iOS:UICollectionView纯自定义的布局:瀑布流布局

#import <UIKit/UIKit.h>

/*
为了体现封装性的特点,我们可以把一些数据设置为公共的，既可以提高扩展性和通用性，
也便于外界按照自己的需求做必要的调整。
*/

@protocol WaterFlowLayoutDelegate; //设置代理传递数据,降低了与其他类的耦合性,通用性更强


@class WaterFlowLayout;
@interface WaterFlowLayout : UICollectionViewLayout
@property (assign,nonatomic)CGFloat columnMargin;//每一列item之间的间距
@property (assign,nonatomic)CGFloat rowMargin;   //每一行item之间的间距
@property (assign,nonatomic)UIEdgeInsets sectionInset;//设置于collectionView边缘的间距
@property (assign,nonatomic)NSInteger columnCount;//设置每一行排列的个数


@property (weak,nonatomic)id<WaterFlowLayoutDelegate> delegate; //设置代理
@end


@protocol WaterFlowLayoutDelegate
-(CGFloat)waterFlowLayout:(WaterFlowLayout *) WaterFlowLayout heightForWidth:(CGFloat)width andIndexPath:(NSIndexPath *)indexPath;
@end

#import "WaterFlowLayout.h"

//每一列item之间的间距
//static const CGFloat columnMargin = 10;
//每一行item之间的间距
//static const CGFloat rowMargin = 10;

@interface WaterFlowLayout()
/** 这个字典用来存储每一列item的高度 */
@property (strong,nonatomic)NSMutableDictionary *maxYDic;
/** 存放每一个item的布局属性 */
@property (strong,nonatomic)NSMutableArray *attrsArray;
@end

@implementation WaterFlowLayout

/** 懒加载 */
-(NSMutableDictionary *)maxYDic
{
    if (!_maxYDic)
    {
        _maxYDic = [NSMutableDictionary dictionary];
    }
    return _maxYDic;
}

/** 懒加载 */
-(NSMutableArray *)attrsArray
{
    if (!_attrsArray)
    {
        _attrsArray = [NSMutableArray array];
    }
    return _attrsArray;
}

//初始化
-(instancetype)init
{
    if (self = [super init]){
        self.columnMargin = 10;
        self.rowMargin = 10;
        self.sectionInset = UIEdgeInsetsMake(10, 10, 10, 10);
        self.columnCount = 3;
    }
    return self;
}

//每一次布局前的准备工作
-(void)prepareLayout
{
    [super prepareLayout];
    
    //清空最大的y值
    for (int i =0; i < self.columnCount; i++)
    {
        NSString *column = [NSString stringWithFormat:@"%d",i];
        self.maxYDic[column] = @(self.sectionInset.top);
    }

    //计算所有item的属性
    [self.attrsArray removeAllObjects];
    NSInteger count = [self.collectionView numberOfItemsInSection:0];
    for (int i=0; i<count; i++)
    {
        UICollectionViewLayoutAttributes *attrs = [self layoutAttributesForItemAtIndexPath:[NSIndexPath indexPathForItem:i inSection:0]];
        
        [self.attrsArray addObject:attrs];
    }
}

//设置collectionView滚动区域
-(CGSize)collectionViewContentSize
{
    //假设最长的那一列为第0列
    __block NSString *maxColumn = @"0";
    
    //遍历字典,找出最长的那一列
    [self.maxYDic enumerateKeysAndObjectsUsingBlock:^(NSString *column, NSNumber *maxY, BOOL *stop) {
        
        if ([maxY floatValue] > [self.maxYDic[maxColumn] floatValue])
        {
            maxColumn = column;
        }
    }];
    return CGSizeMake(0, [self.maxYDic[maxColumn]floatValue]+self.sectionInset.bottom);
}

//允许每一次重新布局
-(BOOL)shouldInvalidateLayoutForBoundsChange:(CGRect)newBounds
{
    return YES;
}

//布局每一个属性
-(UICollectionViewLayoutAttributes *)layoutAttributesForItemAtIndexPath:(NSIndexPath *)indexPath
{
    //假设最短的那一列为第0列
    __block NSString *minColumn = @"0";
    
    //遍历字典,找出最短的那一列
    [self.maxYDic enumerateKeysAndObjectsUsingBlock:^(NSString *column, NSNumber *maxY, BOOL *stop) {
        
        if ([maxY floatValue] < [self.maxYDic[minColumn] floatValue])
        {
            minColumn = column;
        }
    }];
    
    //计算每一个item的宽度和高度
    CGFloat width = (self.collectionView.frame.size.width - self.columnMargin*(self.columnCount - 1) - self.sectionInset.left - self.sectionInset.right) / self.columnCount;
    
    CGFloat height = [self.delegate waterFlowLayout:self heightForWidth:width andIndexPath:indexPath] ;
    
    
    //计算每一个item的位置
    CGFloat x = self.sectionInset.left + (width + self.columnMargin) * [minColumn floatValue];
    CGFloat y = [self.maxYDic[minColumn] floatValue] + self.rowMargin;
    
    
    //更新这一列的y值
    self.maxYDic[minColumn] = @(y + height);
    
    
    //创建布局属性
    UICollectionViewLayoutAttributes *attrs = [UICollectionViewLayoutAttributes layoutAttributesForCellWithIndexPath:indexPath];
    
    //设置item的frame
    attrs.frame = CGRectMake(x, y, width, height);
    
    return attrs;
}

//布局所有item的属性,包括header、footer
-(NSArray *)layoutAttributesForElementsInRect:(CGRect)rect
{
    return self.attrsArray;
}
@end