iOS 自定义 ViewController 过渡动画

// UITabBarControllerDelegate 的这个方法，用于返回一个负责管理过渡动画的 UIViewControllerAnimatedTransitioning

        optional func tabBarController(_ tabBarController:     UITabBarController,
animationControllerForTransitionFromViewController fromVC: UIViewController,
                                    toViewController toVC: UIViewController) -> UIViewControllerAnimatedTransitioning?

// 另一个新东西就是 UIViewControllerAnimatedTransitioning 自己。。。有两个方法需要实现

// 这个方法负责做真正的动画，输入参数是过渡的上下文，从哪个VC过渡到哪个VC这些东西都可以从它得到。
func animateTransition(_ transitionContext: UIViewControllerContextTransitioning)

// 这个返回本过渡的持续时间
func transitionDuration(_ transitionContext: UIViewControllerContextTransitioning) -> NSTimeInterval

class MainVC: UITabBarController, UITabBarControllerDelegate { // 实现UITabBarControllerDelegate接口

    override func viewDidLoad() {
        super.viewDidLoad()
        self.delegate = self  // delegate设置为self
    }

    /* 如GIF中那样在切换时改变状态栏颜色，这里可选，与过渡无关
    func tabBarController(tabBarController: UITabBarController, didSelectViewController viewController: UIViewController) {
        let index = find(tabBarController.viewControllers! as [UIViewController], viewController)!
        switch index {
        case 1:
            UIApplication.sharedApplication().setStatusBarStyle(UIStatusBarStyle.LightContent, animated: true)
        default:
            UIApplication.sharedApplication().setStatusBarStyle(UIStatusBarStyle.Default, animated: true)
        }
    }
    */

    // 这里我们只要返回一个UIViewControllerAnimatedTransitioning，tabbarcontroller就会根据它去执行过渡动画
    func tabBarController(tabBarController: UITabBarController, animationControllerForTransitionFromViewController fromVC: UIViewController, toViewController toVC: UIViewController) -> UIViewControllerAnimatedTransitioning? {

        // 下面会给出TransitioningObject的实现，这里我们可以也可以选择用本方法的fromVC、toVC、tabBarController这三个参数构造出一个TransitioningObject
        let transitionObject: TransitioningObject = TransitioningObject();
        transitionObject.tabBarController = self

        return transitionObject
    }

}

class TransitioningObject:NSObject, UIViewControllerAnimatedTransitioning {
    private weak var tabBarController: MainVC!

    // 动画在这里！
    func animateTransition(transitionContext: UIViewControllerContextTransitioning) {

        // 由context获得参与过渡的各个角色
        let fromView: UIView = transitionContext.viewForKey(UITransitionContextFromViewKey)!
        let fromViewController: UIViewController = transitionContext.viewControllerForKey(UITransitionContextFromViewControllerKey)!
        let toView: UIView = transitionContext.viewForKey(UITransitionContextToViewKey)!
        let toViewController: UIViewController = transitionContext.viewControllerForKey(UITransitionContextToViewControllerKey)!

        // 可以吧containerView理解成一个舞台，参与过渡动画的角色在这个舞台上表演。。。所以要让他们上台先。
        transitionContext.containerView().addSubview(fromView)
        transitionContext.containerView().addSubview(toView)

        // 找出各个VC在tabBar上的位置。
        let fromViewControllerIndex = find(self.tabBarController.viewControllers! as [UIViewController], fromViewController)
        let toViewControllerIndex = find(self.tabBarController.viewControllers! as [UIViewController], toViewController)

        // 计算出点击的tab的位置，作为动画的圆心
        let tabBarFrame = tabBarController.tabBar.frame
        let tabBarItemCount = tabBarController.tabBar.items!.count
        let tabBarItemWidth = tabBarFrame.size.width / CGFloat(tabBarItemCount)

        let tappedItemY = tabBarFrame.origin.y
        let tappedItemX = tabBarItemWidth * CGFloat(toViewControllerIndex!) + tabBarItemWidth / 2

        // 圆要放大到的半径，勾股定理算出toView的对角线长度
        var finalRadius = sqrt(pow(toView.frame.height, 2) + pow(toView.frame.width, 2))

        // 构造开始时和结束时的圆的贝赛尔曲线。
        let start = UIBezierPath(ovalInRect: CGRect(x: tappedItemX, y: tappedItemY, width: 0, height: 0)).CGPath
        let final = UIBezierPath(ovalInRect: CGRect(x: tappedItemX - finalRadius, y: tappedItemY - finalRadius, width: finalRadius * 2, height: finalRadius * 2)).CGPath

        // 新建一个CAShapeLayer，用作toView的遮罩。并且开始时的path设置为上面的start——位置在点击的tab上的一个半径为0的圆。
        // 下文中就要给这个path加特技，让他变化到包含整个界面那么大。
        var circleMask = CAShapeLayer()
        circleMask.path = start
        toView.layer.mask = circleMask

        // 给circleMask的path属性加动画
        let animation = CABasicAnimation(keyPath: "path")
        animation.fromValue = start
        animation.toValue = final
        animation.duration = self.transitionDuration(transitionContext)
        animation.delegate = self // 设置CABasicAnimation的delegate为self，好在动画结束后通知系统过渡完成了。
        animation.setValue(transitionContext, forKey: "transitionContext") // 待会需要用到transitionContext的completeTransition方法
        circleMask.addAnimation(animation, forKey: "circleAnimation")
        circleMask.path = final
    }

    // 过渡动画完成后，调用completeTransition说明过渡完成。
    override func animationDidStop(anim: CAAnimation!, finished flag: Bool) {
        if let context = anim.valueForKey("transitionContext") as? UIViewControllerContextTransitioning {
            context.completeTransition(true)
        }
    }

    // 持续1秒钟
    func transitionDuration(transitionContext: UIViewControllerContextTransitioning) -> NSTimeInterval {
        return 1;
    }
}