在 Swift 中绘制圆形动画时出现的奇怪问题

Question

I'm trying to animate drawing a circle with:

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)
}

and to animate it I call this function:

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")
}

but I do not know why it does not animate it. The canvas is empty and in 2 seconds it just appears on the canvas. Without any animation.

What is the problem? Why it does not animate drawing?

Answer 1

Replace the animation formValue & toValue by:

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

Basically the CABasicAnimation for keyPath stroke does not know that you are drawing a circle, it just animate from 0 to 1. if you give the toValue a value of 0.5 it will just animate to the half of the path.

it's unlike the CABasicAnimation for keyPath position which take CGPoint for the fromValue and toValue .