ios - 自定义 UIViewController 动画问题

Question

我使用 UIViewControllerAnimatedTransitioning 为 UIViewController 创建了一个简单的自定义过渡动画。一切正常，但是当目标 View 出现时，内容从右下角开始出现，当 View 消失时，内容从左上角消失。我附上了 gif 示例。这是我的代码:

CustomViewAnimator.swift

class CustomViewAnimator: NSObject, UIViewControllerAnimatedTransitioning {

    private let interval = 0.5
    var forward = false
    var originFrame: CGRect = .zero

    func transitionDuration(using transitionContext: UIViewControllerContextTransitioning?) -> TimeInterval {
        return interval
    }

    func animateTransition(using transitionContext: UIViewControllerContextTransitioning) {

        let containerView = transitionContext.containerView

        guard let toView = transitionContext.view(forKey: .to) else {return}
        guard let fromView = transitionContext.view(forKey: .from) else {return}
        let destinationView = forward ? toView : fromView
        destinationView.alpha = forward ? 0 : 1

        forward ? containerView.addSubview(toView) : containerView.insertSubview(toView, belowSubview: fromView)
        forward ? (destinationView.frame = originFrame) : (destinationView.frame = fromView.frame)

        UIView.animate(withDuration: interval, animations: {

            self.forward ? (destinationView.frame = fromView.frame) : (destinationView.frame = self.originFrame)
            destinationView.alpha = self.forward ? 1 : 0

        }) { (finished) in
            transitionContext.completeTransition(!transitionContext.transitionWasCancelled)
        }

    }

}

MainViewController.self

class MainViewController: UIViewController {

    private var button: UIButton!

    override func viewDidLoad() {
        super.viewDidLoad()
        setup()
    }

    @objc private func openView() {
        let viewController = TargetViewController()
        viewController.transitioningDelegate = self
        navigationController?.pushViewController(viewController, animated: true)

    }
}

extension MainViewController: UINavigationControllerDelegate {
    func navigationController(_ navigationController: UINavigationController, animationControllerFor operation: UINavigationController.Operation, from fromVC: UIViewController, to toVC: UIViewController) -> UIViewControllerAnimatedTransitioning? {

        let animator = CustomViewAnimator()
        animator.originFrame = button.frame
        animator.forward = (operation == .push)
        return animator
    }
}

extension MainViewController: UIViewControllerTransitioningDelegate {
    func animationController(forPresented presented: UIViewController, presenting: UIViewController, source: UIViewController) -> UIViewControllerAnimatedTransitioning? {
        let animator = CustomViewAnimator()
        animator.forward = true
        return animator
    }

    func animationController(forDismissed dismissed: UIViewController) -> UIViewControllerAnimatedTransitioning? {
        let animator = CustomViewAnimator()
        animator.forward = false
        return animator
    }
}

TargetViewController.swift

class TargetViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()
        setup()
    }

}

extension TargetViewController {

    fileprivate func setup() {
        view.backgroundColor = .orange
        navigationItem.title = "Target view"

        let label: UILabel = {
            let label = UILabel()
            label.translatesAutoresizingMaskIntoConstraints = false
            label.text = "Hello from target view :)"
            label.textColor = .white
            label.font = UIFont.boldSystemFont(ofSize: 18)
            return label
        }()
        view.addSubview(label)

        label.centerXAnchor.constraint(equalTo: view.centerXAnchor).isActive = true
        label.centerYAnchor.constraint(equalTo: view.centerYAnchor).isActive = true
    }

}

谁能告诉我如何解决这个问题，使 TargetViewController 中的标签(或任何其他内容)保持在原位？

Answer 1

基本上你的问题是因为混合了自动布局和框架。您在 ViewController 的 View 中使用约束定位 subview (按钮和标签)，但对于动画，您使用帧更新。因此，据我正确理解布局的工作原理，您的标签约束在转换期间未激活，或者至少布局引擎未使用它来计算标签的位置。

因此，最好的建议是避免过渡中的帧动画，并尝试为 .from 和 .to View 创建适当的约束，并对约束常量进行动画处理。这将需要更多代码，但您可以确定您不会混合使用两种不同的布局策略。

更简单的解决方案是告诉 containerView 计算约束并在动画期间适本地定位元素，只需将 containerView.layoutIfNeeded() 添加到动画 block 即可。所以它会是:

UIView.animate(withDuration: interval, animations: {
    self.forward ? (destinationView.frame = fromView.frame) : (destinationView.frame = self.originFrame)
    destinationView.alpha = self.forward ? 1 : 0
    containerView.layoutIfNeeded()
}) { (finished) in
    transitionContext.completeTransition(!transitionContext.transitionWasCancelled)
}

但这只能使事情正常进行，并不能解决同时使用两种布局策略的主要问题。