UICollectionViewCell systemLayoutSizeFittingSize返回不正确的宽度

Question

我一直在玩弄动态UICollectionViewCell的和已经注意到,在iOS 8调用cell.contentView.systemLayoutSizeFittingSize(UILayoutFittingCompressedSize)上UICollectionViewCell返回不正确的宽度.目前解决此问题的唯一解决方法是显式添加宽度约束以强制单元格的宽度.以下代码用于Cell子类:

func calculatePreferredHeightForFrame(frame: CGRect) -> CGFloat {
    var newFrame = frame
    self.frame = newFrame
    let widthConstraint = NSLayoutConstraint(item: contentView, attribute: .Width, relatedBy: .Equal, toItem: nil, attribute: .NotAnAttribute, multiplier: 1, constant: CGRectGetWidth(frame))
    contentView.addConstraint(widthConstraint)
    self.setNeedsUpdateConstraints()
    self.updateConstraintsIfNeeded()
    self.setNeedsLayout()
    self.layoutIfNeeded()
    let desiredHeight: CGFloat = self.contentView.systemLayoutSizeFittingSize(UILayoutFittingCompressedSize).height
    newFrame.size.height = CGFloat(ceilf(Float(desiredHeight)))
    contentView.removeConstraint(widthConstraint)
    return CGRectGetHeight(newFrame)
}

我知道,对于iOS 8和动态UICollectionViewFlowLayout,UICollectionViewCell的contentView以不同的方式处理约束,但是我在这里缺少什么？为确保systemLayoutSizeFittingSize在单元格上使用特定宽度,需要做些什么？

我也遇到过这篇文章(使用自动布局在UICollectionView中指定单元格的一个维度)并且相信这可能实际上使我的问题无效.也许UICollectionViewFlowLayout不是为只有一个动态维度的单元格设计的,但仍然无法解释为什么单元格给出了不寻常的宽度.

Answer 1

我遇到了同样的问题.关键是在获取systemLayoutSize时使用优先级.

- (CGSize)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout *)collectionViewLayout sizeForItemAtIndexPath:(NSIndexPath *)indexPath
{
    if (!self.prototypeCell) {
        self.prototypeCell = [TrainingMenuCell new];
        self.prototypeCell.contentView.translatesAutoresizingMaskIntoConstraints = NO;
    }

    [self configureCell:self.prototypeCell forIndexPath:indexPath];
    [self.prototypeCell setNeedsLayout];
    [self.prototypeCell layoutIfNeeded];

    CGFloat width = CGRectGetWidth(collectionView.frame) - 12;
    CGSize fittingSize = UILayoutFittingCompressedSize;
    fittingSize.width = width;

    CGSize size = [self.prototypeCell.contentView systemLayoutSizeFittingSize:fittingSize withHorizontalFittingPriority:UILayoutPriorityRequired verticalFittingPriority:UILayoutPriorityDefaultLow];
    return size;
}