IOS简单实现瀑布流UICollectionView

#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;
}