iOS collectionView自適應高度estimatedItemSize

collectionView自適應高度 有兩個點需要注意
1.設置layout的estimatedItemSize屬性

    UICollectionViewFlowLayout *flowLayout = [[UICollectionViewFlowLayout alloc] init];
    flowLayout.scrollDirection = UICollectionViewScrollDirectionVertical;
    flowLayout.estimatedItemSize = CGSizeMake((SCREEN_WIDTH-24), autoSize(150));

2.在cell中重新並實現preferredLayoutAttributesFittingAttributes:方法【這個方法經常被遺漏，這樣算出來的寬高就會有問題】

-(UICollectionViewLayoutAttributes*)preferredLayoutAttributesFittingAttributes:(UICollectionViewLayoutAttributes*)layoutAttributes {
    [self setNeedsLayout];
    [self layoutIfNeeded];
    CGSize size = [self.contentView systemLayoutSizeFittingSize: layoutAttributes.size];
    CGRect cellFrame = layoutAttributes.frame;
    cellFrame.size.height= size.height;
    layoutAttributes.frame= cellFrame;
    return layoutAttributes;
}