//当前月第一天
func firstDateOfCurrentMonth() ->NSDate{
  let calendar = NSCalendar(identifier:NSCalendarIdentifierGregorian )
  let currentDateComponents = calendar!.components([.Year,.Month], fromDate: self)
  let startOfMonth = calendar!.dateFromComponents(currentDateComponents)
  let date = startOfMonth?.dateByAddingTimeInterval(8*60*60)
  return date!
}

//当前月的第一天是星期几
func firstDayOfCurrentMonth() -> Int {
  let calendar = NSCalendar.currentCalendar()
  let components = calendar.components(.Weekday, fromDate: firstDateOfCurrentMonth())
  return components.weekday-1
}

NSString * const kCATransitionFade;
NSString * const kCATransitionMoveIn;
NSString * const kCATransitionPush;
NSString * const kCATransitionReveal;

//为dayView（代表日历的collectionview）添加一个滑动手势
func addPanGestureToDayView() {
  let swipe = UIPanGestureRecognizer(target: self, action: #selector(self.panOnDayView(_:)))
  dayView.addGestureRecognizer(swipe)
}

//当在dayView上滑动时触发
func panOnDayView(pan: UIPanGestureRecognizer) {
  //如果手势的状态是结束状态
  //或者当前动画已经结束（防止上一个翻页动画还没结束，就开始下一个）
  //添加翻页的转场动画到dayView上
  if pan.state == .Ended && !animatiing{
    addAnimationToDayView(pan)
  }
}

let pageCurlDuration = 0.5  //动画时间
let kPageCurlKey = "pageCurl"   //往上翻页的的type
let kPageUnCurlKey = "pageUnCurl"  //往下翻页的type

//添加动画到日历
func addAnimationToDayView(pan: UIPanGestureRecognizer) {
  let translation = pan.translationInView(dayView)
  //创建一个转场动画
  let transitioin = CATransition()
  transitioin.duration = pageCurlDuration
  transitioin.timingFunction = CAMediaTimingFunction(name: "default")
  //在动画结束之后保证状态不被移除(这个两个属性得同时设置)
  transitioin.fillMode = kCAFillModeForwards
  transitioin.removedOnCompletion = false
  //设置代理，在动画开始和结束的代理方法中可以处理一些事情
  transitioin.delegate = self
  if translation.y < 0 {//手势向上
  *
  *
    if translation.x > 0 {//手势朝右上角滑动,朝右上翻页
      //沿y轴对containerView进行M_PI角度翻转，使containerView的右下角变为左下角
      animationContainerView.layer.transform = CATransform3DMakeRotation(CGFloat(M_PI), 0, 1, 0)
      //因为dayView是加在containerView上面的，如果不把dayView重新翻转回去，显示出来的界面都是反的
      dayView.layer.transform = CATransform3DMakeRotation(CGFloat(-M_PI), 0, 1, 0)
    }
    //转场动画的效果
    transitioin.type = kPageCurlKey
    //转场动画方向
    transitioin.subtype = kCATransitionFromBottom
    //设置一个month的key，为了在动画结束时判断动画代表的是上一个月，还是下一个月
    transitioin.setValue("next", forKey: "month")
  }else{//下
    if translation.x < 0 {//手势朝左下角滑动,朝左下翻页
      animationContainerView.layer.transform = CATransform3DMakeRotation(CGFloat(M_PI), 0, 1, 0)
      dayView.layer.transform = CATransform3DMakeRotation(CGFloat(-M_PI), 0, 1, 0)
    }
    transitioin.type = kPageUnCurlKey
    transitioin.subtype = kCATransitionFromTop
    transitioin.setValue("pre", forKey: "month")
  }
  dayView.layer.addAnimation(transitioin, forKey: "pageCurl")
}

//因为上面设置了 transitioin.delegate = self,这两个代理方法会自动调用
override func animationDidStart(anim: CAAnimation) {
  //动画开始时，判断当前动画是代表往上翻页，还是往下翻页，来刷新日历时间
  animatiing = true
  let components = GregorianCalendar?.components([.Year,.Month,.Day], fromDate: date)
  if anim.valueForKey("month") as! String == "next" {
    components?.month += 1
  }else if anim.valueForKey("month") as! String == "pre"{
    components?.month -= 1
  }
  date = (GregorianCalendar?.dateFromComponents(components!))!
  dateDidChaged!(date: date)
}

//动画结束时，将layer的transform属性设置为初始值,并移除dayView的layer上翻页的动画
override func animationDidStop(anim: CAAnimation, finished flag: Bool) {
  if flag {
    animatiing = false
    animationContainerView.layer.transform = CATransform3DIdentity
    dayView.layer.transform = CATransform3DIdentity
    dayView.layer.removeAnimationForKey("pageCurl")
  }
}