[ALAssetsLibrary enumerateGroupsWithTypes:] Asynchronous

/**
* Wait for [AssestLibrary enumerateGroupsWithTypes: usingBlock:] to be completed, or how other
* people call it, [AssestLibrary enumerateGroupsWithTypes: usingBlock:] Asynchronous
* The following method should not be called on main thread, use
* [self performSelectorInBackground:@selector(getAllAssets) withObject:nil];
* to excecute it
*/


- (void)getAllAssets
{
    data_ = [[NSMutableArray alloc] init];


    enum {
        WDASSETURL_PENDINGREADS = 1,
        WDASSETURL_ALLFINISHED = 0
    };
        
    NSAssert(![NSThread isMainThread], @"can't be called on the main thread due to ALAssetLibrary limitations");
        
    NSConditionLock* photosUrlReadLock = [[NSConditionLock alloc] initWithCondition:WDASSETURL_PENDINGREADS];
        
        
    ALAssetsLibrary *assestLibrary = [AssetsLibraryManager defaultAssetsLibrary];
        
    [assestLibrary enumerateGroupsWithTypes:ALAssetsGroupAll usingBlock:^(ALAssetsGroup *group, BOOL *stop){
        
        if (group == nil) {
            [photosUrlReadLock lock];
            [photosUrlReadLock unlockWithCondition:WDASSETURL_ALLFINISHED];
        }
        
        [group enumerateAssetsUsingBlock:^(ALAsset *result, NSUInteger index, BOOL *stop) {
            ALAssetRepresentation *representation = [result defaultRepresentation];
            
            if ([representation url]) {
                [data_ addObject:[representation url]];
            }
        }];
        } failureBlock:^(NSError *error) {
            NSLog(@"error: %@", error);
            [photosUrlReadLock lock];
            [photosUrlReadLock unlockWithCondition:WDASSETURL_ALLFINISHED];
        }];
    [photosUrlReadLock lockWhenCondition:WDASSETURL_ALLFINISHED];
    [photosUrlReadLock unlock];
     
     //Do other things here
}