iOS動畫：UIKit關鍵幀動畫（2）

對於一些連續的動畫，例如下面這種平移效果：

你可能會想到用前面學到的UIView.animate嵌套使用，做完後你會發現，你的代碼嵌套了很多層，閱讀和修改都不方便，這裏有更好的實現方法，那就是關鍵幀動畫

    let originalCenter = planeImage.center
	//withDuration:動畫持續時間
	//delay:動畫延遲多少秒執行
    UIView.animateKeyframes(withDuration: 1.5, delay: 0.0, animations: {
      //add keyframes
      //1.平移
      //withRelativeStartTime:相對於withDuration的開始時間百分比，爲0時表示動畫開始時立刻執行
      //relativeDuration:執行時間百分比，0.25表示執行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)