Issue 475: Fix a bug when perssing on back button

Src/GoogleApis.Auth.WP/OAuth2/WebAuthenticationBrokerUserControl.xaml.cs

 /*
 Copyright 2013 Google Inc
 
 Licensed under the Apache License, Version 2.0 (the "License");
 you may not use this file except in compliance with the License.
 You may obtain a copy of the License at
 
 http://www.apache.org/licenses/LICENSE-2.0
 
 Unless required by applicable law or agreed to in writing, software
@@ skipping 43 matching lines @@
 {
 Loader.Visibility = Visibility.Collapsed;
 }
 
 /// <summary>Callback of browser navigating.</summary>
 private void OnBrowserNavigating(object sender, NavigatingEventArgs e)
 {
 // The code or the error should be received by localhost.
 if (e.Uri.Host == "localhost")
 {
-RemoveBackKeyPressCallback();
 var query = e.Uri.Query.Substring(1);
 tcsAuthorizationCodeResponse.SetResult(new AuthorizationCodeResp
onseUrl(query));
 }
 }
 
 /// <summary>Callback of browser navigation failed.</summary>
 private void OnBrowserNavigationFailed(object sender, NavigationFailedEv
entArgs e)
 {
 if (!tcsAuthorizationCodeResponse.Task.IsCompleted)
 {
-RemoveBackKeyPressCallback();
-
 // See https://code.google.com/p/google-api-dotnet-client/issues
/detail?id=431.
 // If we encounter a null exception, cancel the task because the
Windows Phone app crashed.
 if (e.Exception != null)
 {
 tcsAuthorizationCodeResponse.SetException(e.Exception);
 }
 else
 {
 tcsAuthorizationCodeResponse.SetCanceled();
 }
 }
 }
 
 /// <summary>Callback of browser navigated.</summary>
 private void OnBrowserNavigated(object sender, NavigationEventArgs e)
 {
 StopLoading();
 }
 
 /// <summary>The window launcher that starts browse the browser controll
er to the given URI.</summary>
 /// <param name="uri">The authorization code request URI</param>
 /// <returns>The authorization code response</returns>
 public Task<AuthorizationCodeResponseUrl> Launch(Uri uri)
 {
 tcsAuthorizationCodeResponse = new TaskCompletionSource<Authorizatio
nCodeResponseUrl>();
+tcsAuthorizationCodeResponse.Task.ContinueWith(t =>
+{
+RemoveBackKeyPressCallback();
+}, TaskScheduler.FromCurrentSynchronizationContext());
 StartLoading();
 browser.Navigate(uri);
 return tcsAuthorizationCodeResponse.Task;
 }
 
 /// <summary>A callback handler for when the user presses the back key.<
/summary>
 void RootPage_BackKeyPress(object sender, System.ComponentModel.CancelEv
entArgs e)
 {
- RemoveBackKeyPressCallback();
-
- if (tcsAuthorizationCodeResponse.Task.Status == TaskStatus.Running)
- {
-  e.Cancel = true;
-  tcsAuthorizationCodeResponse.SetCanceled();
- }
+ e.Cancel = true;
+ tcsAuthorizationCodeResponse.SetCanceled();
 }
 
 /// <summary>Removes <see cref="RootPage_BackKeyPress" as the root page
callback./></summary>
 void RemoveBackKeyPressCallback()
 {
 PhoneApplicationFrame rootFrame = Application.Current.RootVisual as
PhoneApplicationFrame;
 PhoneApplicationPage rootPage = rootFrame.Content as PhoneApplicatio
nPage;
 rootPage.BackKeyPress -= RootPage_BackKeyPress;
 }
 }
 }