ALAssetsLibrary enumerateGroupsWithTypes: - Thread synchronization

Question

I'm using [ALAssetsLibrary enumerateGroupsWithTypes:] to store the ALAssets in an array. As this is an asynchronous operation, I need to wait for it to finish before continuing my work.

I read Cocoa thread synchronisation when using [ALAssetsLibrary enumerateGroupsWithTypes:] and tried the recommended NSConditionLock. However, the blocks are always performed in the main thread, thus if I wait using the conditionlock, the main thread is blocked and the blocks won't get executed -> I'm stuck. I even tried running the method loadAssets on a new thread, but still the blocks get executed on the main thread.

I can't find a way to actually wait for the enumeration to finish. Is there a way to force the blocks to a different thread than the main thread or anything else I can do?

Here's the code:

- (void)loadAssets
{
    assets = [NSMutableArray array];
    NSConditionLock *threadLock = [[NSConditionLock alloc] initWithCondition:THREADRUNNING];

    void (^assetEnumerator)(ALAsset *, NSUInteger, BOOL *) = ^(ALAsset *result, NSUInteger index, BOOL *stop)
    {
        if(result != nil)
        {
            [assets addObject:result];
        }
    };

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

        [threadLock lock];
        [threadLock unlockWithCondition:THREADFINISHED];
    };

    void (^assetFailureBlock)(NSError *) = ^(NSError *error)
    {
        [threadLock lock];
        [threadLock unlockWithCondition:THREADFINISHED];
    };

    ALAssetsLibrary *assetsLibrary = [[ALAssetsLibrary alloc] init];
    [assetsLibrary enumerateGroupsWithTypes:ALAssetsGroupAll usingBlock:assetGroupEnumerator failureBlock:assetFailureBlock];

    [threadLock lockWhenCondition:THREADFINISHED];
    [threadLock unlock];

    [assetsLibrary release];
    [threadLock release];
}

Answer 1

you are killing your assetsLibrary object before it finished to enumrate the objects. move it to your .h file and release it in dealloc.

Answer 2

I know this thread is probably dead, but I'm answering it because this is where I landed by Googling

The trick is to lock your background thread until the enumeration yields a nil-group. This category is one example of how to use it, you can use the category method as a drop-in replacement of the ALAssetsLibrary enumeration method.

@implementation ALAssetsLibrary (EEEConcurrency)

- (NSUInteger)eee_enumerateGroupsLockedWithTypes:(ALAssetsGroupType)types
                                      usingBlock:(ALAssetsLibraryGroupsEnumerationResultsBlock)enumerationBlock
                                    failureBlock:(ALAssetsLibraryAccessFailureBlock)failureBlock
{
    NSAssert(![NSThread isMainThread], @"This would create a deadlock (main thread waiting for main thread to complete)");

    enum
    {
        EEEAssetsLibraryDone,
        EEEAssetsLibraryBusy
    };

    NSConditionLock *assetsLibraryConditionLock = [[NSConditionLock alloc] initWithCondition:EEEAssetsLibraryBusy];

    __block NSUInteger numberOfGroups = 0;
    [self enumerateGroupsWithTypes:types
                        usingBlock:^(ALAssetsGroup *group, BOOL *stop) {
                            enumerationBlock(group, stop);
                            if (group) numberOfGroups++;
                            if (!group || *stop)
                            {
                                [assetsLibraryConditionLock lock];
                                [assetsLibraryConditionLock unlockWithCondition:EEEAssetsLibraryDone];
                            }
                        }
                      failureBlock:^(NSError *error) {
                          failureBlock(error);
                          [assetsLibraryConditionLock lock];
                          [assetsLibraryConditionLock unlockWithCondition:EEEAssetsLibraryDone];
                      }];

    [assetsLibraryConditionLock lockWhenCondition:EEEAssetsLibraryDone];
    [assetsLibraryConditionLock unlock];

    return numberOfGroups;
}

@end

Download the category at https://gist.github.com/epologee/8890692