iphone - AlAssetLibrary 返回空结果

Question

我正在尝试挑选一些相机胶卷元数据。当我枚举 Assets 时，我似乎无法检索任何信息并获得一个空数组。有没有我错过的步骤？

我的代码:

assets = [[NSMutableArray array] init]; 

void (^assetEnumerator)(ALAsset *, NSUInteger, BOOL *) = ^(ALAsset *asset, NSUInteger index, BOOL *stop) {
    if(asset != NULL) {
        [assets addObject:asset];
        dispatch_async(dispatch_get_main_queue(), ^{
        });
    }
};

void (^assetGroupEnumerator)(ALAssetsGroup *, BOOL *) =  ^(ALAssetsGroup *group, BOOL *stop) {
    if(group != nil) {
        [group enumerateAssetsUsingBlock:assetEnumerator];
    }
};

library = [[ALAssetsLibrary alloc] init];

[library enumerateGroupsWithTypes:ALAssetsGroupSavedPhotos 
                       usingBlock:assetGroupEnumerator
                     failureBlock: ^(NSError *error) {
                         NSLog(@"Failed.");
                     }];    

NSLog(@"%@", assets); //prints an empty array

Answer 1

Midhun MP 是正确的，您没有等待异步枚举完成。在这种情况下，您有异步 block 调用其他异步 block ，因此要知道所有枚举何时完成并不简单。

如果您想知道何时完成，并最终得到一个包含所有枚举 Assets 的数组，您可以使用 dispatch_groups。这是您可以做到这一点的一种方法(我包含了多个 ALAssetGroup 类型以表明这可以与多个专辑一起使用):

dispatch_group_t loadingGroup = dispatch_group_create();
NSMutableArray * assets = [[NSMutableArray array] init];
NSMutableArray * albums = [[NSMutableArray array] init];

void (^assetEnumerator)(ALAsset *, NSUInteger, BOOL *) = ^(ALAsset *asset, NSUInteger index, BOOL *stop) {
    if(index != NSNotFound) {
        [assets addObject:asset];
        dispatch_async(dispatch_get_main_queue(), ^{ });
    } else {
        dispatch_group_leave(loadingGroup);
    }
};


void (^assetGroupEnumerator)(ALAssetsGroup *, BOOL *) =  ^(ALAssetsGroup *group, BOOL *stop) {
    if(group != nil) {
        [albums addObject: group];
    } else {
         NSLog(@"Found %d albums", [albums count]);
         // album loading is done
        for (ALAssetsGroup * album in albums) {
            dispatch_group_enter(loadingGroup);
            [album enumerateAssetsUsingBlock: assetEnumerator];
        }
        dispatch_group_notify(loadingGroup, dispatch_get_main_queue(), ^{
            NSLog(@"DONE: ALAsset array contains %d elements", [assets count]);
        });
    }
};

ALAssetsLibrary * library = [[ALAssetsLibrary alloc] init];

[library enumerateGroupsWithTypes:ALAssetsGroupSavedPhotos | ALAssetsGroupAlbum
                   usingBlock:assetGroupEnumerator
                 failureBlock: ^(NSError *error) {
                     NSLog(@"Failed.");
                 }];

(在本例中，将各种 block 添加到 assets 和 albums 是安全的，因为枚举都发生在主线程上。)