iOS 瀑布流之栅格布局

/** 返回indexPath位置cell对应的布局属性*/
- (CGRect)itemFrameOfHorizontalGridWaterFlow:(NSIndexPath *)indexPath{
    //collectionView的高度
    CGFloat collectionH = self.collectionView.frame.size.height;
    //设置布局属性item的frame
    CGFloat h = [self.delegate waterFlowLayout:self sizeForItemAtIndexPath:indexPath].height;
    CGFloat w = [self.delegate waterFlowLayout:self sizeForItemAtIndexPath:indexPath].width;
    
    CGFloat x = 0;
    CGFloat y = 0;
  
    //找出宽度最短的那一行
    NSInteger destRow = 0;
    CGFloat minRowWidth = [self.rowWidths[destRow] doubleValue];
    for (NSInteger i = 1; i < self.rowWidths.count; i++) {
        //取出第i行
        CGFloat rowWidth = [self.rowWidths[i] doubleValue];
        if (minRowWidth > rowWidth) {
            minRowWidth = rowWidth;
            destRow = i;
        }
    }
    
    y = destRow == 0 ? self.edgeInsets.top : self.edgeInsets.top + h + self.rowMargin;
    
    x = [self.rowWidths[destRow] doubleValue] == self.edgeInsets.left ? self.edgeInsets.left : [self.rowWidths[destRow] doubleValue] + self.columnMargin;
    //更新最短那行的宽度
    if (h >= collectionH - self.edgeInsets.bottom - self.edgeInsets.top) {
        x = [self.rowWidths[destRow] doubleValue] == self.edgeInsets.left ? self.edgeInsets.left : self.maxRowWidth + self.columnMargin;
        for (NSInteger i = 0; i < 2; i++) {
            self.rowWidths[i] = @(x + w);
        }
    }else{
        self.rowWidths[destRow] = @(x + w);
    }
    //记录最大宽度
    if (self.maxRowWidth < x + w) {
        self.maxRowWidth = x + w ;
    }
    return CGRectMake(x, y, w, h);
}