CAGroupAnimation不显示动画。单独添加动画时效果很好

Question

我正在使用Swift和Core Animation实现一个简单的活动指示器。核心动画循环仅包含两个动画。当我直接将它们添加到图层时，它们可以完美工作。当我将它们添加为CAAnimationGroup时，什么都没有发生。我对这种行为完全感到困惑，并且我已经检查了有关CAAnimationGroup的所有关于stackoverflow的问题，网络上的所有教程，并多次阅读了官方文档。我不知道发生了什么事。请帮助。

动画：

let anim1 = CABasicAnimation(keyPath: "strokeEnd")
anim1.fromValue = 0
anim1.toValue = 1.0
anim1.duration = 2.0
anim1.beginTime = CACurrentMediaTime()

let anim2 = CABasicAnimation(keyPath:"strokeStart")
anim2.fromValue = 0
anim2.toValue = 1.0
anim2.duration = 2.0
anim2.beginTime = CACurrentMediaTime() + 2.0

这完全按预期工作：

shapeLayer.addAnimation(anim1, forKey:nil)
shapeLayer.addAnimation(anim2, forKey:nil)

这对我的图层没有任何作用：

let group = CAAnimationGroup()
group.animations = [anim1, anim2]
group.duration = 4.0
shapeLayer.addAnimation(group, forKey: nil)

我制作了一个简短的演示片段，用于Playground：https://gist.github.com/anonymous/6021295eab4e00b813ce。请亲自看看，并帮助我解决这个问题。 （以防您不确定如何使用Playground制作原型动画的情况：http://possiblemobile.com/2015/03/prototyping-uiview-animations-swift-playground/）

Answer 1

将动画直接添加到图层时，开始时间是基于当前媒体时间。

[将动画添加到动画组时，它们的“本地时间”是动画组。动画组中的开始时间为0是该组的开始，而动画组中的beginTime为1.0是1秒，以此类推。以此类推。将动画添加到动画组中时，您的开始时间应从零开始，不是从CACurrentMediaTime()开始。

Answer 2

import UIKit
import XCPlayground
let view = UIView(frame: CGRect(x: 0, y: 0, width: 500, height: 500))
let shapeLayer = CAShapeLayer()
let size:CGFloat = 52
let rect = CGRect(x: view.bounds.midX - size/2, y: view.bounds.midY-size/2, width: size, height: size)
shapeLayer.path = UIBezierPath(ovalInRect: rect).CGPath
shapeLayer.fillColor = UIColor.clearColor().CGColor
shapeLayer.strokeColor = UIColor.redColor().CGColor
shapeLayer.lineWidth = 3
shapeLayer.strokeStart = 0
shapeLayer.strokeEnd = 0
view.layer.addSublayer(shapeLayer)
XCPShowView("view", view)

let beginTime = CACurrentMediaTime()
let anim1 = CABasicAnimation(keyPath: "strokeEnd")
anim1.fromValue = 0
anim1.toValue = 1
anim1.duration = 2

let anim2 = CABasicAnimation(keyPath:"strokeEnd")
anim2.fromValue = 1
anim2.toValue = 0
anim2.duration = 2
anim2.beginTime =  2

let group = CAAnimationGroup()
group.duration = 4.0
group.removedOnCompletion = true
group.animations = [anim1, anim2]
//shapeLayer.addAnimation(anim1, forKey:nil)
//shapeLayer.addAnimation(anim2, forKey:nil)
shapeLayer.addAnimation(group, forKey: nil)

Answer 3

let group = CAAnimationGroup()
group.beginTime = CACurrentMediaTime() + delay
group.duration = 4.0
group.removedOnCompletion = true
group.animations = [anim1, anim2]