iOS动画：关键帧动画（2）

对于一些连续的动画，例如下面这种平移效果：

你可能会想到用前面学到的UIView.animate嵌套使用，做完后你会发现，你的代码嵌套了很多层，阅读和修改都不方便，这里有更好的实现方法，那就是关键帧动画

    let originalCenter = planeImage.center
	//withDuration:动画持续时间
	//delay:动画延迟多少秒执行
    UIView.animateKeyframes(withDuration: 1.5, delay: 0.0, animations: {
      //add keyframes
      //1.平移
      //withRelativeStartTime:相对于withDuration的开始时间百分比，为0时表示动画开始时立刻执行
      //relativeDuration:执行时间百分比，0.25表示执行1.5*0.25秒
      UIView.addKeyframe(withRelativeStartTime: 0.0, relativeDuration: 0.25, animations: {
        self.planeImage.center.x += 80.0
        self.planeImage.center.y -= 10.0
      })

      UIView.addKeyframe(withRelativeStartTime: 0.1, relativeDuration: 0.4) {
        self.planeImage.transform = CGAffineTransform(rotationAngle: -.pi / 8)
      }
	
      UIView.addKeyframe(withRelativeStartTime: 0.25, relativeDuration: 0.25) {
        self.planeImage.center.x += 100.0
        self.planeImage.center.y -= 50.0
        self.planeImage.alpha = 0.0
      }

      UIView.addKeyframe(withRelativeStartTime: 0.51, relativeDuration: 0.01) {
        self.planeImage.transform = .identity
        self.planeImage.center = CGPoint(x: 0.0, y: originalCenter.y)
      }

      UIView.addKeyframe(withRelativeStartTime: 0.55, relativeDuration: 0.45) {
        self.planeImage.alpha = 1.0
        self.planeImage.center = originalCenter
      }
    }, completion: nil)