1.创建一种布局

   UICollectionViewFlowLayout *flowL = [[UICollectionViewFlowLayout alloc]init];
   //设置每一个item的大小
   flowL.itemSize = CGSizeMake((view2.frame.size.width - 15-20) / 3 , (view2.frame.size.width - 15-20) /3 );
   flowL.sectionInset = UIEdgeInsetsMake(3, 0, 3, 0);
   //列
   flowL.minimumInteritemSpacing = 2;
   //行最小间距，低于该值就会另起一行
   flowL.minimumLineSpacing = 2;

2.创建集合视图

    self.collectionView = [[UICollectionView alloc]initWithFrame:CGRectMake(10, 32, ScreenWidth-40,100) collectionViewLayout:flowL];
    self.collectionView.backgroundColor = [UIColor whiteColor];
    self.collectionView.scrollEnabled = NO;
    self.collectionView.delegate = self;
    self.collectionView.dataSource = self;
    
    //注册对应的cell
    [self.collectionView registerClass:[PhotoCollectionViewCell class] forCellWithReuseIdentifier:@"cell"];

3.UICollectionViewDelegate,UICollectionViewDataSource协议

- (NSInteger)collectionView:(UICollectionView *)collectionView numberOfItemsInSection:(NSInteger)section {
    if (_photoArrayM.count == 0) {
        return 0;
    }
    else{
        return _photoArrayM.count;
    }
}
//返回每一个cell
- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath{
    PhotoCollectionViewCell *cell = [collectionView dequeueReusableCellWithReuseIdentifier:@"cell" forIndexPath:indexPath];
    
    MLSelectPhotoAssets *asset = self.photoArrayM[indexPath.item];
    cell.photoView.image = [MLSelectPhotoPickerViewController getImageWithImageObj:asset];
    return cell;
}
- (void)collectionView:(UICollectionView *)collectionView didSelectItemAtIndexPath:(NSIndexPath *)indexPath {
    SelectedPictureViewController *sVC = [[SelectedPictureViewController alloc]init];
    [sVC returnControl:^(BOOL deleteImage) {
        if (deleteImage) {
            [self.photoArrayM removeObjectAtIndex:indexPath.row];
        }
    }];
    sVC.asset = self.photoArrayM[indexPath.row];
    //NSLog(@"%ld",(long)indexPath.item);
    [self.navigationController pushViewController:sVC animated:YES];
    
//    MLSelectPhotoBrowserViewController *browserVC = [[MLSelectPhotoBrowserViewController alloc]init];
//    browserVC.currentPage = indexPath.item;
//    browserVC.photos = self.photoArrayM;
//    //browserVC.isEditing  = YES;
//    [self.navigationController pushViewController:browserVC animated:YES];

}

4. UICollectionViewDelegateFlowLayout协议。

//设定指定Cell的尺寸
- (CGSize)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout*)collectionViewLayout sizeForItemAtIndexPath:(NSIndexPath *)indexPath
{
    return (CGSize){ScreenWidth/3,ScreenWidth/3+30};
}

- (UIEdgeInsets)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout*)collectionViewLayout insetForSectionAtIndex:(NSInteger)section
{
    return UIEdgeInsetsMake(0, 0, 0, 0);
}

//设定指定区内Cell的最小行距
- (CGFloat)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout*)collectionViewLayout minimumLineSpacingForSectionAtIndex:(NSInteger)section
{
    return 1;
}

//设定指定区内Cell的最小间距
- (CGFloat)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout*)collectionViewLayout minimumInteritemSpacingForSectionAtIndex:(NSInteger)section
{
    return 0;
}