UWP - 介紹 App Service 與新功能

App Service 是一種背景工作運行的服務，提供給其他 Apps 使用就像 Web Service。它本身無使用介面(UI-less)，允許 Apps 在同一個設備被引用，甚至 Windows 10 1607 開始允許 remote devices 使用它。

[重點觀念]

• Windows 10, version 1607 開始， App Service 支持新模式：
  
  • 可以與 host App 運行在相同的 process；(一般屬於 Background Task 執行在不同的 process)
  • 支援從 App 呼叫 Remote Devices 中的 App Service；
• 想要 App Service 每次被啓動都是新的 instance，在 Package.appmanifest 加入宣告；uap4:SupportsMultipleInstances="true"；但需要 Windows 10, version 15063 以上才支援
• App Service 的生命周期，因爲 Process 有所不同：
  
  • Background Task (out-of-process):
    
    • 當它被建立時會進入 Run()，隨著 Run() 執行完畢就會被結束
    • 它被啓動後，基本會維持活著約有 30 秒，可搭配呼叫 GetDeferral() 多加 5 秒來完成任務
  • In-app process model：生命周期則跟著呼叫者一起共存，讓兩個 Apps 之間更容易溝通，不用再分成兩份 code 來串聯與維護
• App Service 的 OnTaskCancel() 被觸發有幾個原因：
  
  1. Client app 釋放AppServiceConnection
  2. Client app 被 suspended
  3. 系統關閉或睡眠
  4. 系統執行該 Task 用過高的資源

大略有概念之後，接著介紹怎麽做基本的 App Service (two process)，再介紹怎麽整合到 App 的 process 裏面；
* 如何建立 App service 並使用它：

分成兩個 App 做説明：一個是擁有 App Service 的 Host App；一個是使用 App Service 的 Client App；

1. 建立一個 Windows Runtime Component，並且加入AppServiceConnection 的處理邏輯：
   

   public sealed class ServiceTask : IBackgroundTask
   {
       private BackgroundTaskDeferral backgroundTaskDeferral;
       private AppServiceConnection appServiceconnection;
   
       public void Run(IBackgroundTaskInstance taskInstance)
       {
           // Background Task 被建立時，取得 deferral 拉長生命周期，避免被結束
           this.backgroundTaskDeferral = taskInstance.GetDeferral();
   
           // 一定要註冊處理 Canceled 事件來正確釋放用到的資源
           taskInstance.Canceled += OnTaskCanceled;
   
           // 根據被啓動的 Instance 類型，建立 App Service Connection，並註冊 Request 事件.
           var details = taskInstance.TriggerDetails as AppServiceTriggerDetails;
           appServiceconnection = details.AppServiceConnection;
           appServiceconnection.RequestReceived += OnRequestReceived;
       }
   
       private void OnTaskCanceled(IBackgroundTaskInstance sender, BackgroundTaskCancellationReason reason)
       {
           if (this.backgroundTaskDeferral != null)
           {
               // Complete the service deferral.
               this.backgroundTaskDeferral.Complete();
           }
       }
   
       private async void OnRequestReceived(AppServiceConnection sender, AppServiceRequestReceivedEventArgs args)
       {
           // 當 App Service 收到請求時，該 method 就會被觸發
           // 先要求取得 取得 deferral 拉長生命周期
           var requestDeferral = args.GetDeferral();
   
           ValueSet message = args.Request.Message;
           string cmd = message["cmd"] as string;
           string id = message["id"] as string;
   
           ValueSet responseMsg = new ValueSet();
   
           switch (cmd)
           {
               case "Query":
                   responseMsg.Add("id", "123456");
                   responseMsg.Add("name", "pou");
                   responseMsg.Add("status", "OK");
                   var result = await args.Request.SendResponseAsync(responseMsg);
                   break;
           }
   
           requestDeferral.Complete();
       }
   }

2. 在 Host App 的 Package.manifest 宣告 App Service 並設定 Entry Point，記得把 App Service 的專案加入到 Host App 的專案參考：
   

   <Applications>
       <Application Id="App" Executable="$targetnametoken$.exe" EntryPoint="ServiceHost.App">
           <uap:VisualElements  />
           <Extensions>
               <uap:Extension Category="windows.appService" EntryPoint="MyAppService.ServiceTask">
                   <uap:AppService Name="com.pou.MyApService" />
               </uap:Extension>
           </Extensions>
       </Application>
   </Applications>

   加入專案參考這樣在 Host App 被安裝的時候才會一并加入 App Service。 利用 Windows.ApplicationModel.Package.Current.Id.FamilyName 在 Host App 拿到 package family name，準備交給 Client App。
3. 在 Client App 利用 AppServiceConnection 呼叫 App Service：
   

   private async void MainPage_Loaded(object sender, RoutedEventArgs e)
   {
       AppServiceConnection connection = new AppServiceConnection();
       connection.AppServiceName = "com.pou.MyApService";
       connection.PackageFamilyName = "f9842749-e4c8-4c15-bac8-bc018db1b2ea_s1mb6h805jdtj";
   
       var status = await connection.OpenAsync();
   
       if (status != AppServiceConnectionStatus.Success)
       {
           Debug.WriteLine("Failed to connect");
           return;
       }
   
       var message = new ValueSet();
       message.Add("cmd", "Query");
       message.Add("id", "1234");
   
       AppServiceResponse response = await connection.SendMessageAsync(message);
       string result = "";
   
       if (response.Status == AppServiceResponseStatus.Success)
       {
           if (response.Message["status"] as string == "OK")
           {
               result = response.Message["name"] as string;
           }
       }
   }

上面介紹的 App Service 是比較一般的用法， 把 App Service 放到 Background Task 的架構。

* 把 App Service 合併到 App.xaml.cs 裏面，作爲 Same Process：
AppServiceConnection 允許其他 App 叫醒在背景中自己的 App 並傳入指令。它與上方的 out-of-process 最大不同有兩個：

1. Package.manifest 宣告 <uap:Extension Category="windows.appService"> 不用 Entry Point，改用 OnBackgroundActivated()。
   

   <Package>
     <Applications>
         <Application>
             <Extensions>
                 <uap:Extension Category="windows.appService">
                     <uap:AppService Name="com.pou.MyApService" />
                 </uap:Extension>
             </Extensions>
         </Application>
     </Applications>

   OnBackgroundActivated 代表 App 在背景時被啓動，Life cycle 可以參考 Windows 10 universal Windows platform (UWP) app lifecycle。
2. 在 App.xaml.cs 加入 OnBackgroundActivated() 的處理邏輯。
   

   sealed partial class App : Application
   {
       private AppServiceConnection appServiceConnection;
       private BackgroundTaskDeferral appServiceDeferral;
   
       protected override void OnBackgroundActivated(BackgroundActivatedEventArgs args)
       {
           base.OnBackgroundActivated(args);
   
           AppServiceTriggerDetails appService = args.TaskInstance.TriggerDetails as AppServiceTriggerDetails;
               
           if (appService ==null)
           {
               return;
           }
               
           args.TaskInstance.Canceled += OnAppServicesCanceled;
   
           // appServiceDeferral 與 appServiceConnection 需要變成公用變數
           // 因爲其他時間需要用到，已維持連線的一致性
           appServiceDeferral = args.TaskInstance.GetDeferral();
           appServiceConnection = appService.AppServiceConnection;
   
           appServiceConnection.RequestReceived += AppServiceConnection_RequestReceived;
           appServiceConnection.ServiceClosed += AppServiceConnection_ServiceClosed;
       }
   
       private async void AppServiceConnection_RequestReceived(AppServiceConnection sender, AppServiceRequestReceivedEventArgs args)
       {
           // 當 App Service 收到請求時，該 method 就會被觸發
   
           // 先要求取得 取得 deferral 拉長生命周期
           var requestDeferral = args.GetDeferral();
   
           ValueSet message = args.Request.Message;
   
           string cmd = message["cmd"] as string;
           string id = message["id"] as string;
   
           ValueSet responseMsg = new ValueSet();
   
           switch (cmd)
           {
               case "Query":
                   responseMsg.Add("id", "123456");
                   responseMsg.Add("name", "pou");
                   responseMsg.Add("status", "OK");
                   var result = await args.Request.SendResponseAsync(responseMsg);
                   break;
           }
   
           requestDeferral.Complete();
       }
   
       private void AppServiceConnection_ServiceClosed(AppServiceConnection sender, AppServiceClosedEventArgs args)
       {
           appServiceDeferral?.Complete();
           appServiceConnection?.Dispose();
       }
   
       private void OnAppServicesCanceled(IBackgroundTaskInstance sender, BackgroundTaskCancellationReason reason)
       {
           appServiceDeferral?.Complete();
           appServiceConnection?.Dispose();
       }
   }

要支援 in-process model 就是這樣簡單，而且讓原本的 App Service 邏輯回到 App 本身，讓邏輯更乾净。
OnBackgroundActivated() 負責處理 App Service 的啓用，並儲存 Deferral 保持服務的生命周期。
詳細可以參考 Windows 10 通用 Windows 平台 (UWP) app 週期。

介紹完怎麽實作之後，下面補充幾個重要的元件：

• AppServiceConnection
  
  代表連線到 App Service 的端點，App Service 允許 UWP App 之間互相溝通。幾個重點：
  

  Type	Name	Description
  Properties	AppServiceName	取得或設定想要操作的 App Service Name
  	PackageFamilyName	取得或設定該 App Service 所屬 package 的 family name
  Methods	Dispose()	Performs application-defined tasks associated with freeing, releasing, or resetting unmanaged resources.
  	OpenAsync()	Opens a connection to the endpoint for the app service.
  	OpenRemoteAsync(RemoteSystemConnectionRequest)	Opens a connection to the endpoint on another device for the app service.
  	SendMessageAsync(ValueSet)	傳送 ValueSet 内容到 App Service
  Events	RequestReceived	Occurs when a message arrives from the other endpoint of the app service connection.
  	ServiceClosed	Occurs when the other endpoint closes the connection to the app service.

• ValueSet Class
  
  實現來傳遞交換訊息用的結構，利用 string 做為 Key，Value 則是 Object。這個結構不能放不可被序列化的結構。
• AppServiceResponseStatus
  
  代表 App 傳送訊息到 App Service 的結果
• AppServiceClosedStatus
  
  代表 App Service 被關閉連線時的描述

======
從上面的介紹，如果您的 App Service 是比較工具型跟 App 本身不需要共用資料或邏輯，建議獨立成 Background Task；相反地，需要讓 App Service 跟 App 有互動，建議改為 Same Process 的架構。更多介紹可以參考 Extend your app with services, extensions, and packages。
Connected apps and devices (Project Rome) 支援更豐富的功能，下一篇將有更詳細的説明。
希望對大家有所幫助。

References：

• Create and consume an app service
• Convert an app service to run in the same process as its host app
• Extend your app with app services, extensions, and packages
• Create and consume an app extension
• Communicate with a remote app service
• Universal Windows Platform (UWP) app samples for App Service
• Calling Windows 10 APIs From a Desktop Application
• qmatteoq/DesktopBridge
• Support your app with background tasks
• Windows 10 universal Windows platform (UWP) app lifecycle
• Communicate with a remote app service
• Connected apps and devices (Project Rome)
• [UWP] Windows 10 IoT App Service 簡單實作
• 偵錯背景工作