#import "WaterfallCollectionLayout.h"
#define colMargin 5
#define colCount 4
#define rolMargin 5
@interface WaterfallCollectionLayout ()
//数组存放每列的总高度
@property(nonatomic,strong)NSMutableArray* colsHeight;
//单元格宽度
@property(nonatomic,assign)CGFloat colWidth;
@end

//完成布局前的初始工作
-(void)prepareLayout;

//collectionView的内容尺寸
-(CGSize)collectionViewContentSize;

//为每个item设置属性
-(UICollectionViewLayoutAttributes *)layoutAttributesForItemAtIndexPath:(NSIndexPath *)indexPath;

//获取制定范围的所有item的属性
-(NSArray<UICollectionViewLayoutAttributes *> *)layoutAttributesForElementsInRect:(CGRect)rect;

-(BOOL)shouldInvalidateLayoutForBoundsChange:(CGRect)newBounds;

//完成布局前的初始工作
-(void)prepareLayout{
  [super prepareLayout];
  self.colWidth =( self.collectionView.frame.size.width - (colCount+1)*colMargin )/colCount;
  //让它重新加载
  self.colsHeight = nil;
}
通过遍历colHeight数组里的所有列来获得最长的那一列，返回contentsize
//collectionView的内容尺寸
-(CGSize)collectionViewContentSize{
  NSNumber * longest = self.colsHeight[0];
  for (NSInteger i =0;i<self.colsHeight.count;i++) {
    NSNumber* rolHeight = self.colsHeight[i];
    if(longest.floatValue<rolHeight.floatValue){
      longest = rolHeight;
    }
  }
  return CGSizeMake(self.collectionView.frame.size.width, longest.floatValue);
}

//为每个item设置属性
-(UICollectionViewLayoutAttributes *)layoutAttributesForItemAtIndexPath:(NSIndexPath *)indexPath{
  UICollectionViewLayoutAttributes* attr = [UICollectionViewLayoutAttributes layoutAttributesForCellWithIndexPath:indexPath];
  NSNumber * shortest = self.colsHeight[0];
  NSInteger shortCol = 0;
  for (NSInteger i =0;i<self.colsHeight.count;i++) {
    NSNumber* rolHeight = self.colsHeight[i];
    if(shortest.floatValue>rolHeight.floatValue){
      shortest = rolHeight;
      shortCol=i;
    }
  }
  CGFloat x = (shortCol+1)*colMargin+ shortCol * self.colWidth;
  CGFloat y = shortest.floatValue+colMargin;
  
  //获取cell高度
  CGFloat height=0;
  NSAssert(self.heightBlock!=nil, @"未实现计算高度的block ");
  if(self.heightBlock){
    height = self.heightBlock(indexPath);
  }
  attr.frame= CGRectMake(x, y, self.colWidth, height);
  self.colsHeight[shortCol]=@(shortest.floatValue+colMargin+height);
  
  return attr;
}

//获取所有item的属性
-(NSArray<UICollectionViewLayoutAttributes *> *)layoutAttributesForElementsInRect:(CGRect)rect{
  NSMutableArray* array = [NSMutableArray array];
  NSInteger items = [self.collectionView numberOfItemsInSection:0];
  for (int i = 0; i<items;i++) {
    UICollectionViewLayoutAttributes* attr = [self layoutAttributesForItemAtIndexPath:[NSIndexPath indexPathForItem:i inSection:0]];
    [array addObject:attr];
  }
  return array;
}

-(BOOL)shouldInvalidateLayoutForBoundsChange:(CGRect)newBounds{
  return YES;
}

-(NSMutableArray *)colsHeight{
  if(!_colsHeight){
    NSMutableArray * array = [NSMutableArray array];
    for(int i =0;i<colCount;i++){
      //这里可以设置初始高度
      [array addObject:@(0)];
    }
    _colsHeight = [array mutableCopy];
  }
  return _colsHeight;
}

#pragma mark getter-setter
-(UICollectionView *)collectionView{
  if(!_collectionView){
    _collectionView = [[UICollectionView alloc]initWithFrame:self.view.frame collectionViewLayout:self.layout];
    _collectionView.backgroundColor = [UIColor whiteColor];
    _collectionView.delegate=self;
    _collectionView.dataSource=self;
    [_collectionView registerClass:[CollectionViewCell class] forCellWithReuseIdentifier:identifer];
  }
  return _collectionView;
}
-(UICollectionViewLayout *)layout{
  if(!_layout){
    _layout = [[WaterfallCollectionLayout alloc]initWithItemsHeightBlock:^CGFloat(NSIndexPath *index) {
      return [self.heightArr[index.item] floatValue];
    }];
    
  }
  return _layout;
}
-(NSArray *)heightArr{
  if(!_heightArr){
    //随机生成高度
    NSMutableArray *arr = [NSMutableArray array];
    for (int i = 0; i<100; i++) {
      [arr addObject:@(arc4random()%50+80)];
    }
    _heightArr = [arr copy];
  }
  return _heightArr;
}