ios基于UICollectionView实现横向瀑布流

WaterfallFlowLayout为布局类，继承与NSObject。.m文件入下

#import "WaterfallFlowLayout.h"

@interface WaterfallFlowLayout ()
{
  // 用于记录每一列布局到的宽度
  NSMutableArray * _widthOfColumns;
  // 用于保存所有item的属性 (frame)
  NSMutableArray * _itemsAttributes;
}

@end


@implementation WaterfallFlowLayout
- (void) setNumberOfColumns:(NSInteger)numberOfColumns {
  if (_numberOfColumns != numberOfColumns) {
    _numberOfColumns = numberOfColumns;
    // 让原有布局失效，需要重新布局
    [self invalidateLayout];
  }
}

- (void)setMinimumLineSpacing:(CGFloat)minimumLineSpacing {
  if (_minimumLineSpacing != minimumLineSpacing) {
    _minimumLineSpacing = minimumLineSpacing;
    [self invalidateLayout];
  }
}
- (void)setMinimumInteritemSpacing:(CGFloat)minimumInteritemSpacing {
  if (_minimumInteritemSpacing != minimumInteritemSpacing) {
    _minimumInteritemSpacing = minimumInteritemSpacing;
    [self invalidateLayout];
  }
}

- (void)setSectionInset:(UIEdgeInsets)sectionInset {
  if (!UIEdgeInsetsEqualToEdgeInsets(_sectionInset, sectionInset)) {
    _sectionInset = sectionInset;
    [self invalidateLayout];
  }
}

//重写方法 1: 准备布局
-(void)prepareLayout {
  [super prepareLayout];
  // 真正的布局在这里完成
  if (_itemsAttributes) {
    [_itemsAttributes removeAllObjects];
  }else {
    _itemsAttributes = [[NSMutableArray alloc] init];
  }
  if (_widthOfColumns) {
    [_widthOfColumns removeAllObjects];
  }else {
    _widthOfColumns = [[NSMutableArray alloc] init];
  }
  for (NSInteger i = 0; i < self.numberOfColumns; i++) {
    // 初始化每一列的宽度(默认为上边距)
//    _heightOfColumns[i] = @(self.sectionInset.top);
    [_widthOfColumns addObject:@(self.sectionInset.left)];
  }
  // item的总数
  NSInteger count = [self.collectionView numberOfItemsInSection:0];

//  CGFloat itemWidth = (self.collectionView.frame.size.width - self.sectionInset.left - self.sectionInset.right - (_numberOfColumns-1) * _minimumInteritemSpacing )/_numberOfColumns;

  // 总的高度 (集合视图的宽度)
  CGFloat totalHeight = self.collectionView.frame.size.height;
  // 有效的高度 (出去间隔及边界)
  CGFloat validHeight = totalHeight - self.sectionInset.top - self.self.sectionInset.bottom - (self.numberOfColumns-1) * self.minimumInteritemSpacing;
  // 每一个item的高度
  CGFloat itemHeight = validHeight/self.numberOfColumns;


  // 设置item的默认宽度
  CGFloat itemWidth = itemHeight;
  for (NSInteger i = 0; i<count; i++) {
    // 最短列的下标
    NSInteger index = [self indexOfShortestColumn];
    CGFloat originY = self.sectionInset.top + index * (itemHeight +self.minimumInteritemSpacing);
    CGFloat originX = [_widthOfColumns[index] floatValue];
    // 构造 indexPath
    NSIndexPath * indexPath = [NSIndexPath indexPathForItem:i inSection:0];
    // 动态的获取宽度
    if ([self.delegate respondsToSelector:@selector(WaterfallFlowLayout:widthForItemAtIndexPath:)]) {
      itemWidth = [self.delegate WaterfallFlowLayout:self widthForItemAtIndexPath:indexPath];
    }
    UICollectionViewLayoutAttributes * attr = [UICollectionViewLayoutAttributes layoutAttributesForCellWithIndexPath:indexPath];

    attr.frame = CGRectMake(originX, originY, itemWidth, itemHeight);
    // 保存 item 的属性 到数组中
    [_itemsAttributes addObject:attr];
    // 更新布局到的一列(最短列) 的高度
    _widthOfColumns[index] = @(originX + itemWidth + self.minimumLineSpacing);
  }
  // 刷新显示
  [self.collectionView reloadData];
}


//重写方法 2: 返回指定区域的item的属性(frame)
- (NSArray<UICollectionViewLayoutAttributes *> *)layoutAttributesForElementsInRect:(CGRect)rect {
  NSMutableArray * array = [NSMutableArray array];

  for (UICollectionViewLayoutAttributes * attr in _itemsAttributes) {
    // 判断两个矩形是否有交集
    if (CGRectIntersectsRect(attr.frame, rect)) {
      [array addObject:attr];
    }
  }
  return array;
}


//重写方法 3: 返回内容的尺寸
-(CGSize)collectionViewContentSize {
  CGFloat height = self.collectionView.frame.size.height;
  NSInteger index = [self indexOfLongestColumn];
  CGFloat width = [_widthOfColumns[index] floatValue] + self.sectionInset.right - self.minimumLineSpacing;
  return CGSizeMake(width, height);
}


- (NSInteger) indexOfLongestColumn {
  NSInteger index = 0;
  for (NSInteger i = 0; i<_numberOfColumns; i++) {
    if ([_widthOfColumns[i] floatValue] > [_widthOfColumns[index] floatValue]) {
      index = i;
    }
  }

  return index;
}

- (NSInteger) indexOfShortestColumn {
  NSInteger index = 0;
  for (NSInteger i = 0; i<_numberOfColumns; i++) {
    if ([_widthOfColumns[i] floatValue] < [_widthOfColumns[index] floatValue]) {
      index = i;
    }
  }

  return index;
}
@end								 
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