initialLayoutAttributesForAppearingItemAtIndexPath 为所有可见单元格触发，而不仅仅是插入的单元格 [英] initialLayoutAttributesForAppearingItemAtIndexPath fired for all visible cells, not just inserted cells

问题描述

有人看过这个问题的正确答案吗?

Has anyone seen a decent answer to this problem?

initialLayoutAttributesForAppearingItemAtIndexPath 似乎被所有可见单元格调用，而不仅仅是被插入的单元格.根据 Apple 自己的文档:

initialLayoutAttributesForAppearingItemAtIndexPath seems to be being called for all visible cells, not just the cell being inserted. According to Apple's own docs:

对于移动的项目，集合视图使用标准方法来检索项目的更新布局属性.对于插入或删除的项目，集合视图会调用一些不同的方法，您应该重写这些方法以提供适当的布局信息

For moved items, the collection view uses the standard methods to retrieve the item’s updated layout attributes. For items being inserted or deleted, the collection view calls some different methods, which you should override to provide the appropriate layout information

这听起来不像正在发生的事情......其他单元格没有被插入，它们正在被移动，但它正在为 被移动的那些调用 initialLayoutAttributesForAppearingItemAtIndexPath强>也是.

Which doesn't sound like what is happening... the other cells aren't being inserted, they are being moved, but it's calling initialLayoutAttributesForAppearingItemAtIndexPath for the ones being moved too.

我已经看到使用 prepareForCollectionViewUpdates: 来跟踪哪些 indexPaths 正在更新并且只更改那些的变通方法，但这似乎有点奇怪，因为它会再次出现在他们自己的文档中.有其他人找到解决此问题的更好方法吗?

I have seen work arounds using prepareForCollectionViewUpdates: to trace which indexPaths are being updated and only changing those, but this seems a bit odd that it's going agains their own docs. Has anyone else found a better way around this?

推荐答案

我找到了 Mark Pospesel 的这篇博文 会有所帮助.
作者还修复了 WWDC CircleLayout 示例并发布了 它在 Github 上.

I found this blog post by Mark Pospesel to be helpful.
The author also fixed WWDC CircleLayout sample and posted it on Github.

感兴趣的方法:

- (void)prepareForCollectionViewUpdates:(NSArray *)updateItems
{
    // Keep track of insert and delete index paths
    [super prepareForCollectionViewUpdates:updateItems];

    self.deleteIndexPaths = [NSMutableArray array];
    self.insertIndexPaths = [NSMutableArray array];

    for (UICollectionViewUpdateItem *update in updateItems)
    {
        if (update.updateAction == UICollectionUpdateActionDelete)
        {
            [self.deleteIndexPaths addObject:update.indexPathBeforeUpdate];
        }
        else if (update.updateAction == UICollectionUpdateActionInsert)
        {
            [self.insertIndexPaths addObject:update.indexPathAfterUpdate];
        }
    }
}

- (void)finalizeCollectionViewUpdates
{
    [super finalizeCollectionViewUpdates];
    // release the insert and delete index paths
    self.deleteIndexPaths = nil;
    self.insertIndexPaths = nil;
}

// Note: name of method changed
// Also this gets called for all visible cells (not just the inserted ones) and
// even gets called when deleting cells!
- (UICollectionViewLayoutAttributes *)initialLayoutAttributesForAppearingItemAtIndexPath:(NSIndexPath *)itemIndexPath
{
    // Must call super
    UICollectionViewLayoutAttributes *attributes = [super initialLayoutAttributesForAppearingItemAtIndexPath:itemIndexPath];

    if ([self.insertIndexPaths containsObject:itemIndexPath])
    {
        // only change attributes on inserted cells
        if (!attributes)
            attributes = [self layoutAttributesForItemAtIndexPath:itemIndexPath];

        // Configure attributes ...
        attributes.alpha = 0.0;
        attributes.center = CGPointMake(_center.x, _center.y);
    }

    return attributes;
}

// Note: name of method changed
// Also this gets called for all visible cells (not just the deleted ones) and
// even gets called when inserting cells!
- (UICollectionViewLayoutAttributes *)finalLayoutAttributesForDisappearingItemAtIndexPath:(NSIndexPath *)itemIndexPath
{
    // So far, calling super hasn't been strictly necessary here, but leaving it in
    // for good measure
    UICollectionViewLayoutAttributes *attributes = [super finalLayoutAttributesForDisappearingItemAtIndexPath:itemIndexPath];

    if ([self.deleteIndexPaths containsObject:itemIndexPath])
    {
        // only change attributes on deleted cells
        if (!attributes)
            attributes = [self layoutAttributesForItemAtIndexPath:itemIndexPath];

        // Configure attributes ...
        attributes.alpha = 0.0;
        attributes.center = CGPointMake(_center.x, _center.y);
        attributes.transform3D = CATransform3DMakeScale(0.1, 0.1, 1.0);
    }

    return attributes;
}

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