1）- (void)prepareLayout
2）- (CGSize)collectionViewContentSize
3）- (UICollectionViewLayoutAttributes *)layoutAttributesForItemAtIndexPath:(NSIndexPath *)indexPath
4）- (NSArray *)layoutAttributesForElementsInRect:(CGRect)rect

#import <UIKit/UIKit.h>@class JQWaterflowLayout;@protocol JQWaterflowLayoutDelegate <NSObject>
@required
- (CGFloat)waterflowLayout:(JQWaterflowLayout *)waterflowLayout heightForItemAtIndex:(NSUInteger)index itemWidth:(CGFloat)itemWidth;@optional
- (CGFloat)columnCountInWaterflowLayout:(JQWaterflowLayout *)waterflowLayout;
- (CGFloat)columnMarginInWaterflowLayout:(JQWaterflowLayout *)waterflowLayout;
- (CGFloat)rowMarginInWaterflowLayout:(JQWaterflowLayout *)waterflowLayout;
- (UIEdgeInsets)edgeInsetsInWaterflowLayout:(JQWaterflowLayout *)waterflowLayout;
@end@interface JQWaterflowLayout : UICollectionViewLayout
/** 代理 */
@property (nonatomic, weak) id<JQWaterflowLayoutDelegate> delegate;
@end

#import "XMGWaterflowLayout.h"/** 默认的列数 */
static const NSInteger JQDefaultColumnCount = 3;
/** 每一列之间的间距 */
static const CGFloat JQDefaultColumnMargin = 10;
/** 每一行之间的间距 */
static const CGFloat JQDefaultRowMargin = 10;
/** 边缘间距 */
static const UIEdgeInsets JQDefaultEdgeInsets = {10, 10, 10, 10};@interface JQWaterflowLayout()
/** 存放所有cell的布局属性 */
@property (nonatomic, strong) NSMutableArray *attrsArray;
/** 存放所有列的当前高度 */
@property (nonatomic, strong) NSMutableArray *columnHeights;
/** 内容的高度 */
@property (nonatomic, assign) CGFloat contentHeight;- (CGFloat)rowMargin;
- (CGFloat)columnMargin;
- (NSInteger)columnCount;
- (UIEdgeInsets)edgeInsets;
@end@implementation JQWaterflowLayout#pragma mark - 常见数据处理
- (CGFloat)rowMargin
{if ([self.delegate respondsToSelector:@selector(rowMarginInWaterflowLayout:)]) {return [self.delegate rowMarginInWaterflowLayout:self];} else {return JQDefaultRowMargin;}
}- (CGFloat)columnMargin
{if ([self.delegate respondsToSelector:@selector(columnMarginInWaterflowLayout:)]) {return [self.delegate columnMarginInWaterflowLayout:self];} else {return JQDefaultColumnMargin;}
}- (NSInteger)columnCount
{if ([self.delegate respondsToSelector:@selector(columnCountInWaterflowLayout:)]) {return [self.delegate columnCountInWaterflowLayout:self];} else {return JQDefaultColumnCount;}
}- (UIEdgeInsets)edgeInsets
{if ([self.delegate respondsToSelector:@selector(edgeInsetsInWaterflowLayout:)]) {return [self.delegate edgeInsetsInWaterflowLayout:self];} else {return JQDefaultEdgeInsets;}
}#pragma mark - 懒加载
- (NSMutableArray *)columnHeights
{if (!_columnHeights) {_columnHeights = [NSMutableArray array];}return _columnHeights;
}- (NSMutableArray *)attrsArray
{if (!_attrsArray) {_attrsArray = [NSMutableArray array];}return _attrsArray;
}/*** 初始化*/
- (void)prepareLayout
{[super prepareLayout];self.contentHeight = 0;// 清除以前计算的所有高度
    [self.columnHeights removeAllObjects];for (NSInteger i = 0; i < self.columnCount; i++) {[self.columnHeights addObject:@(self.edgeInsets.top)];}// 清除之前所有的布局属性
    [self.attrsArray removeAllObjects];// 开始创建每一个cell对应的布局属性NSInteger count = [self.collectionView numberOfItemsInSection:0];for (NSInteger i = 0; i < count; i++) {// 创建位置NSIndexPath *indexPath = [NSIndexPath indexPathForItem:i inSection:0];// 获取indexPath位置cell对应的布局属性UICollectionViewLayoutAttributes *attrs = [self layoutAttributesForItemAtIndexPath:indexPath];[self.attrsArray addObject:attrs];}
}/*** 决定cell的排布*/
- (NSArray *)layoutAttributesForElementsInRect:(CGRect)rect
{return self.attrsArray;
}/*** 返回indexPath位置cell对应的布局属性*/
- (UICollectionViewLayoutAttributes *)layoutAttributesForItemAtIndexPath:(NSIndexPath *)indexPath
{// 创建布局属性UICollectionViewLayoutAttributes *attrs = [UICollectionViewLayoutAttributes layoutAttributesForCellWithIndexPath:indexPath];// collectionView的宽度CGFloat collectionViewW = self.collectionView.frame.size.width;// 设置布局属性的frameCGFloat w = (collectionViewW - self.edgeInsets.left - self.edgeInsets.right - (self.columnCount - 1) * self.columnMargin) / self.columnCount;CGFloat h = [self.delegate waterflowLayout:self heightForItemAtIndex:indexPath.item itemWidth:w];// 找出高度最短的那一列NSInteger destColumn = 0;CGFloat minColumnHeight = [self.columnHeights[0] doubleValue];for (NSInteger i = 1; i < self.columnCount; i++) {// 取得第i列的高度CGFloat columnHeight = [self.columnHeights[i] doubleValue];if (minColumnHeight > columnHeight) {minColumnHeight = columnHeight;destColumn = i;}}CGFloat x = self.edgeInsets.left + destColumn * (w + self.columnMargin);CGFloat y = minColumnHeight;if (y != self.edgeInsets.top) {y += self.rowMargin;}attrs.frame = CGRectMake(x, y, w, h);// 更新最短那列的高度self.columnHeights[destColumn] = @(CGRectGetMaxY(attrs.frame));// 记录内容的高度CGFloat columnHeight = [self.columnHeights[destColumn] doubleValue];if (self.contentHeight < columnHeight) {self.contentHeight = columnHeight;}return attrs;
}- (CGSize)collectionViewContentSize
{
//    CGFloat maxColumnHeight = [self.columnHeights[0] doubleValue];
//    for (NSInteger i = 1; i < self.columnCount; i++) {
//        // 取得第i列的高度
//        CGFloat columnHeight = [self.columnHeights[i] doubleValue];
//
//        if (maxColumnHeight < columnHeight) {
//            maxColumnHeight = columnHeight;
//        }
//    }return CGSizeMake(0, self.contentHeight + self.edgeInsets.bottom);
}@end