动画的关键在于animator如何实现，它实现了UIViewControllerAnimatedTransitioning协议，至少需要实现两个方法，我建议您仔细阅读animateTransition方法中的注释，它是整个动画逻辑的核心：
class HalfWaySpringAnimator: NSObject, UIViewControllerAnimatedTransitioning {
/// 设置动画的持续时间
func transitionDuration(transitionContext: UIViewControllerContextTransitioning?) -> NSTimeInterval {
return 2
}
/// 设置动画的进行方式，附有详细注释，demo中其他地方的这个方法不再解释
func animateTransition(transitionContext: UIViewControllerContextTransitioning) {
let fromViewController = transitionContext.viewControllerForKey(UITransitionContextFromViewControllerKey)
let toViewController = transitionContext.viewControllerForKey(UITransitionContextToViewControllerKey)
let containerView = transitionContext.containerView()
// 需要关注一下from/to和presented/presenting的关系
// For a Presentation:
// fromView = The presenting view.
// toView = The presented view.
// For a Dismissal:
// fromView = The presented view.
// toView = The presenting view.
var fromView = fromViewController?.view
var toView = toViewController?.view
// iOS8引入了viewForKey方法，尽可能使用这个方法而不是直接访问controller的view属性
// 比如在form sheet样式中，我们为presentedViewController的view添加阴影或其他decoration，animator会对整个decoration view
// 添加动画效果，而此时presentedViewController的view只是decoration view的一个子视图
if transitionContext.respondsToSelector(Selector("viewForKey:")) {
fromView = transitionContext.viewForKey(UITransitionContextFromViewKey)
toView = transitionContext.viewForKey(UITransitionContextToViewKey)
}
// 我们让toview的origin.y在屏幕的一半处，这样它从屏幕的中间位置弹起而不是从屏幕底部弹起，弹起过程中逐渐变为不透明
toView?.frame = CGRectMake(fromView!.frame.origin.x, fromView!.frame.maxY / 2, fromView!.frame.width, fromView!.frame.height)
toView?.alpha = 0.0
// 在present和，dismiss时，必须将toview添加到视图层次中
containerView?.addSubview(toView!)
let transitionDuration = self.transitionDuration(transitionContext)
// 使用spring动画，有弹簧效果，动画结束后一定要调用completeTransition方法
UIView.animateWithDuration(transitionDuration, delay: 0, usingSpringWithDamping: 0.6, initialSpringVelocity: 0, options: .CurveLinear, animations: { () -> Void in
toView!.alpha = 1.0 // 逐渐变为不透明
toView?.frame = transitionContext.finalFrameForViewController(toViewController!) // 移动到指定位置
}) { (finished: Bool) -> Void in
let wasCancelled = transitionContext.transitionWasCancelled()
transitionContext.completeTransition(!wasCancelled)
}
}
}
