如何在自定义动画中访问 toView 对象的框架

Question

想做一个iOS主屏文件夹之类的动画，要知道“文件夹”最终位置的frame：

我正在使用自定义过渡，这里是动画文件的代码：

class FolderAnimationController: NSObject, UIViewControllerAnimatedTransitioning {

    let duration    = 5.0
    var presenting  = true

    func transitionDuration(_ transitionContext: UIViewControllerContextTransitioning?) -> TimeInterval {
        return duration
    }

    func animateTransition(_ transitionContext: UIViewControllerContextTransitioning) {

        let containerView = transitionContext.containerView()
        let toViewC = transitionContext.viewController(forKey: UITransitionContextToViewControllerKey)! as! ProjectViewController
        let fromViewC = transitionContext.viewController(forKey: UITransitionContextFromViewControllerKey)!as! ViewController

        let cell : FolderCollectionViewCell = fromViewC.folderCollectionView.cellForItem(at: (fromViewC.folderCollectionView.indexPathsForSelectedItems()?.first!)!) as! FolderCollectionViewCell
        let cellSnapshot: UIView = cell.snapshotView(afterScreenUpdates: false)!
        let cellFrame = containerView.convert(cell.frame, from: cell.superview)
        cellSnapshot.frame = cellFrame
        cell.isHidden = true

        toViewC.view.frame = transitionContext.finalFrame(for: toViewC)
        toViewC.view.alpha = 0
        containerView.addSubview(toViewC.view)
        containerView.addSubview(cellSnapshot)

        UIView.animate(withDuration: duration, animations: {
            toViewC.view.alpha = 1.0

            let finalFrame = containerView.convert(toViewC.containerView.frame, from: toViewC.view)
            cellSnapshot.frame = finalFrame
            }) { (_) in
                cellSnapshot.removeFromSuperview()
                transitionContext.completeTransition(true)
        }

    }
}

除了将finalFrame 值（是一个CGRect 变量）设置为(origin = (x = 0, y = 0), size = (width = 0, height = 0)) 的let finalFrame = containerView.convert(toViewC.containerView.frame, from: toViewC.view) 之外，一切正常。

我遵循this 教程，用 Swift 编写代码

那么，我怎样才能正确访问该属性？

Answer 1

框架是零矩形，因为在您访问它时，还没有发生视图的布局传递。设置 toVC.view 框架后，添加以下行：

toVC.view.layoutIfNeeded()

如果您有兴趣，我已经写过关于在视图控制器转换here 中使用快照的文章。