ios6 - initialLayoutAttributesForAppearingItemAtIndexPath 为所有可见单元格触发，而不仅仅是插入的单元格

Question

有没有人看到这个问题的体面答案？
initialLayoutAttributesForAppearingItemAtIndexPath似乎正在为所有可见的单元格调用，而不仅仅是插入的单元格。根据Apple's own docs :

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，但它正在调用 initialLayoutAttributesForAppearingItemAtIndexPath对于那些 正在移动 也。

我见过使用 prepareForCollectionViewUpdates: 的变通方法跟踪哪些 indexPaths 正在更新并且只更改那些，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。有没有其他人找到解决这个问题的更好方法？

Answer 1

我找到了this blog post by Mark Pospesel乐于助人。
作者还修复了 WWDC CircleLayout sample 和 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;
}