Question

I’m trying to pick out some camera roll meta data. When I enumerate through the assets, I cannot seem to retrieve any information and get an empty array. Is there a step I’m missing?

My code:

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 is right that you are not waiting for the asynchronous enumeration to complete. In this case, you have asynchronous blocks calling other asynchronous blocks, so it is not simple to know when all enumeration is done.

If you would like to know when this is done, and end up with an array that contains all of the enumerated assets, you could use dispatch_groups. Here is one way you could do that (I’ve included multiple ALAssetGroup types to show that this can work with multiple albums):

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.");
                 }];

(In this example, it is safe to have various blocks adding to assets and albums because the enumeration is all happening on the main thread.)