iOS 转场动画封装

转场（transition）动画

1.转场种类

分为4种：present,dismiss; push, pop.
前两者是presentViewController时的方式.
后两者是在navigationController中的方式.

2.针对不同种类的转场动画

（1）对于前两者, 需要实现UIViewControllerTransitioningDelegate,
并在viewController中设置该delegate.

设置transitioningDelegate：

override func viewDidLoad() {
        super.viewDidLoad()
        self.transitioningDelegate = self
}

实现UIViewControllerTransitioningDelegate：

extension GreenViewController: UIViewControllerTransitioningDelegate {
    func animationController(forDismissed dismissed: UIViewController) -> UIViewControllerAnimatedTransitioning? {
        return ShapeAnimator(duration: TimeInterval(animationDurationSlider.value), viewCenter: self.view.center, viewSize: self.view.bounds.size, startPoint: self.transitionButton.center, transitionMode: .push)
    }
    func animationController(forPresented presented: UIViewController, presenting: UIViewController, source: UIViewController) -> UIViewControllerAnimatedTransitioning? {
        return ShapeAnimator(duration: TimeInterval(animationDurationSlider.value), viewCenter: self.view.center, viewSize: self.view.bounds.size, startPoint: self.transitionButton.center, transitionMode: .push)
    }
}

forPresented 和 forDismissed 方法 分别对应present 和 dismiss.

注意到方法有返回值，且返回值类型为UIViewControllerAnimatedTransitioning

这实际上就是转场动画需要实现的协议,转场动画的关键也在于此.
下文会着重讲解这个部分.

(2)对于后两者，需要实现UINavigationControllerDelegate,并在ViewController中设置navigationController的delegate.

override func viewDidAppear(_ animated: Bool) {
    super.viewDidAppear(animated)
    self.navigationController?.delegate = self
}

实现UINavigationControllerDelegate

extension GreenViewController: UINavigationControllerDelegate {
    func navigationController(_ navigationController: UINavigationController, animationControllerFor operation: UINavigationControllerOperation, from fromVC: UIViewController, to toVC: UIViewController) -> UIViewControllerAnimatedTransitioning? {
        if operation == .push {
            if isAnimationSpring {
                return SpringAnimator(duration: TimeInterval(animationDurationSlider.value), velocity: CGFloat(initialVelocitySlider.value), damping: CGFloat(springSlider.value))
            } else {
                return ShapeAnimator(duration: TimeInterval(animationDurationSlider.value), viewCenter: fromVC.view.center, viewSize: fromVC.view.bounds.size, startPoint: self.transitionButton.center, transitionMode: .push)
            }
        } else {
            return nil
        }
    }
}

push以及pop的类型可以通过operation获取.

同样针对不同的类型，也需要返回实现了UIViewControllerAnimatedTransitioning的对象.

所以，核心就是封装一个实现了UIViewControllerAnimatedTransitioning的对象.

而这个对象是可以在present／navigate动画中复用的.

3.封装一个实现UIViewControllerAnimatedTransitioning的对象.

直接上代码：
关键就是实现UIViewControllerAnimatedTransitioning的协议,
transitionContext是转场的上下文,
可以通过它获取到动画容器视图，转场前的视图，转场后的视图.
并在动画容器视图中实现动画效果即可.
在transitionDuration()可以设置转场动画的时长.

import UIKit

class SpringAnimator: NSObject {
    
    var animationDuration: TimeInterval = 2
    var initialVelocity: CGFloat = 0
    var springDamping: CGFloat = 0.4
    
    convenience init(duration: TimeInterval, velocity: CGFloat, damping: CGFloat) {
        self.init()
        animationDuration = duration
        initialVelocity = velocity
        springDamping = damping
    }
}

extension SpringAnimator: UIViewControllerAnimatedTransitioning {
    func transitionDuration(using transitionContext: UIViewControllerContextTransitioning?) -> TimeInterval {
        return animationDuration
    }
    
    func animateTransition(using transitionContext: UIViewControllerContextTransitioning) {
        let toViewController = transitionContext.viewController(forKey: UITransitionContextViewControllerKey.to)
        let containerView = transitionContext.containerView
        
        let fromView = transitionContext.view(forKey: UITransitionContextViewKey.from)
        let toView = transitionContext.view(forKey: UITransitionContextViewKey.to)
        
        toView?.frame = fromView?.frame.offsetBy(dx: 0, dy: -(fromView?.frame.size.height ?? 0)) ?? .zero
        toView?.alpha = 0
        
        containerView.addSubview(toView!)
        
        let duration = self.transitionDuration(using: transitionContext)
        
        UIView.animate(withDuration: duration, delay: 0, usingSpringWithDamping: springDamping, initialSpringVelocity: initialVelocity, options: .curveLinear, animations: {
            toView!.alpha = 1.0
            toView?.frame = transitionContext.finalFrame(for: toViewController!)
        }) { (isFinished) in
            let wasCancelled = transitionContext.transitionWasCancelled
            transitionContext.completeTransition(!wasCancelled)
        }
    }
}

至此，一个转场动画效果就封装好啦，demo可以去我的github下载：https://github.com/LoKiLyn/TransitionAnimation