回調中的異步方法

Question

我要做的是做一個后台任務，記錄手機的每次解鎖，但是我無法使日志內容正常工作。

這就是我注冊后台任務的方式：

public App()
    {
        this.InitializeComponent();
        this.Suspending += this.OnSuspending;

        backgroundTask = registerBackgroundTask();
    }

    private BackgroundTaskRegistration registerBackgroundTask()
    {
        // -----------------------------
        const string taskName = "BackgroundTask_Task";
        // -----------------------------


        IAsyncOperation<BackgroundAccessStatus> accessStatus = BackgroundExecutionManager.RequestAccessAsync();

        SystemTrigger systemTrigger = new SystemTrigger(SystemTriggerType.UserPresent, false);

        foreach( var cur in BackgroundTaskRegistration.AllTasks )
        {
            if( cur.Value.Name == taskName )
            {
                // The Task is already registered.
                return (BackgroundTaskRegistration)(cur.Value);
            }
        }

        BackgroundTaskBuilder builder = new BackgroundTaskBuilder();

        builder.Name = taskName;
        builder.TaskEntryPoint = typeof(BackgroundTask_Task.Task).FullName;
        builder.SetTrigger(systemTrigger);

        return builder.Register();

    }

然后我嘗試這個：

public async void Run(IBackgroundTaskInstance taskInstance)
    {
        Debug.WriteLine("Test");
        Debug.WriteLine(DateTime.Now.ToString("g"));

        StorageFolder dataFolder = ApplicationData.Current.LocalFolder;
        StorageFile logFile = await dataFolder.CreateFileAsync("logFile.txt", CreationCollisionOption.OpenIfExists);

        IList<string> logLines = await FileIO.ReadLinesAsync(logFile);

        foreach( var s in logLines )
        {
            Debug.WriteLine(s);
        }

        logLines.Add(DateTime.Now.ToString("g"));
        if( logLines.Count > 5 )
        {
            logLines.RemoveAt(0);
        }

        await FileIO.AppendLinesAsync(logFile, logLines);

    }

但是問題是它不會從logFile.txt中讀取/寫入。 （有時會，但並非每次都如此）。

我以為我不能僅僅讓系統調用async在其中使用異步方法就可以了。 但是后來我嘗試使用ThreadPool.RunAsync將其運行到異步lamdbda中，也沒有成功。 當您要在非異步函數中調用異步方法時，通常會做什么？

Answer 1

如本博客文章所述，異步工作需要封裝在后台任務延遲中

public async void Run(IBackgroundTaskInstance taskInstance)
{
    BackgroundTaskDeferral _deferral = taskInstance.GetDeferral();

    Debug.WriteLine("Test");
        Debug.WriteLine(DateTime.Now.ToString("g"));

        StorageFolder dataFolder = ApplicationData.Current.LocalFolder;
        StorageFile logFile = await dataFolder.CreateFileAsync("logFile.txt", CreationCollisionOption.OpenIfExists);

        IList<string> logLines = await FileIO.ReadLinesAsync(logFile);

        foreach( var s in logLines )
        {
            Debug.WriteLine(s);
        }

        logLines.Add(DateTime.Now.ToString("g"));
        if( logLines.Count > 5 )
        {
            logLines.RemoveAt(0);
        }

        await FileIO.AppendLinesAsync(logFile, logLines);

    _deferral.Complete(); 
}