frame.width, y: frame.height))
        halfBeneathFill.addLine(to: CGPoint(x: 0, y: frame.height))
        halfBeneathFill.addLine(to: CGPoint(x: 0, y: frame.height / 2))
        return halfBeneathFill.cgPath
    }
    
    private func addShapeLayer() {
        isShapeLayerAdded = true
        shapeLayer.path = getBezierPath(forFrame: frame)
        shapeLayer.fillColor = UIColor.red.cgColor
        shapeLayer.opacity = 0.3
        layer.addSublayer(shapeLayer)
    }
}

PlaygroundPage.current.liveView = MyViewController()