ios - 具有自定义 anchorPoint 的 UICollectionViewFlowLayout Cell 重用过早

Question

我将 UICollectionViewFlowLayout 子类化以在水平滚动期间实现小的缩放效果。因此，我不得不继承 UICollectionViewCell 并更改单元格的 layer.anchorPoint(我的缩放比例是从单元格的左下角开始，而不是从默认中心开始).现在一切都很好，除了当我水平滚动时，我的单元格被过早地重复使用(当它突然消失时我仍然可以看到半个单元格)。

我感觉 collection view 的计算仍然基于位于单元格中心的 anchor 来重用单元格...

但是，这是我的收藏 View 。您可以看到项目在到达 Collection View 的左侧时如何变大。这是我想要的缩放比例。

现在我向左滚动。您可以看到右边的项目如何变大，而左边的项目如何离开屏幕。

在这里您可以看到左边的项目并没有离开屏幕，而是已经消失了。只有正确的项目仍然可见:/

所以我想要的是，只有当它的右边界到达屏幕的最左边时，才让左边的项目消失。简单地说，只有当我不再看到它时才消失。

这是我的代码:

class SongsCollectionViewCell : UICollectionViewCell {

@IBOutlet weak var imgAlbumCover: UIImageView!


override func applyLayoutAttributes(layoutAttributes: UICollectionViewLayoutAttributes) {
    super.applyLayoutAttributes(layoutAttributes)

    //we must change the anchor point for propper cells positioning and scaling
    self.layer.anchorPoint.x = 0
    self.layer.anchorPoint.y = 1
}

这是布局本身:

class SongsCollectionViewLayout : UICollectionViewFlowLayout {

override func prepareLayout() {
    collectionView?.decelerationRate = UIScrollViewDecelerationRateFast
    self.scrollDirection = UICollectionViewScrollDirection.Horizontal;

    //size of the viewport
    let size:CGSize = self.collectionView!.frame.size;
    let itemWidth:CGFloat  = size.width * 0.7//0.7//3.0;
    self.itemSize = CGSizeMake(itemWidth, itemWidth);
    self.sectionInset = UIEdgeInsetsMake(0,0,0,0);
    self.minimumLineSpacing = 0
    self.minimumInteritemSpacing = 0

}

override func shouldInvalidateLayoutForBoundsChange(newBounds: CGRect) -> Bool {
    return true
}


override func layoutAttributesForElementsInRect(rect: CGRect) -> [UICollectionViewLayoutAttributes]? {

    let attributes:[UICollectionViewLayoutAttributes] = super.layoutAttributesForElementsInRect(rect)!

    var visibleRect:CGRect = CGRect()
    visibleRect.origin = self.collectionView!.contentOffset;
    visibleRect.size = self.collectionView!.bounds.size;

    for layoutAttributes in attributes {

        if (CGRectIntersectsRect(layoutAttributes.frame, rect)) {

            //we must align items to the bottom of the collection view on y axis
            let frameHeight = self.collectionView!.bounds.size.height
            layoutAttributes.center.y = frameHeight
            layoutAttributes.center.x = layoutAttributes.center.x - self.itemSize.width/2

            //find where ite, left x is
            let itemLeftX:CGFloat = layoutAttributes.center.x

            //distance of the item from the left of the viewport
            let distanceFromTheLeft:CGFloat = itemLeftX - CGRectGetMinX(visibleRect)
            let normalizedDistanceFromTheLeft = abs(distanceFromTheLeft) / self.collectionView!.frame.size.width

            //item that is closer to the left is most visible one
            layoutAttributes.alpha = 1 - normalizedDistanceFromTheLeft
            layoutAttributes.zIndex = abs(Int(layoutAttributes.alpha)) * 10;

            //scale items
            let scale =  min(layoutAttributes.alpha + 0.5, 1)
            layoutAttributes.transform = CGAffineTransformMakeScale(scale, scale)

        }

    }

    return attributes;
}

override func targetContentOffsetForProposedContentOffset(proposedContentOffset: CGPoint, withScrollingVelocity velocity: CGPoint) -> CGPoint {
    // Snap cells to centre
    var newOffset = CGPoint()
    let layout = collectionView!.collectionViewLayout as! UICollectionViewFlowLayout
    let width = layout.itemSize.width + layout.minimumLineSpacing
    var offset = proposedContentOffset.x + collectionView!.contentInset.left

    if velocity.x > 0 {
        //ceil returns next biggest number
        offset = width * ceil(offset / width)
    } else if velocity.x == 0 { //6
        //rounds the argument
        offset = width * round(offset / width)
    } else if velocity.x < 0 { //7
        //removes decimal part of argument
        offset = width * floor(offset / width)
    }
    newOffset.x = offset - collectionView!.contentInset.left
    newOffset.y = proposedContentOffset.y //y will always be the same...
    return newOffset
}

Answer 1

回答我自己的问题。所以正如我所怀疑的那样，布局考虑到了旧的中心，这就是为什么我必须在更改 anchor 后立即更正单元格的中心。所以我的自定义单元格现在看起来像这样:

class SongsCollectionViewCell : UICollectionViewCell {

@IBOutlet weak var imgAlbumCover: UIImageView!

override func prepareForReuse() {
    imgAlbumCover.hnk_cancelSetImage()
    imgAlbumCover.image = nil
}

override func applyLayoutAttributes(layoutAttributes: UICollectionViewLayoutAttributes) {
    super.applyLayoutAttributes(layoutAttributes)

    //we must change the anchor point for propper cells positioning and scaling
    self.layer.anchorPoint.x = 0
    self.layer.anchorPoint.y = 1

    //we need to adjust a center now
    self.center.x = self.center.x - layoutAttributes.size.width/2
}

希望对大家有帮助