// 每一列的間距
static const CGFloat MKJDefaultColumnMargin = 10;
// 每一行間距
static const CGFloat MKJDefaultRowMargin = 10;
// 整體的上間距，左間距，下間距，右間距
static const UIEdgeInsets MKJDefaultEdgeInsets = {10,10,10,10};
// 默認是多少列
static const NSUInteger MKJDefaultColumnCounts = 2;

@interface MKJWaterFallLayout ()

@property (nonatomic,strong) NSMutableArray *attributeArr; // cell屬性的數組
@property (nonatomic,strong) NSMutableArray *columnHeightArr; // 每列的高度數組

@end

// 每次刷新會調用一次
- (void)prepareLayout
{
  [super prepareLayout];

  // 每次重新刷新的時候清除之前的所有高度值，默認就是UIEdg給定的top
  [self.columnHeightArr removeAllObjects];
  for (NSInteger i = 0; i < [self columnCount]; i++) {
    [self.columnHeightArr addObject:@([self insetMargin].top)];
  }
  // 每次刷新把對應的att屬性清空
  [self.attributeArr removeAllObjects];
  // 初始化一次每個cell對應的attribute屬性
  NSInteger count = [self.collectionView numberOfItemsInSection:0];
  for (NSInteger i = 0; i < count; i++) {
    NSIndexPath *indexpath = [NSIndexPath indexPathForItem:i inSection:0];
    UICollectionViewLayoutAttributes *attribute = [self layoutAttributesForItemAtIndexPath:indexpath];
    [self.attributeArr addObject:attribute];
  }
}

// 返回attribute屬性數組決定最后的排布
- (NSArray<UICollectionViewLayoutAttributes *> *)layoutAttributesForElementsInRect:(CGRect)rect
{
  return self.attributeArr;
}

// 返回對應的indexpath下每個cell的屬性 cell的出現會一直刷新該方法
- (UICollectionViewLayoutAttributes *)layoutAttributesForItemAtIndexPath:(NSIndexPath *)indexPath
{
  // 初始化布局屬性---> 對應的indexpath
  UICollectionViewLayoutAttributes *att = [UICollectionViewLayoutAttributes layoutAttributesForCellWithIndexPath:indexPath];

  CGFloat collectionW = self.collectionView.frame.size.width;

  // 寬度是根據列數和間距固定算出來的
  CGFloat width = (collectionW - [self insetMargin].left - [self insetMargin].right - ([self columnCount] - 1) * [self columnMargin]) / [self columnCount];

  // 高度是根據代理的數據源返回比例計算的
  CGFloat height = [self.delegate MKJWaterFallLayout:self heightForItemAtIndexPath:indexPath] * width;
  // X 和 Y值都是在找出最小column之后才能確定，核心就是根據列數，找出最小高度的那一列
  // 先取出第一個默認是最小的
  CGFloat minColumnHeight = [self.columnHeightArr[0] doubleValue];
  // 默認最小的是第0列
  NSUInteger finalCol = 0;
  for (NSInteger i = 1 ; i < [self columnCount]; i++) {
    CGFloat currentColHeight = [self.columnHeightArr[i] doubleValue];
    if (minColumnHeight > currentColHeight) {
      minColumnHeight = currentColHeight;
      finalCol = i;
    }
  }

  // x，y值是根據最小高度列算出來的
  CGFloat x = [self insetMargin].left + (width + [self columnMargin]) * finalCol;
  CGFloat y = minColumnHeight;
  // 當你是一個行排布的時候 默認是top值，不需要加間距
  NSInteger count = indexPath.item;
  if ((count / ([self columnCount])) >= 1) {
    y += [self rowMargin];
  }
  att.frame = CGRectMake(x, y, width, height);
  self.columnHeightArr[finalCol] = @(CGRectGetMaxY(att.frame));
  return att;

}

// 計算出滾動區域的大小
- (CGSize)collectionViewContentSize
{
  CGFloat maxColumHeight = [self.columnHeightArr[0] doubleValue];
  for (NSInteger i = 1; i < [self columnCount]; i++) {
    CGFloat currentColHeight = [self.columnHeightArr[i] doubleValue];
    if (maxColumHeight < currentColHeight) {
      maxColumHeight = currentColHeight;
    }
  }
  return CGSizeMake(0, maxColumHeight + [self insetMargin].bottom);

}

// 聲明如下
- (UICollectionView *)colletionView
{
  if (_colletionView == nil) {
    MKJWaterFallLayout *layout = [[MKJWaterFallLayout alloc] init];
    layout.delegate = self;
    UICollectionView *collectionV = [[UICollectionView alloc] initWithFrame:[UIScreen mainScreen].bounds collectionViewLayout:layout];
    collectionV.backgroundColor = [UIColor redColor];
    [collectionV registerNib:[UINib nibWithNibName:productID bundle:nil] forCellWithReuseIdentifier:productID];
    collectionV.delegate = self;
    collectionV.dataSource = self;
    _colletionView = collectionV;

  }
  return _colletionView;
}

// 內部行數，間距的控制
#pragma mark - waterfallLayoutDelegate
- (CGFloat)MKJWaterFallLayout:(MKJWaterFallLayout *)layout heightForItemAtIndexPath:(NSIndexPath *)indexPath
{
  // 返回寬度和高度比例
  MKJProductModel *product = self.dataSource[indexPath.item];
  return product.h / product.w;
}

// 控制列間距
- (CGFloat)columnMarginForWaterFallLayout:(MKJWaterFallLayout *)collectionViewLayout
{
  return 10;
}
// 控制行間距
- (CGFloat)rowMarginForWaterFallLayout:(MKJWaterFallLayout *)collectionViewLayout
{
  return 30;
}
// 控制列數
- (NSUInteger)columnCountForWaterFallLayout:(MKJWaterFallLayout *)collectionViewLayout
{
//  if (self.dataSource.count > 50) {
//    return 3;
//  }
  return 3;
}
// 控制整體上左下右間距
- (UIEdgeInsets)insetForWaterFallLayout:(MKJWaterFallLayout *)collectionViewLayout
{
  return UIEdgeInsetsMake(10, 10, 10, 10);
}