ios - 在 Swift 中动画绘制圆圈的奇怪问题

Question

我正在尝试用动画绘制一个圆圈:

func drawProgressCircle(with endAngle: CGFloat) {
    let progressBezierPath = UIBezierPath(arcCenter: CGPoint(x: self.frame.width / 2, y: self.frame.width / 2), radius: self.frame.width / 2.5, startAngle: -CGFloat(Double.pi / 2), endAngle: endAngle, clockwise: true)
    let progressShapeLayer = CAShapeLayer()
    progressShapeLayer.path = progressBezierPath.cgPath

    progressShapeLayer.strokeColor = UIColor.brown.cgColor
    progressShapeLayer.fillColor = UIColor.clear.cgColor
    progressShapeLayer.lineWidth = 15.0

    // Do not draw initially. Wait for animation
    progressShapeLayer.strokeEnd = 0.0

    // Make corners rounded
    progressShapeLayer.lineCap = kCALineCapRound

    self.layer.addSublayer(progressShapeLayer)
    self.animate(circleWith: progressShapeLayer, from: -CGFloat(Double.pi / 2), to: endAngle, with: 2.0)
}

为了让它动起来，我调用了这个函数:

func animate(circleWith shapeLayer: CAShapeLayer, from: CGFloat, to: CGFloat, with duration: TimeInterval) {
    // We want to animate the strokeEnd property of the circleLayer
    let animation = CABasicAnimation(keyPath: "strokeEnd")

    // Set the animation duration appropriately
    animation.duration = duration

    // Animate from start point to end point
    animation.fromValue = from
    animation.toValue = to

    // Do a linear animation (i.e. the speed of the animation stays the same)
    animation.timingFunction = CAMediaTimingFunction(name: kCAMediaTimingFunctionEaseInEaseOut)

    // Set the circleLayer's strokeEnd property to 1.0 now so that it's the
    // right value when the animation ends.
    shapeLayer.strokeEnd = 1.0

    // Do the actual animation
    shapeLayer.add(animation, forKey: "animate")
}

但我不知道为什么它没有动画。 Canvas 是空的，2 秒后它就出现在 Canvas 上。没有任何动画。

问题是什么？为什么它没有动画绘图？

Answer 1

将动画 formValue 和 toValue 替换为:

// Animate from start point to end point
animation.fromValue = 0
animation.toValue = 1

基本上 keyPath stroke 的 CABasicAnimation 并不知道您正在绘制一个圆，它只是从 0 到 1 进行动画处理。如果你给 toValue 一个 0.5 的值，它只会动画到路径的一半。

它不同于 CABasicAnimation for keyPath position，它采用 CGPoint 作为 fromValue 和 toValue。