iOS动画教你编写Slack的Loading动画进阶篇

前几天看了一篇关于动画的博客叫手摸手教你写 Slack 的 Loading 动画，看着挺炫，但是是安卓版的，寻思的着仿造着写一篇iOS版的，下面是我写这个动画的分解~

老规矩先上图和demo地址：

刚看到这个动画的时候，脑海里出现了两个方案，一种是通过drawRect画出来，然后配合CADisplayLink不停的绘制线的样式；第二种是通过CAShapeLayer配合CAAnimation来实现动画效果。再三考虑觉得使用后者，因为前者需要计算很多，比较复杂，而且经过测试前者相比于后者消耗更多的CPU，下面将我的思路写下来：

相关配置和初始化方法

在写这个动画之前，我们把先需要的属性写好，比如线条的粗细，动画的时间等等，下面是相关的配置和初识化方法：

  //线的宽度
  var lineWidth:CGFloat = 0
  //线的长度
  var lineLength:CGFloat = 0
  //边距
  var margin:CGFloat = 0
  //动画时间
  var duration:Double = 2
  //动画的间隔时间
  var interval:Double = 1
  //四条线的颜色
  var colors:[UIColor] = [UIColor.init(rgba: "#9DD4E9") , UIColor.init(rgba: "#F5BD58"), UIColor.init(rgba: "#FF317E") , UIColor.init(rgba: "#6FC9B5")]
  //动画的状态
  private(set) var status:AnimationStatus = .Normal
  //四条线
  private var lines:[CAShapeLayer] = []

  enum AnimationStatus {
    //普通状态
    case Normal
    //动画中
    case Animating
    //暂停
    case pause
  }

   //MARK: Initial Methods
  convenience init(fram: CGRect , colors: [UIColor]) {
    self.init()
    self.frame = frame
    self.colors = colors
    config()
  }

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

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

  private func config() {
    lineLength = max(frame.width, frame.height)
    lineWidth = lineLength/6.0
    margin   = lineLength/4.5 + lineWidth/2
    drawLineShapeLayer()
    transform = CGAffineTransformRotate(CGAffineTransformIdentity, angle(-30))
  }

通过CAShapeLayer绘制线条

看到这个线条我就想到了用CAShapeLayer来处理，因为CAShapeLayer完全可以实现这种效果，而且它的strokeEnd的属性可以用来实现线条的长度变化的动画，下面上绘制四根线条的代码：

//MARK: 绘制线
  /**
   绘制四条线
   */
  private func drawLineShapeLayer() {
    //开始点
    let startPoint = [point(lineWidth/2, y: margin),
             point(lineLength - margin, y: lineWidth/2),
             point(lineLength - lineWidth/2, y: lineLength - margin),
             point(margin, y: lineLength - lineWidth/2)]
    //结束点
    let endPoint  = [point(lineLength - lineWidth/2, y: margin) ,
             point(lineLength - margin, y: lineLength - lineWidth/2) ,
             point(lineWidth/2, y: lineLength - margin) ,
             point(margin, y: lineWidth/2)]
    for i in 0...3 {
      let line:CAShapeLayer = CAShapeLayer()
      line.lineWidth  = lineWidth
      line.lineCap   = kCALineCapRound
      line.opacity   = 0.8
      line.strokeColor = colors[i].CGColor
      line.path    = getLinePath(startPoint[i], endPoint: endPoint[i]).CGPath
      layer.addSublayer(line)
      lines.append(line)
    }

  }

  /**
   获取线的路径

   - parameter startPoint: 开始点
   - parameter endPoint:  结束点

   - returns: 线的路径
   */
  private func getLinePath(startPoint: CGPoint, endPoint: CGPoint) -> UIBezierPath {
    let path = UIBezierPath()
    path.moveToPoint(startPoint)
    path.addLineToPoint(endPoint)
    return path
  }

  private func point(x:CGFloat , y:CGFloat) -> CGPoint {
    return CGPointMake(x, y)
  }

  private func angle(angle: Double) -> CGFloat {
    return CGFloat(angle * (M_PI/180))
  }

执行完后就跟上图一样的效果了~~~

动画分解

经过分析，可以将动画分为四个步骤：
 •画布的旋转动画，旋转两圈
 •线条由长变短的动画，更画布选择的动画一起执行，旋转一圈的时候结束
 •线条的位移动画，线条逐渐向中间靠拢，再画笔旋转完一圈的时候执行，两圈的时候结束
 •线条由短变长的动画，画布旋转完两圈的时候执行

第一步画布旋转动画

这里我们使用CABasicAnimation基础动画，keyPath作用于画布的transform.rotation.z，以z轴为目标进行旋转，下面是效果图和代码：

//MARK: 动画步骤
  /**
   旋转的动画，旋转两圈
   */
  private func angleAnimation() {
    let angleAnimation         = CABasicAnimation.init(keyPath: "transform.rotation.z")
    angleAnimation.fromValue      = angle(-30)
    angleAnimation.toValue       = angle(690)
    angleAnimation.fillMode      = kCAFillModeForwards
    angleAnimation.removedOnCompletion = false
    angleAnimation.duration      = duration
    angleAnimation.delegate      = self
    layer.addAnimation(angleAnimation, forKey: "angleAnimation")
  }

第二步线条由长变短的动画

这里我们还是使用CABasicAnimation基础动画，keyPath作用于线条的strokeEnd属性，让strokeEnd从1到0来实现线条长短的动画，下面是效果图和代码：

/**
   线的第一步动画，线长从长变短
   */
  private func lineAnimationOne() {
    let lineAnimationOne         = CABasicAnimation.init(keyPath: "strokeEnd")
    lineAnimationOne.duration      = duration/2
    lineAnimationOne.fillMode      = kCAFillModeForwards
    lineAnimationOne.removedOnCompletion = false
    lineAnimationOne.fromValue      = 1
    lineAnimationOne.toValue       = 0
    for i in 0...3 {
      let lineLayer = lines[i]
      lineLayer.addAnimation(lineAnimationOne, forKey: "lineAnimationOne")
    }
  }

第三步线条的位移动画

这里我们也是使用CABasicAnimation基础动画，keyPath作用于线条的transform.translation.x和transform.translation.y属性，来实现向中间聚拢的效果，下面是效果图和代码：

/**
   线的第二步动画，线向中间平移
   */
  private func lineAnimationTwo() {
    for i in 0...3 {
      var keypath = "transform.translation.x"
      if i%2 == 1 {
        keypath = "transform.translation.y"
      }
      let lineAnimationTwo = CABasicAnimation.init(keyPath: keypath)
      lineAnimationTwo.beginTime = CACurrentMediaTime() + duration/2
      lineAnimationTwo.duration = duration/4
      lineAnimationTwo.fillMode = kCAFillModeForwards
      lineAnimationTwo.removedOnCompletion = false
      lineAnimationTwo.autoreverses = true
      lineAnimationTwo.fromValue = 0
      if i < 2 {
        lineAnimationTwo.toValue = lineLength/4
      }else {
        lineAnimationTwo.toValue = -lineLength/4
      }
      let lineLayer = lines[i]
      lineLayer.addAnimation(lineAnimationTwo, forKey: "lineAnimationTwo")
    }

    //三角形两边的比例
    let scale = (lineLength - 2*margin)/(lineLength - lineWidth)
    for i in 0...3 {
      var keypath = "transform.translation.y"
      if i%2 == 1 {
        keypath = "transform.translation.x"
      }
      let lineAnimationTwo = CABasicAnimation.init(keyPath: keypath)
      lineAnimationTwo.beginTime = CACurrentMediaTime() + duration/2
      lineAnimationTwo.duration = duration/4
      lineAnimationTwo.fillMode = kCAFillModeForwards
      lineAnimationTwo.removedOnCompletion = false
      lineAnimationTwo.autoreverses = true
      lineAnimationTwo.fromValue = 0
      if i == 0 || i == 3 {
        lineAnimationTwo.toValue = lineLength/4 * scale
      }else {
        lineAnimationTwo.toValue = -lineLength/4 * scale
      }
      let lineLayer = lines[i]
      lineLayer.addAnimation(lineAnimationTwo, forKey: "lineAnimationThree")
    }
  }

第四步线条恢复的原来长度的动画

这里我们还是使用CABasicAnimation基础动画，keyPath作用于线条的strokeEnd属性，让strokeEnd从0到1来实现线条长短的动画，下面是效果图和代码：

/**
   线的第三步动画，线由短变长
   */
  private func lineAnimationThree() {
    //线移动的动画
    let lineAnimationFour         = CABasicAnimation.init(keyPath: "strokeEnd")
    lineAnimationFour.beginTime      = CACurrentMediaTime() + duration
    lineAnimationFour.duration      = duration/4
    lineAnimationFour.fillMode      = kCAFillModeForwards
    lineAnimationFour.removedOnCompletion = false
    lineAnimationFour.fromValue      = 0
    lineAnimationFour.toValue       = 1
    for i in 0...3 {
      if i == 3 {
        lineAnimationFour.delegate = self
      }
      let lineLayer = lines[i]
      lineLayer.addAnimation(lineAnimationFour, forKey: "lineAnimationFour")
    }
  }

最后一步需要将动画组合起来

关于动画组合我没用到CAAnimationGroup，因为这些动画并不是加到同一个layer上，再加上动画类型有点多加起来也比较麻烦，我就通过动画的beginTime属性来控制动画的执行顺序，还加了动画暂停和继续的功能，效果和代码见下图：

//MARK: Public Methods
  /**
   开始动画
   */
  func startAnimation() {
    angleAnimation()
    lineAnimationOne()
    lineAnimationTwo()
    lineAnimationThree()
  }

  /**
   暂停动画
   */
  func pauseAnimation() {
    layer.pauseAnimation()
    for lineLayer in lines {
      lineLayer.pauseAnimation()
    }
    status = .pause
  }

  /**
   继续动画
   */
  func resumeAnimation() {
    layer.resumeAnimation()
    for lineLayer in lines {
      lineLayer.resumeAnimation()
    }
    status = .Animating
  }

  extension CALayer {
  //暂停动画
  func pauseAnimation() {
    // 将当前时间CACurrentMediaTime转换为layer上的时间, 即将parent time转换为localtime
    let pauseTime = convertTime(CACurrentMediaTime(), fromLayer: nil)
    // 设置layer的timeOffset, 在继续操作也会使用到
    timeOffset  = pauseTime
    // localtime与parenttime的比例为0, 意味着localtime暂停了
    speed     = 0;
  }

  //继续动画
  func resumeAnimation() {
    let pausedTime = timeOffset
    speed     = 1
    timeOffset   = 0;
    beginTime   = 0
    // 计算暂停时间
    let sincePause = convertTime(CACurrentMediaTime(), fromLayer: nil) - pausedTime
    // local time相对于parent time时间的beginTime
    beginTime   = sincePause
  }
}

//MARK: Animation Delegate
  override func animationDidStart(anim: CAAnimation) {
    if let animation = anim as? CABasicAnimation {
      if animation.keyPath == "transform.rotation.z" {
        status = .Animating
      }
    }
  }

  override func animationDidStop(anim: CAAnimation, finished flag: Bool) {
    if let animation = anim as? CABasicAnimation {
      if animation.keyPath == "strokeEnd" {
        if flag {
          status = .Normal
          dispatch_after(dispatch_time(DISPATCH_TIME_NOW, Int64(interval) * Int64(NSEC_PER_SEC)), dispatch_get_main_queue(), {
            if self.status != .Animating {
              self.startAnimation()
            }
          })
        }
      }
    }
  }

   //MARK: Override
  override func touchesEnded(touches: Set<UITouch>, withEvent event: UIEvent?) {
    switch status {
    case .Animating:
      pauseAnimation()
    case .pause:
      resumeAnimation()
    case .Normal:
      startAnimation()
    }
  }

总结

动画看起来挺复杂，但是细细划分出来也就那么回事，在写动画之前要先想好动画的步骤，这个很关键，希望大家通过这篇博文章可以学到东西，有什么好的建议可以随时提出来，谢谢大家阅读~~demo地址

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