UWP - 介绍App Service 与新功能

App Service 是一种背景工作运行的服务，提供给其他Apps 使用就像Web Service。它本身无使用介面(UI-less)，允许Apps 在同一个设备被引用，甚至Windows 10 1607 开始允许remote devices 使用它。

[ 重点观念 ]

Windows 10, version 1607 开始， App Service 支持新模式：
可以与host App 运行在相同的process；(一般属于Background Task 执行在不同的process)
支援从App呼叫Remote Devices中的App Service；
想要App Service每次被启动都是新的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，因为Process 有所不同：
后台任务（进程外）：
当它被建立时会进入Run()，随着Run()执行完毕就会被结束
它被启动后，基本会维持活着约有30秒，可搭配呼叫GetDeferral()多加5秒来完成任务
In-app process model：生命周期则跟着呼叫者一起共存，让两个Apps 之间更容易沟通，不用再分成两份code 来串联与维护
App Service 的OnTaskCancel() 被触发有几个原因：
Client app释放AppServiceConnection
Client app 被 suspended
系统关闭或睡眠
系统执行该Task 用过高的资源
大略有概念之后，接着介绍怎么做基本的App Service (two process)，再介绍怎么整合到App 的process 里面；

如何建立App service 并使用它：

分成两个App 做说明：一个是拥有App Service 的Host App；一个是使用App Service 的Client App；

1建立一个Windows Runtime Component，并且加入AppServiceConnection 的处理逻辑：

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 并设定Entry Point，记得把App Service 的专案加入到Host App 的专案参考：

<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>

加入专案参考这样在Host App被安装的时候才会一并加入App Service。利用Windows.ApplicationModel.Package.Current.Id.FamilyName在Host App拿到package family name，准备交给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;

        }

    }

}

上面介绍的App Service 是比较一般的用法，把App Service 放到Background Task 的架构。

把App Service 合并到App.xaml.cs 里面，作为Same Process：

AppServiceConnection允许其他App叫醒在背景中自己的App并传入指令。它与上方的out-of-process最大不同有两个：

1Package.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>

2在App.xaml.cs加入OnBackgroundActivated()的处理逻辑。

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就是这样简单，而且让原本的App Service逻辑回到App本身，让逻辑更干净。OnBackgroundActivated()负责处理App Service的启用，并储存Deferral保持服务的生命周期，详细可以参考Windows 10通用Windows平台(UWP) app周期。

巴特西

UWP - 介绍App Service 与新功能

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