回调中的异步方法

本文关键字：异步方法 回调 | 更新日期: 2023-09-27 17:57:30

我想做的是做一个后台任务，记录手机的每次解锁，但我无法写入日志。

这就是我注册后台任务的方式：

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();
    }

然后我尝试这个：

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);
    }

但问题是它不会从logFile.txt中读/写。

我想我不能只是让系统调用async来使用其中的异步方法，这是有意义的。但后来我尝试用ThreadPool.RunAsync将它运行到异步lamdbda中，但也没有成功。当你想在非异步函数中调用异步方法时，你通常会怎么做？

回调中的异步方法

如本文所述，异步工作需要封装在后台任务延迟中

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(); 
}
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