Swift：沿动画路径对对象进行动画处理

使用CoreAnimation根据路径为红点的位置设置动画时，假设路径没有变化。从理论上讲，您可以定义一个反映扩展圆的螺旋路径。就我个人而言，我将使用CADisplayLink，它是为屏幕刷新而优化设计的特殊计时器，并完全淘汰了CoreAnimation调用。例如

func startDisplayLink() {
    let displayLink = CADisplayLink(target: self, selector: #selector(handleDisplayLink(_:)))
    displayLink.add(to: .main, forMode: .common)
}

@objc func handleDisplayLink(_ displayLink: CADisplayLink) {
    let percent = CGFloat(displayLink.timestamp).truncatingRemainder(dividingBy: duration) / duration
    let radius = ...
    let center = containerView.bounds.center
    circleLayer.path = makeArcPath(arcCenter: center, radius: radius)
    let angle = percent * .pi * 2
    let dotCenter = CGPoint(x: center.x + cos(angle) * radius, y: center.y + sin(angle) * radius)
    redDot.path = makeArcPath(arcCenter: dotCenter, radius: 5)
}

产生：

完整的例子：

class ViewController: UIViewController {

    private let radiusRange: ClosedRange<CGFloat> = 10...100
    private let duration: CGFloat = 1.5

    private lazy var containerView: UIView = {
        let containerView = UIView()
        containerView.translatesAutoresizingMaskIntoConstraints = false
        return containerView
    }()

    private lazy var redDot: CAShapeLayer = {
        let layer = CAShapeLayer()
        layer.fillColor = UIColor.red.cgColor
        return layer
    }()

    private lazy var circleLayer: CAShapeLayer = {
        let layer = CAShapeLayer()
        layer.lineWidth = 1.5
        layer.strokeColor = UIColor.black.cgColor
        layer.fillColor = UIColor.clear.cgColor
        return layer
    }()

    private weak var displayLink: CADisplayLink?

    override func viewDidLoad() {
        super.viewDidLoad()

        setup()
    }

    override func viewDidAppear(_ animated: Bool) {
        super.viewDidAppear(animated)
        startDisplayLink()
    }

    override func viewDidDisappear(_ animated: Bool) {
        super.viewDidDisappear(animated)
        stopDisplayLink()
    }
}

// MARK: Private utility methods

private extension ViewController {
    func setup() {
        addContainer()

        containerView.layer.addSublayer(circleLayer)
        containerView.layer.addSublayer(redDot)
    }

    func addContainer() {
        view.addSubview(containerView)

        NSLayoutConstraint.activate([
            containerView.leadingAnchor.constraint(equalTo: view.leadingAnchor),
            containerView.trailingAnchor.constraint(equalTo: view.trailingAnchor),
            containerView.topAnchor.constraint(equalTo: view.topAnchor),
            containerView.bottomAnchor.constraint(equalTo: view.bottomAnchor)
        ])
    }

    func makeArcPath(arcCenter: CGPoint, radius: CGFloat) -> CGPath {
        UIBezierPath(arcCenter: arcCenter, radius: radius, startAngle: 0, endAngle: .pi * 2, clockwise: true).cgPath
    }
}

// MARK: - DisplayLink related methods

private extension ViewController {
    func startDisplayLink() {
        stopDisplayLink()  // stop existing display link, if any

        let displayLink = CADisplayLink(target: self, selector: #selector(handleDisplayLink(_:)))
        displayLink.add(to: .main, forMode: .common)
        self.displayLink = displayLink
    }

    func stopDisplayLink() {
        displayLink?.invalidate()
    }

    @objc func handleDisplayLink(_ displayLink: CADisplayLink) {
        let percent = CGFloat(displayLink.timestamp).truncatingRemainder(dividingBy: duration) / duration
        let radius = radiusRange.percent(percent)
        let center = containerView.bounds.center
        circleLayer.path = makeArcPath(arcCenter: center, radius: radius)
        let angle = percent * .pi * 2
        let dotCenter = CGPoint(x: center.x + cos(angle) * radius, y: center.y + sin(angle) * radius)
        redDot.path = makeArcPath(arcCenter: dotCenter, radius: 5)
    }
}

// MARK: - CGRect extension

extension CGRect {
    var center: CGPoint { return CGPoint(x: midX, y: midY) }
}

// MARK: - ClosedRange extension

extension ClosedRange where Bound: FloatingPoint {
    func percent(_ percent: Bound) -> Bound {
        (upperBound - lowerBound) * percent + lowerBound
    }
}