Objectivec 如何确定 UICollectionView flowLayout 中单元格之间的间距

更新:此答案的Swift版本:https://github.com/fanpyi/UICollectionViewLeftAlignedLayout-Swift

取@matt's lead，我修改了他的代码，以确保项目始终保持左对齐.我发现，如果一个项目单独结束在一条线上，它会被流程布局居中.为了解决这个问题，我做了以下修改.

只有当单元格的宽度不同时，才会出现这种情况，这可能会导致如下布局.由于UICollectionViewFlowLayout的行为，最后一行总是左对齐，问题在于除了最后一行之外的任何一行中的项目本身.

我看到了@matt的代码.

在这个例子中，我们看到，如果细胞自己最终在直线上，它们就会居中.下面的代码确保你的Collection 视图是这样的.

#import "CWDLeftAlignedCollectionViewFlowLayout.h"

const NSInteger kMaxCellSpacing = 9;

@implementation CWDLeftAlignedCollectionViewFlowLayout

- (NSArray *)layoutAttributesForElementsInRect:(CGRect)rect {
    NSArray* attributesToReturn = [super layoutAttributesForElementsInRect:rect];
    for (UICollectionViewLayoutAttributes* attributes in attributesToReturn) {
        if (nil == attributes.representedElementKind) {
            NSIndexPath* indexPath = attributes.indexPath;
            attributes.frame = [self layoutAttributesForItemAtIndexPath:indexPath].frame;
        }
    }
    return attributesToReturn;
}

- (UICollectionViewLayoutAttributes *)layoutAttributesForItemAtIndexPath:(NSIndexPath *)indexPath {
    UICollectionViewLayoutAttributes* currentItemAttributes =
    [super layoutAttributesForItemAtIndexPath:indexPath];

    UIEdgeInsets sectionInset = [(UICollectionViewFlowLayout *)self.collectionView.collectionViewLayout sectionInset];

    CGRect currentFrame = currentItemAttributes.frame;

    if (indexPath.item == 0) { // first item of section
        currentFrame.origin.x = sectionInset.left; // first item of the section should always be left aligned
        currentItemAttributes.frame = currentFrame;

        return currentItemAttributes;
    }

    NSIndexPath* previousIndexPath = [NSIndexPath indexPathForItem:indexPath.item-1 inSection:indexPath.section];
    CGRect previousFrame = [self layoutAttributesForItemAtIndexPath:previousIndexPath].frame;
    CGFloat previousFrameRightPoint = CGRectGetMaxX(previousFrame) + kMaxCellSpacing;

    CGRect strecthedCurrentFrame = CGRectMake(0,
                                              currentFrame.origin.y,
                                              self.collectionView.frame.size.width,
                                              currentFrame.size.height);

    if (!CGRectIntersectsRect(previousFrame, strecthedCurrentFrame)) { // if current item is the first item on the line
        // the approach here is to take the current frame, left align it to the edge of the view
        // then stretch it the width of the collection view, if it intersects with the previous frame then that means it
        // is on the same line, otherwise it is on it's own new line
        currentFrame.origin.x = sectionInset.left; // first item on the line should always be left aligned
        currentItemAttributes.frame = currentFrame;
        return currentItemAttributes;
    }

    currentFrame.origin.x = previousFrameRightPoint;
    currentItemAttributes.frame = currentFrame;
    return currentItemAttributes;
}

@end