CustomLayout.h

#import <UIKit/UIKit.h>

@protocol CustomLayoutDelegate <NSObject>

- (CGFloat)collectionViewLayout:(UICollectionViewLayout*)layout andIndexPath:(NSIndexPath*)indexPath;
@end
@interface CustomLayout : UICollectionViewLayout
@property (nonatomic,assign)CGFloat itemWidth;
@property (nonatomic,assign)CGFloat itemSpace;
@property (nonatomic,weak)id<CustomLayoutDelegate>delegate;
@end

CustomLayout.m

#import "CustomLayout.h"

@interface CustomLayout ()
{

    NSMutableArray *_attrs;
    NSMutableArray *_infos;
}
@end

@implementation CustomLayout

- (instancetype)init{
    self = [super init];
    if (self) {
        _attrs = [NSMutableArray array];
        //给每一列第一个的上一个的y;
        _infos = @[@0,@0,@0].mutableCopy;
    }
    return self;
}
//给attributes数组赋值
- (void)prepareLayout{

    [super prepareLayout];
    //有多个Section
    NSInteger sections = [self.collectionView numberOfSections];
    for (NSInteger se = 0; se<sections; se++) {
        NSInteger itC = [self.collectionView numberOfItemsInSection:se];       
        for (NSInteger j = 0; j<itC; j++) {
        }
    }
    
    //只有一个Sections
    NSInteger itemCounts = [self.collectionView numberOfItemsInSection:0];
   //算出列数
    NSInteger columCount  = (self.collectionView.bounds.size.width - self.itemSpace)/(self.itemSpace + self.itemWidth);
   //算出每个item之间的间距
    self.itemSpace = (self.collectionView.bounds.size.width - columCount * self.itemWidth)/(columCount + 1);
    
    for (NSInteger i = 0; i<itemCounts; i++) {
        //算出item处于哪一列
        NSInteger columIndex = i % columCount;
       
        NSIndexPath *indexPath = [NSIndexPath indexPathForRow:i inSection:0];
        //根据下标找到item的高度
        CGFloat itemHeight =  [self.delegate collectionViewLayout:self andIndexPath:indexPath];
        //创建attributes数组
        UICollectionViewLayoutAttributes *attr = [UICollectionViewLayoutAttributes layoutAttributesForCellWithIndexPath:indexPath];
        //计算出x坐标
        CGFloat p_x = (columIndex + 1)*self.itemSpace + columIndex*self.itemWidth;
        //根据数组拿出上一个点高的y
        NSNumber *lastP = _infos[columIndex];
        CGFloat lastPValue = lastP.floatValue;
        //计算出y坐标
        CGFloat p_y = lastPValue + self.itemSpace;
        
#if 0
        if(i == 10){
            attr.frame = CGRectMake(p_x, p_y, self.itemWidth+100, itemHeight);
        }else{
            attr.frame = CGRectMake(p_x, p_y, self.itemWidth, itemHeight);
        }
#endif
        
        attr.frame = CGRectMake(p_x, p_y, self.itemWidth, itemHeight);
        //将自己高的y放到数组中去
        _infos[columIndex] = @(p_y+itemHeight);
        
        //放入数组
        [_attrs addObject:attr];
    }   
}
//返回数组attributes
- (NSArray<UICollectionViewLayoutAttributes *> *)layoutAttributesForElementsInRect:(CGRect)rect{
    //返回数组
    return _attrs;
}
//使collectionView可以滑动
- (CGSize)collectionViewContentSize{

    //找出数组中的最大值
    NSArray *tempArray = [_infos sortedArrayUsingSelector:@selector(compare:)];
    //上面函数所返回的数组是升序的
    NSNumber *maxNum = tempArray.lastObject;
    //返回整个collectionView的宽高
    return CGSizeMake(self.collectionView.bounds.size.width, maxNum.floatValue);
}
@end