为什么角半径仅应用于自定义视图的左上角和右上角？

编辑：澄清最终目标后，这里有一个更简单的方法。您可以直接在操场页面中运行它...

import UIKit
import PlaygroundSupport

class ProgressBar: UIView {

    var progressView: UIView = {
        let v = UIView()
        v.backgroundColor = .white
        return v
    }()

    override init(frame: CGRect) {
        super.init(frame: frame)
        configure()
    }

    required init?(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)
        configure()
    }

    func configure() {
        backgroundColor = UIColor(white: 1.0, alpha: 0.1)
        layer.cornerRadius = 8.0
        clipsToBounds = true

        progressView.frame = bounds
        progressView.frame.size.width = 0.0
        addSubview(progressView)
    }

    func animate(_ pct: CGFloat) -> Void {
        UIView.animate(withDuration: 1.0, animations: {
            self.progressView.frame.size.width = self.bounds.size.width * pct
        })
    }

}

class MyViewController : UIViewController {

    var theButton: UIButton = {
        let b = UIButton()
        b.setTitle("Tap Me", for: .normal)
        b.translatesAutoresizingMaskIntoConstraints = false
        b.backgroundColor = .red
        return b
    }()

    var myProgressBar = ProgressBar(frame: CGRect(x: 40, y: 300, width: 300, height: 30))

    // on button tap, set the progress bar to 80%
    @objc func didTap(_ sender: Any?) -> Void {
        myProgressBar.animate(0.8)
    }

    override func loadView() {
        let view = UIView()
        self.view = view
        view.backgroundColor = UIColor.purple

        view.addSubview(theButton)
        // constrain button to Top: 32 and centerX
        NSLayoutConstraint.activate([
            theButton.topAnchor.constraint(equalTo: view.topAnchor, constant: 32.0),
            theButton.centerXAnchor.constraint(equalTo: view.centerXAnchor, constant: 0.0),
            ])

        // add an action for the button tap
        theButton.addTarget(self, action: #selector(didTap(_:)), for: .touchUpInside)

        view.addSubview(myProgressBar)

    }

}

// Present the view controller in the Live View window
PlaygroundPage.current.liveView = MyViewController()

点击按钮将使“进度条”从零到 80% 动画

结果：

原答案：

首先：您要添加一条从...开始的“线”，并且您将 lineWidth0,0设为width,0视图的高度。

这给你这个结果：

黄色矩形是您的视图，白色矩形是您的“线”。正如你看到的，中心的线沿运行的顶视图，使该行的上半部分居然是外面的视图的框架。因此，当您圆化视图的角时，看起来好像只有顶角被圆化了。如果您为视图设置背景颜色，您将看到底角也被圆角，但那里没有“线”。

如果这是您真正想要的（顶部栏为白色，底部栏为绿色）：

您想将线宽设置为视图高度的 1/2，并将线中心设置为 1/4 和 3/4：

class ProgressBar: UIView {

    var bottomProgressBar = CAShapeLayer()
    var topProgressBar = CAShapeLayer()

    override init(frame: CGRect) {
        super.init(frame: frame)
        configure()
    }

    required init?(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)
        configure()
    }

    func configure() {
        bottomProgressBar.strokeColor = UIColor.green.cgColor
        bottomProgressBar.opacity = 1
        bottomProgressBar.lineWidth = self.frame.height
        bottomProgressBar.strokeStart = 0
        bottomProgressBar.strokeEnd = 1
        self.layer.addSublayer(bottomProgressBar)

        topProgressBar.strokeColor = UIColor.white.cgColor
        topProgressBar.lineWidth = self.frame.height
        topProgressBar.strokeStart = 0
        topProgressBar.strokeEnd = 1
        self.layer.addSublayer(topProgressBar)

        //      animate(toValue: 1)
    }

    override func layoutSubviews() {
        super.layoutSubviews()

        var path = UIBezierPath()
        path.move(to: CGPoint(x: 0, y: self.frame.height * 0.25))
        path.addLine(to: CGPoint(x: self.frame.width, y: self.frame.height * 0.25))

        topProgressBar.path = path.cgPath

        path = UIBezierPath()
        path.move(to: CGPoint(x: 0, y: self.frame.height * 0.75))
        path.addLine(to: CGPoint(x: self.frame.width, y: self.frame.height * 0.75))

        bottomProgressBar.path = path.cgPath

        bottomProgressBar.lineWidth = self.frame.height / 2.0 // Update lineWidth
        topProgressBar.lineWidth = self.frame.height / 2.0    // Update lineWidth

        self.clipsToBounds = true
        self.layer.cornerRadius = 8

    }

    func animate(toValue: Double) {
        let animation = CABasicAnimation(keyPath: "strokeEnd")

        animation.duration = 3.0
        animation.fromValue = 0
        animation.toValue = toValue

        topProgressBar.strokeEnd = CGFloat(toValue)
        topProgressBar.animation(forKey: "animate")
    }
}

当然，如果你想要的只是一个带圆角的矩形，那么你不需要任何一个子层......只需创建一个白色背景的视图，并设置其层的角半径......