问iOS:从url下载视频并保存在画廊.？

EN

    -(void)DownloadVideo
{
//download the file in a seperate thread.
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
NSLog(@"Downloading Started");
NSString *urlToDownload = @"http://www.somewhere.com/thefile.mp4";
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,@"thefile.mp4"];

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

});
}