Index: Src/GoogleApis.Auth.WP81/OAuth2/AuthorizationCodeBroker.cs
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+/*
+Copyright 2014 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
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+using System;
+using System.Threading;
+using System.Threading.Tasks;
+using Windows.Security.Authentication.Web;
+using Windows.ApplicationModel.Core;
+using Windows.UI.Core;
+
+using Google.Apis.Auth.OAuth2.Requests;
+using Google.Apis.Auth.OAuth2.Responses;
+using Google.Apis.Util.Store;
+
+namespace Google.Apis.Auth.OAuth2
+{
+    /// <summary>
+    /// OAuth 2.0 verification code receiver for Windows Phone 8.1 application that opens a Google account form so the
+    /// users can enter their credentials and accept the application access to their token.
+    /// 
+    /// This broker's most important method is <see cref="ReceiveCodeAsync"/>.
+    /// The first time it is being called, it calls <code>WebAuthenticationBroker.AuthenticateAndContinue</code>
+    /// so the users can authorize the app to access their private resources. While calling this method the app is
+    /// suspended.
+    /// 
+    /// When the app is being activated again, the developer MUST store the auth result (which may contain the access
+    /// code or an error) in the storage with the <code>SerializableWebAuthResult.Name</code> key.
+    /// Then when the auth framework calls again the <see cref="ReceiveCodeAsync"/> it looks in the storage to find
+    /// the auth result, and if a code was received the framework will convert it in turn to access and refresh tokens.
+    /// </summary>
+    public class AuthorizationCodeBroker : ICodeReceiver
+    {
+        #region ICodeReceiver Members
+
+        public string RedirectUri
+        {
+            get { return GoogleAuthConsts.LocalhostRedirectUri; }
+        }
+
+        public async Task<AuthorizationCodeResponseUrl> ReceiveCodeAsync(AuthorizationCodeRequestUrl url,
+            CancellationToken taskCancellationToken)
+        {
+            TaskCompletionSource<AuthorizationCodeResponseUrl> tcs =
+                new TaskCompletionSource<AuthorizationCodeResponseUrl>();
+            await ReceiveCodeAsync(url, tcs);
+            return tcs.Task.Result;
+        }
+
+        #endregion
+
+        private async Task InvokeFromUIThread(Action action)
+        {
+            await CoreApplication.MainView.CoreWindow.Dispatcher.RunAsync(CoreDispatcherPriority.Normal,
+                () => action());
+        }
+
+        /// <summary>Asynchronously receives the authorization code.</summary>
+        /// <param name="url">The authorization code request URL.</param>
+        /// <param name="tcs">Task completion source whose result will be set to the authorization code.</param>
+        private async Task ReceiveCodeAsync(AuthorizationCodeRequestUrl url,
+            TaskCompletionSource<AuthorizationCodeResponseUrl> tcs)
+        {
+            var result = await PasswordVaultDataStore.Default.GetAsync<SerializableWebAuthResult>(
+                SerializableWebAuthResult.Name);
+            if (result == null)
+            {
+                // We should run WebAuthenticationBroker.AuthenticateAndContinue from the UI thread ONLY.
+                await InvokeFromUIThread(() => WebAuthenticationBroker.AuthenticateAndContinue(url.Build(),
+                    new Uri(GoogleAuthConsts.LocalhostRedirectUri), null, WebAuthenticationOptions.None));
+                
+                // No need to return anything, cause the application is going to be suspended now.
+                return;
+            }
+
+            const string Code = "code=";
+            const string Error = "error=";
+            // Get the index of the error or the code.
+            var index = result.ResponseData.IndexOf(Code);
+            index = index != -1 ? index : result.ResponseData.IndexOf(Error);
+
+            if (index != -1)
+            {
+                tcs.SetResult(new AuthorizationCodeResponseUrl(result.ResponseData.Substring(index)));
+                return;
+            }
+
+            tcs.SetException(new TokenResponseException(
+                new TokenErrorResponse
+                {
+                    Error = result.ResponseStatus.ToString(),
+                    ErrorDescription = "The WebAuthenticationBroker didn't return a code or an error. Details: " +
+                        result.ResponseErrorDetail,
+                }));
+        }
+    }
+}
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