title: What does [urlString componentsSeperatedByString: @"?"] [0]? mean in Swift?
body: <p>I'm not entirely sure what</p>
  
  <pre><code>NSString * fileName = [self cachedFileNameForKey:[urlString componentsSeparatedByString:@"?"][0]];
  </code></pre>
  
  <p>Means from this code.  I am primely writing in Swift so this notation is a bit confusing for me.  What does the double [ ] notation mean? 3d array?</p>
  
  <pre><code>+ (AVPlayerItem *)localDownloadedVideoFromUrl:(NSURL *)url {
        NSString * urlString = url.absoluteString;
        NSString * fileName = [self cachedFileNameForKey:[urlString componentsSeparatedByString:@"?"][0]];
        TWRDownloadManager * manager = [TWRDownloadManager sharedManager];
        if ([manager fileExistsWithName:fileName]) {
             AVPlayerItem * item = [AVPlayerItem playerItemWithURL:[NSURL fileURLWithPath:[manager localPathForFile:fileName]]];
             return item;
        }
        return [NSNull null];
     }
  </code></pre>