func handleGesture(recognizer: UIPanGestureRecognizer) {
    let translation = recognizer.translationInView(recognizer.view!.superview!)

    // (isOpening ? 1.0 : -1.0) isOpening为BOOL值,表示打开或者关闭
    var progress = translation.x / menuWidth * (isOpening ? 1.0 : -1.0)

    // 保证在0~1之间
    progress = min(max(progress, 0), 1.0)

    switch recognizer.state {
    case .Began:

      let isOpen = mainVC.view.frame.origin.x/menuWidth
      isOpening = isOpen == 1.0 ? false:true

      // 为layer的效果添加一个缓存,作用是避免旋转的时候产生锯齿
      sideMenuVC.view.layer.shouldRasterize = true;
      // 设置渲染的范围
      sideMenuVC.view.layer.rasterizationScale = UIScreen.mainScreen().scale

    case .Changed:

      // 核心代码:调整容器中视图的位置 3D效果 以及透明度 以及左上角按钮的翻转效果
      setToPercent(isOpening ? progress: (1.0 - progress))

    case .Ended: fallthrough
    case .Cancelled: fallthrough
    case .Failed:
      // 分页效果
      var targetProgress: CGFloat
      if (isOpening) {
        targetProgress = progress < 0.5 ? 0.0 : 1.0
      }else {
        targetProgress = progress < 0.5 ? 1.0 : 0.0
      }

      // 设置failed状态下的sideMenuVC的view的效果
      UIView.animateWithDuration(animationTime, animations: { () -> Void in
        self.setToPercent(targetProgress)
        }, completion: { (_) -> Void in
        // 记得关闭layer的缓存渲染
        self.sideMenuVC.view.layer.shouldRasterize = false
      })
    default: break
    }
  }

  func setToPercent(percent: CGFloat) {

    // 调整mainVC.view的位置
    mainVC.view.frame.origin.x = menuWidth * CGFloat(percent)
    // 设置3D效果及透明度
    sideMenuVC.view.layer.transform = menuTransformForPercent(percent)
    sideMenuVC.view.alpha = CGFloat(max(0.2, percent))

    // 左上角按钮的翻转设置
    let mainVc = (mainVC as! UINavigationController).viewControllers.first as? MainViewController
    if let menuButton = mainVc?.menuButton {
      menuButton.imageView.layer.transform = buttonTransformForPercent(percent)
    }
  }

  // 根据百分比添加一个3D特效
  func menuTransformForPercent(percent: CGFloat) -> CATransform3D {

    var identify = CATransform3DIdentity
    // m34负责z轴方向的translation（移动），m34= -1/D, 默认值是0, D越小透视效果越明显,这边的1000视情况调整的
    identify.m34 = -1.0/1000

    let remainingPercent = 1.0 - percent
    let angle = remainingPercent * CGFloat(-M_PI_2)

    // 后面3个数为 x y z
    let rotationTransform = CATransform3DRotate(identify, angle, 0.0, 1.0, 0.0)
    // 将值转换成一个矩阵
    let translationTransform =
    CATransform3DMakeTranslation(menuWidth * percent, 0, 0)
    // 将上面两者结合起来
    return CATransform3DConcat(rotationTransform,
      translationTransform)
  }

  // 为按钮添加一个3D效果
  func buttonTransformForPercent(percent: CGFloat) -> CATransform3D {

    var identity = CATransform3DIdentity
    identity.m34 = -1.0/1000

    let angle = percent * CGFloat(-M_PI)
    let rotationTransform = CATransform3DRotate(identity, angle, 1.0, 1.0, 0.0)

    return rotationTransform
  }