The Replacement of the Semaphore

In order to synchronize between threads, i used semaphore in here. Today, I find a replacement of the semaphore. I think it is more efficient and cool. Now, i note it as follow:

    - (id)initWithLibraryChangedHandler:(void (^)(void))libraryChangedHandler
{
    self = [super init];
    if (self) 
    {
        _assetItems = [NSMutableArray array];
        //Creates a new dispatch queue to which blocks can be submitted.  创建一个新的分发队列用于提交blocks
        _assetItemsQueue = dispatch_queue_create("com.apple.avmovieexporter.assetItemLibraryQueue", DISPATCH_QUEUE_SERIAL);

        _libraryGroup = dispatch_group_create();
        //Returns a well-known global concurrent queue of a given priority level.
        //The well-known global concurrent queues cannot be modified
        //Blocks submitted to these global concurrent queues may be executed concurrently with respect to each other.
        _libraryQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0);

        // Update the table view whenever the library changes
        [[NSNotificationCenter defaultCenter] addObserverForName:ALAssetsLibraryChangedNotification 
                                                          object:nil 
                                                           queue:[NSOperationQueue mainQueue] 
                                                      usingBlock:^(NSNotification *block){
                                                               libraryChangedHandler();
                                                           }];
    }

    return self;
}

- (void)dealloc
{
    dispatch_release(_assetItemsQueue);

    dispatch_release(_libraryQueue);
    dispatch_release(_libraryGroup);

    [[NSNotificationCenter defaultCenter] removeObserver:self name:ALAssetsLibraryChangedNotification object:nil];
}

- (void)loadLibraryWithCompletionBlock:(void (^)(void))completionHandler
{
    // Load content using the Media Library and AssetLibrary APIs, also check for content included in the application bundle
    [self.assetItems removeAllObjects];

    [self buildMediaLibrary];
    [self buildAssetLibrary];
    [self buildApplicationBundleLibrary];


    //Schedules a block object to be submitted to a queue when a group of previously submitted block objects have completed.
    dispatch_group_notify(self.libraryGroup, self.libraryQueue, ^{
        dispatch_async(dispatch_get_main_queue(), ^{
            completionHandler();
        });
    });
}

- (void)addURL:(NSURL *)url
{
    __unsafe_unretained __block VideoLibrary *weakSelf = (VideoLibrary *)self;

    if (url == nil)
        return;

    dispatch_async(self.assetItemsQueue, ^{
        [weakSelf.assetItems addObject:[[AssetItem alloc] initWithURL:url]];
    });
}

#pragma mark - iPod Library

- (void)buildMediaLibrary
{
    __unsafe_unretained __block VideoLibrary *weakSelf = (VideoLibrary *)self;

    //将blocks绑定到libraryGroup，然后放在libraryQueue并发队列中
    dispatch_group_async(self.libraryGroup, self.libraryQueue, ^{
        NSLog(@"started building media library...");

        // Search for video content in the Media Library
#if  __IPHONE_OS_VERSION_MAX_ALLOWED >= 50000
        NSNumber *videoTypeNum = [NSNumber numberWithInteger:MPMediaTypeAnyVideo];
#else
        NSNumber *videoTypeNum = [NSNumber numberWithInteger:(MPMediaTypeAny ^ MPMediaTypeAnyAudio)];
#endif
        MPMediaPropertyPredicate *videoPredicate = [MPMediaPropertyPredicate predicateWithValue:videoTypeNum forProperty:MPMediaItemPropertyMediaType];
        MPMediaQuery *videoQuery = [[MPMediaQuery alloc] init];
        [videoQuery addFilterPredicate: videoPredicate];
        NSArray *items = [videoQuery items];

        for (MPMediaItem *mediaItem in items) 
            [weakSelf addURL:[mediaItem valueForProperty:MPMediaItemPropertyAssetURL]];

        NSLog(@"done building media library...");
    });
}

- (void)buildAssetLibrary
{
    NSLog(@"started building asset library...");

    __unsafe_unretained __block VideoLibrary *weakSelf = (VideoLibrary *)self;
    dispatch_group_enter(weakSelf.libraryGroup);

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

    // Enumerate through all the groups in the Asset Library
    [assetLibrary enumerateGroupsWithTypes:ALAssetsGroupAll 
                                usingBlock:
     ^(ALAssetsGroup *group, BOOL *stop)
     {
         if (group != nil)
         {
             // Filter by groups that contain video
             [group setAssetsFilter:[ALAssetsFilter allVideos]];
             [group enumerateAssetsUsingBlock:
              ^(ALAsset *asset, NSUInteger index, BOOL *stop)
              {
                  if (asset)
                      [weakSelf addURL:[[asset defaultRepresentation] url]];
              }];
         }
         else
         {
             dispatch_group_leave(weakSelf.libraryGroup);
             NSLog(@"done building asset library...");
         }
     }
                              failureBlock:^(NSError *error)
     {
         dispatch_group_leave(weakSelf.libraryGroup);
         NSLog(@"error enumerating AssetLibrary groups %@\n", error);
     }];

}

The above is used for understanding the whole plot, now you can see the detail:

dispatch_group_notify(self.libraryGroup, self.libraryQueue, ^{
        dispatch_async(dispatch_get_main_queue(), ^{
            completionHandler();
        });
    });
}

The explain of the document for the selector is that “Schedules a block object to be submitted to a queue when a group of previously submitted block objects have completed.” So, we can wait the tasks to complete, then to do something we want. The dispatch_group_enter is cool as well. it can let the block enter the queue and execute as a task associated group.

So, that is all. See you.