居中对齐

@implementation WYTCollectionViewCenterFlowLayout
- (NSArray<__kindof UICollectionViewLayoutAttributes *> *)layoutAttributesForElementsInRect:(CGRect)rect {
    NSArray *attrsArry = [super layoutAttributesForElementsInRect:rect];
    // 最后一行有几个
    NSInteger last = attrsArry.count % kLineNum;
    if (last == 0) {
        return attrsArry;
    }
    CGFloat itemW = self.itemSize.width;
    CGFloat spacing = self.minimumInteritemSpacing;
    CGFloat totalW = spacing*(last-1) + itemW*last;
    CGFloat startX = (kCollectionViewW - totalW) * 0.5;
    // 最后一行起始位置
    NSInteger startIndex = attrsArry.count - last;
    for (NSInteger i=startIndex; i<attrsArry.count; i++) {
        CGFloat x = startX + (attrsArry.count-i-1) * (itemW+spacing);
        UICollectionViewLayoutAttributes *curAttr = attrsArry[i];
        CGRect frame = curAttr.frame;
        frame.origin.x = x;
        curAttr.frame = frame;
    }
    return attrsArry;
}
@end

左对齐

@implementation WYTCollectionViewLeftFlowLayout
-(NSArray<__kindof UICollectionViewLayoutAttributes *> *)layoutAttributesForElementsInRect:(CGRect)rect {
    NSArray *attrsArry = [super layoutAttributesForElementsInRect:rect];
    for (int i=0; i<attrsArry.count; i++) {
        if (i+1 < attrsArry.count) {
            UICollectionViewLayoutAttributes *curAttr = attrsArry[i];
            UICollectionViewLayoutAttributes *nextAttr = attrsArry[i+1];
            //如果下一个在同一行则调整，不在同一行则跳过
            if (CGRectGetMinY(curAttr.frame) == CGRectGetMinY(nextAttr.frame)) {
                if (CGRectGetMinX(nextAttr.frame) - CGRectGetMaxX(curAttr.frame) > self.minimumInteritemSpacing){
                    CGRect frame = nextAttr.frame;
                    frame.origin.x = CGRectGetMaxX(curAttr.frame) + self.minimumInteritemSpacing;
                    nextAttr.frame = frame;
                }
            }
        }
    }
    return attrsArry;
}
@end