自定义UICollectionViewFlowLayout布局
代码直接奉上：

----h文件---

import <UIKit/UIKit.h>

@interface VerticalCollectionAnyCenterLayout : UICollectionViewFlowLayout
//传入一个item'的大小跟、行距离、自动计算布局等、
-(instancetype)initWithItemSize:(CGSize)size lineSpace:(CGFloat)margin_line;
@end
----m文件---

import "VerticalCollectionAnyCenterLayout.h"

@interface VerticalCollectionAnyCenterLayout()
@property(nonatomic,strong)NSMutableArray * attrsArr;
@property (nonatomic,assign)NSInteger rows_line;//需要手动计算
@property (nonatomic,assign)CGFloat margin_inter;//需要手动计算item之间的距离
@property (nonatomic,assign)CGFloat margin_line;
@property (nonatomic,assign)CGSize size_item;

@end
@implementation VerticalCollectionAnyCenterLayout

-(instancetype)initWithItemSize:(CGSize)size lineSpace:(CGFloat)margin_line{
if (self = [super init]){
self.margin_line = margin_line;
self.size_item = size;
}
return self;
}
-(void)caculation{
[self.collectionView setNeedsLayout];
CGSize size_collection = self.collectionView.bounds.size;

self.rows_line = (NSInteger)size_collection.width / self.size_item.width;
self.margin_inter  = ((CGFloat)(size_collection.width - self.rows_line * self.size_item.width)) / ((CGFloat)(self.rows_line + 1));

self.itemSize = self.size_item;
self.minimumLineSpacing =  self.margin_line;
self.minimumInteritemSpacing = self.margin_inter;

}

pragma mark ---- 懒加载

-(NSMutableArray *)attrsArr
{
if(!_attrsArr){
_attrsArr=[[NSMutableArray alloc] init];
}
return _attrsArr;
}

-(void)prepareLayout
{
[super prepareLayout];
[self caculation];

    [self.attrsArr removeAllObjects];
    [self creatAttrs];

}
-(void)creatAttrs{
//计算出每组有多少个
NSInteger count=[self.collectionView numberOfItemsInSection:0];
/**
* 因为不是继承流水布局 UICollectionViewFlowLayout
* 所以我们需要自己创建 UICollectionViewLayoutAttributes
*/
//如果是多组的话 需要2层循环
for (int i=0; i<count; i++) {
//创建UICollectionViewLayoutAttributes
NSIndexPath * indexPath = [NSIndexPath indexPathForItem:i inSection:0];
//这里需要 告诉 UICollectionViewLayoutAttributes 是哪里的attrs
UICollectionViewLayoutAttributes * attrs=[self layoutAttributesForItemAtIndexPath:indexPath];
[self.attrsArr addObject:attrs];
}
}

-(NSArray<UICollectionViewLayoutAttributes *> *)layoutAttributesForElementsInRect:(CGRect)rect
{
// 特别注意 在这个方法里 可以边滑动边刷新（添加） attrs 一劳永逸 如果只需要添加一次的话 可以把这些 prepareLayout方法中去
return self.attrsArr;
}

-(BOOL)shouldInvalidateLayoutForBoundsChange:(CGRect)newBounds
{
return YES;

}
-(BOOL)shouldInvalidateLayoutForBoundsChange{
return YES;
}

pragma mark ---- 这个方法需要返回indexPath位置对应cell的布局属性

/**
 *  //TODO:  这个方法主要用于 切换布局的时候 如果不适用该方法 就不会切换布局的时候会报错
 *   reason: 'no UICollectionViewLayoutAttributes instance for -layoutAttributesForItemAtIndexPath: <NSIndexPath: 0xc000000000400016> {length = 2, path = 0 - 2}'
 */

-(UICollectionViewLayoutAttributes *)layoutAttributesForItemAtIndexPath:(NSIndexPath *)indexPath{
// 主要是返回每个indexPath的attrs

    //创建UICollectionViewLayoutAttributes
    //这里需要 告诉 UICollectionViewLayoutAttributes 是哪里的attrs
    //计算出每组有多少个
    NSInteger section = indexPath.section;
    NSInteger count=[self.collectionView numberOfItemsInSection:section];


    UICollectionViewLayoutAttributes * attrs=[UICollectionViewLayoutAttributes layoutAttributesForCellWithIndexPath:indexPath];
    NSInteger yushu =  count % self.rows_line;
NSInteger zhengshu  = count / self.rows_line;
NSInteger start = zhengshu * self.rows_line;
if (indexPath.row >= start){
    //默认单行每个item都是等宽度
    CGFloat left_right_margin  = (self.collectionView.bounds.size.width  -
                                  self.size_item.width * yushu - (yushu - 1) * self.margin_inter) /2;
    NSInteger index_last_line =indexPath.row - start;
    CGRect frame  = attrs.frame;
    attrs.size = self.size_item;
    frame.size =  self.size_item;
    frame.origin.x = left_right_margin + (self.margin_inter + self.size_item.width)  * index_last_line;
    frame.origin.y =  (self.margin_line + self.size_item.height) * (indexPath.row / self.rows_line)+ self.margin_line;
    attrs.frame = frame;
    
}
else
{
    CGRect frame  = attrs.frame;
    frame.size = self.size_item;
    attrs.size = self.size_item;
    frame.origin.x = (self.margin_inter + self.size_item.width) * (indexPath.row % self.rows_line) + self.margin_inter;
    frame.origin.y =  (self.margin_line + self.size_item.height) * (indexPath.row / self.rows_line) + self.margin_line;
    attrs.frame = frame;
    
}
if (indexPath.row == (count - 1)){///最后一个、计算contenSize
    CGSize contentSize = self.collectionView.contentSize;
    CGRect frame_last = attrs.frame;
    contentSize.height = CGRectGetMaxY(frame_last);
   self.collectionView.contentSize = contentSize;

}
    return attrs;

}

@end

QQ20180810-0.png

使用方法直接给collectionView 一个layout即可、不用在代理中返回偏移量和itemSize