void Update()
 {
     // Call Update() on AccessTokenGetter and on WwwRequestInProgress to trigger execution of pending tasks
     // if there are any.
     AccessTokenGetter.Update();
     WwwRequestInProgress.Update();
 }
 /// <summary>
 /// Invokes an initial action that requires access token to send an HTTP requests, and schedules invocation of
 /// the post completion action on the WWW instance holding the request for when the
 /// response to the request is received. Updates access token before making this request if necessary.
 /// </summary>
 /// <param name="initialAction">An action that will need an updated access token.</param>
 /// <param name="postResponseAction">An action to execute when the response to the request has been received.</param>
 private static void InvokeAccessRestrictedAction(
     Action <Action <WWW> > initialAction, Action <WWW> postResponseAction)
 {
     if (!AccessTokenGetter.AccessToken.IsValid())
     {
         AccessTokenGetter.ValidateAccessToken(() => initialAction(postResponseAction));
     }
     else
     {
         initialAction(postResponseAction);
     }
 }