ios - UICollectionView InteractiveTransitionToCollectionViewLayout 崩溃并出现异常

Question

我有一个带有水平 collectionView 的 collectionViewController，就像在 Paper App 中一样。我添加了平移手势以从一种布局更改为另一种布局，并且我使用了交互式过渡。如果您在动画完成时拖动并等待，效果很好，但如果您拖动得更快并且不等待动画完成或取消应用程序抛出异常:

Assertion failure in -[UICollectionView _finishInteractiveTransitionShouldFinish:finalAnimation:], /SourceCache/UIKit_Sim/UIKit-2935.137/UICollectionView.m:2691 
*** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'the collection was not prepared for an interactive transition. see startInteractiveTransitionToCollectionViewLayout:completion:'

手势处理程序代码:

- (void)oneFingerGesture:(UIPanGestureRecognizer *)sender
 {

if (sender.state == UIGestureRecognizerStateEnded ||
    sender.state == UIGestureRecognizerStateCancelled)
{
    if (self.transitionLayout.transitionProgress > 0.2) {
        [self.collectionView finishInteractiveTransition];
    } else {
        [self.collectionView cancelInteractiveTransition];
    }
}else
{
    CGPoint point = [sender locationInView:sender.view];

    if (sender.state == UIGestureRecognizerStateBegan && !self.transitionLayout && !_isInTransition)
    {
        invertPan = self.largeLayout == self.collectionView.collectionViewLayout;

        UICollectionViewLayout *toLayout = invertPan ? self.smallLayout : self.largeLayout;

        self.transitionLayout = [self.collectionView startInteractiveTransitionToCollectionViewLayout:toLayout
                                                 completion:^(BOOL completed, BOOL finish) {
                                                     self.transitionLayout = nil;
                            _isInTransition = NO; }];


        self.initialTapPoint = point;
        _isInTransition = YES;

    }else if(sender.state == UIGestureRecognizerStateChanged && self.transitionLayout && _isInTransition)
    {
        CGFloat distance = _initialTapPoint.y - point.y;
        if (invertPan) {
            distance = -distance;
        }
        CGFloat dimension = self.collectionView.bounds.size.height - 200;
        CGFloat progress = MAX(MIN(((distance)/ dimension), 1.0), 0.0);
       [self.transitionLayout setTransitionProgress:progress];

    }
}
 }

Answer 1

文档指出，调用 finishInteractiveTransition: 和 cancelInteractiveTransition: 将分别安装过渡将要到达和离开的布局对象。然而，这似乎不会立即发生。因此，如果触发和驱动过渡的手势可以快速连续发生，则在调用两种方法之一结束过渡之前检查当前布局是否为 UICollectionViewTransitionLayout 或其子类是不够的。

我通过引入 BOOL ivar (_isFinishingOrCancellingTransition) 来解决这个问题，以避免结束已经在结束过程中的转换:

if (_isFinishingOrCancellingTransition) return;

if (!self.transitionLayout) return;
if (self.collectionView.collectionViewLayout != self.transitionLayout) return;

_isFinishingOrCancellingTransition = YES;

if (self.transitionLayout.transitionProgress > 0.5)
{
    [self.collectionView finishInteractiveTransition];
}
else
{
    [self.collectionView cancelInteractiveTransition];
}

然后在转换完成时重置 BOOL:

self.transitionLayout = [self.collectionView startInteractiveTransitionToCollectionViewLayout:toLayout completion:^(BOOL completed, BOOL finish) {
    self.transitionLayout = nil;
    _isFinishingOrCancellingTransition = NO;
}];