AssetsLibrary 实现访问相册,选取多张照片显示

 #import "ViewController.h"
 #import <AssetsLibrary/AssetsLibrary.h>

 /**
  *  ALAssetsLibrary.h 代表资源库(所有的视频,照片)
     ALAssetsGroup.h   代表资源库中的相册
     ALAsset.h         代表相册中一个视频或者一张照片
     ALAssetRepresentation.h 代表一个资源的描述,可以获取到原始图片
  */

 @interface ViewController ()

 @property (weak, nonatomic) IBOutlet UICollectionView *collectionView;

 @property (nonatomic,strong)NSMutableArray *array;
 @end

 @implementation ViewController{

     ALAssetsLibrary *library;

 }

 - (void)viewDidLoad {
     [super viewDidLoad];

     //创建可变数组,存储资源文件
     _array = [NSMutableArray array];

     //创建资源库,用于访问相册资源
     library = [[ALAssetsLibrary alloc] init];

     //遍历资源库中所有的相册,有多少个相册,usingBlock会调用多少次
     [library enumerateGroupsWithTypes:ALAssetsGroupSavedPhotos usingBlock:^(ALAssetsGroup *group, BOOL *stop) {

         //如果存在相册,再遍历
         if (group) {

             //遍历相册中所有的资源(照片,视频)
            [group enumerateAssetsUsingBlock:^(ALAsset *result, NSUInteger index, BOOL *stop) {

                /*
                 资源的索引
                if (index == 2) {

                 //停止遍历
                    *stop = YES;
                }
                 */

                if (result) {
                    //将资源存储到数组中
                    [_array addObject:result];
                }

            }];
         }

         //刷新_collectionView reloadData;
          [_collectionView reloadData];

     } failureBlock:^(NSError *error) {

         NSLog(@"访问失败");
     }];

 }

 #pragma mark -UICollectionViewDelegate

 //行的个数
 - (NSInteger)collectionView:(UICollectionView *)collectionView numberOfItemsInSection:(NSInteger)section {

     return _array.count;

 }

 //创建UICollectionViewCell
 - (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath {

     //如果单元格是在故事版中画出来的,不需要注册,需要在单元格中指定标识符
     UICollectionViewCell *cell = [collectionView dequeueReusableCellWithReuseIdentifier:@"cell" forIndexPath:indexPath];

     //取得图片视图
     UIImageView *imgView = (UIImageView *)[cell.contentView viewWithTag:];

     //取出对应的资源数据
      ALAsset *result =_array[indexPath.row];

     //获取到缩略图
     CGImageRef cimg = [result thumbnail];

     //转换为UIImage
     UIImage *img = [UIImage imageWithCGImage:cimg];

     //显示图片
     imgView.image = img;

     /**
      *  获取到原始图片
      ALAssetRepresentation *presentation = [result defaultRepresentation];

     CGImageRef resolutionImg = [presentation fullResolutionImage];
      */

     return cell;

 }

 //单元格大小
 - (CGSize)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout *)collectionViewLayout sizeForItemAtIndexPath:(NSIndexPath *)indexPath{

     return CGSizeMake(, );

 }