如何显示在ios中下载视频的进度？

Question

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        NSLog(@"Downloading Started");
        NSString *urlToDownload = @"http://km.support.apple.com/library/APPLE/APPLECARE_ALLGEOS/HT1211/sample_iTunes.mov";
        NSURL  *url = [NSURL URLWithString:urlToDownload];
        NSData *urlData = [NSData dataWithContentsOfURL:url];
        if ( urlData )
        {
            NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
            NSString *documentsDirectory = [paths objectAtIndex:0];

            NSString *filePath = [NSString stringWithFormat:@"%@/%@", documentsDirectory,@"prakash.mov"];

            //saving is done on main thread
            dispatch_async(dispatch_get_main_queue(), ^{
                [urlData writeToFile:filePath atomically:YES];
                NSLog(@"File Saved !");
            });
        }

    });