//聲明LYPWaterFlowLayout為一個類
@class LYPWaterFlowLayout;
@protocol LYPWaterFlowLayoutDelegate <NSObject>
//必須實現(xiàn)的方法
@required
/**獲取瀑布流每個元素的高度*/
- (CGFloat)waterFlowLayout:(LYPWaterFlowLayout *)waterFlowLayout heightForItemAtIndex:(NSInteger)index itemWith:(CGFloat)itemWith;
//可選實現(xiàn)的方法
@optional
/**獲取瀑布流的列數(shù)*/
- (NSInteger)columnCountInWaterFlowLayout:(LYPWaterFlowLayout *)waterFlowLayout;
/**獲取瀑布流列間距*/
- (CGFloat)columnMarginInWaterFlowLayout:(LYPWaterFlowLayout *)waterFlowLayout;
/**獲取瀑布流的行間距*/
- (CGFloat)rowMarginInWaterFlowLayout:(LYPWaterFlowLayout *)waterFlowLayout;
/**獲取瀑布流的內邊距*/
- (UIEdgeInsets)edgeInsetsInWaterFlowLayout:(LYPWaterFlowLayout *)waterFlowLayout;
@end

@interface LYPWaterFlowLayout : UICollectionViewLayout
/**代理*/
@property (nonatomic, weak) id<LYPWaterFlowLayoutDelegate> delegate;
@end

/**默認的列數(shù)*/
static const NSInteger LYPDefaultColumnCount = 3;
/**默認每一列之間的間距*/
static const CGFloat LYPDefaultColumMargin = 10;
/**默認每一行之間的間距*/
static const CGFloat LYPDefaultRowMargin = 10;
/**默認邊緣間距*/
static const UIEdgeInsets LYPDefaultEdgeInsets = {10, 10, 10, 10};

- (NSInteger)columnCount
{
  //判斷代理是否實現(xiàn)了獲取列數(shù)的可選方法
  if ([self.delegate respondsToSelector:@selector(columnCountInWaterFlowLayout:)])
  {
    //實現(xiàn)，返回通過代理設置的列數(shù)
    return [self.delegate columnCountInWaterFlowLayout:self];
  }
  else
  {
    //為實現(xiàn)，返回默認的列數(shù)
    return LYPDefaultColumnCount;
  }
}

/**所有cell的布局屬性*/
@property (nonatomic, strong) NSMutableArray *attrsArray;
/**所有列的當前高度*/
@property (nonatomic, strong) NSMutableArray *columnHeights;

/**--attrsArray--懶加載*/
- (NSMutableArray *)attrsArray
{
  if (_attrsArray == nil)
  {
    _attrsArray = [NSMutableArray array];
  }
  return _attrsArray;
}
/**--columnHeights--懶加載*/
- (NSMutableArray *)columnHeights
{
  if (_columnHeights == nil)
  {
    _columnHeights = [NSMutableArray array];
  }
  return _columnHeights;
}

- (void)prepareLayout
{
  [super prepareLayout];

  /**清除之前跟布局相關的所有屬性，重新設置新的布局*/
  //清除之前計算的所有列的高度
  [self.columnHeights removeAllObjects];
  //設置所有列的初始高度
  for (NSInteger i = 0; i<self.columnCount; i++)
  {
    self.columnHeights[i] = @(self.edgeInsets.top);
  }
  //清除之前所有的布局屬性
  [self.attrsArray removeAllObjects];

  /**開始創(chuàng)建每一個cell對應的布局屬性*/
  NSInteger count = [self.collectionView numberOfItemsInSection:0];
  for (NSInteger i = 0; i<count; i++)
  {
    NSIndexPath *indexPath = [NSIndexPath indexPathForItem:i inSection:0];
    //獲取indexPath位置cell對應的布局屬性
    UICollectionViewLayoutAttributes *attrs = [self layoutAttributesForItemAtIndexPath:indexPath];
    //將indexPath位置的cell的布局屬性添加到所有cell的布局屬性數(shù)組中
    [self.attrsArray addObject:attrs];
  }
}

- (nullable NSArray<UICollectionViewLayoutAttributes *> *)layoutAttributesForElementsInRect:(CGRect)rect
{
  return self.attrsArray;
}
設置每一個cell的布局屬性

- (nullable UICollectionViewLayoutAttributes *)layoutAttributesForItemAtIndexPath:(nonnull NSIndexPath *)indexPath
{
  //獲取indexPath位置的布局屬性
  UICollectionViewLayoutAttributes *attrs = [UICollectionViewLayoutAttributes layoutAttributesForCellWithIndexPath:indexPath];

  /**設置cell布局屬性的frame*/

  /***確定cell的尺寸***/
  //獲取collectionView的寬度
  CGFloat collectionViewWidth = self.collectionView.frame.size.width;
  //cell寬度
  CGFloat width = ((collectionViewWidth - self.edgeInsets.left - self.edgeInsets.right - (self.columnCount - 1) * self.columMargin)) / self.columnCount;
  //cell高度
  CGFloat height = [self.delegate waterFlowLayout:self heightForItemAtIndex:indexPath.item itemWith:width];

  /***設置cell的位置***/
  NSInteger destColumn = 0;
  CGFloat minColumnHeight = [self.columnHeights[0] doubleValue];
  for (NSInteger i = 1; i<self.columnCount; i++)
  {
    CGFloat columnHeight = [self.columnHeights[i] doubleValue];
    if (minColumnHeight > columnHeight)
    {
      minColumnHeight = columnHeight;
      destColumn = i;
    }
  }
  //計算cell的位置
  CGFloat x = self.edgeInsets.left + destColumn * (width + self.columMargin);
  CGFloat y = minColumnHeight;
  //判斷是不是第一行
  if (y != self.edgeInsets.top)
  {
    //若不是第一行，需要加上行間距
    y += self.rowMargin;
  }

  /**給cell的布局屬性的frame賦值*/
  attrs.frame = CGRectMake(x, y, width, height);

  //更新最短那列的高度
  self.columnHeights[destColumn] = @(CGRectGetMaxY(attrs.frame));

  /**返回indexPath位置的cell的布局屬性*/
  return attrs;
}

- (CGSize)collectionViewContentSize
{
  //獲取最高的那一列的高度
  CGFloat maxColumnHeight = [self.columnHeights[0] doubleValue];
  for (NSInteger i = 1; i<self.columnCount; i++)
  {
    CGFloat columnHeight = [self.columnHeights[i] doubleValue];
    if (maxColumnHeight < columnHeight)
    {
      maxColumnHeight = columnHeight;
    }
  }
  //返回collectionView的contentSize，高度為最高的高度加上一個行間距
  return CGSizeMake(0, maxColumnHeight + self.rowMargin);
}