ios - 自定义 UIViewController 转换的 UIView 快照

Question

我正在为我的应用程序创建一个自定义过渡，但是当我尝试创建目标 View 的快照时，它总是显示为一个空白的白色矩形。重要的是，我注意到这是一个自定义的 push 转换，而不是 View 的模态呈现。以模态方式呈现时，快照似乎有效。这是自定义 push/pop 转换的正常行为吗？

我写的代码如下:

func animateTransition(using transitionContext: UIViewControllerContextTransitioning) {
    guard let toViewController = transitionContext.viewController(forKey: .to) as? CultureViewController else {
        return
    }

    let containerView = transitionContext.containerView
    let finalFrame = transitionContext.finalFrame(for: toViewController)

    let snapshot = toViewController.view.snapshotView(afterScreenUpdates: true)
    snapshot?.frame = UIScreen.main.bounds
    containerView.addSubview(snapshot!)

    toViewController.view.transform = CGAffineTransform(translationX: 0, y: toViewController.view.bounds.height)
    //containerView.addSubview(toViewController.view)

    let duration = transitionDuration(using: transitionContext)
    UIView.animateKeyframes(withDuration: duration, delay: 0, options: .calculationModeCubic, animations: { 
        UIView.addKeyframe(withRelativeStartTime: 0, relativeDuration: 1, animations: { 
            toViewController.view.frame = finalFrame
        })
    }, completion: { _ in
        transitionContext.completeTransition(!transitionContext.transitionWasCancelled)
    })
}

Answer 1

在 View 呈现之前对其进行快照，例如在调用 containerView.addSubview(toViewController.view) 之前进行转换的情况，根据 UIView's documentation 返回一个空白 View .

If the current view is not yet rendered, perhaps because it is not yet onscreen, the snapshot view has no visible content.