C# Windows Phone ScheduleAgent在设置时间段抓取图像

我正试图遵循一个，调整它一点，以满足我的需要。我试图每X秒抓取一张新图片（30秒用于调试），并将其设置为新的锁屏背景。我创建了一个ScheduleAgent对象来调用它，但是，它似乎跳过了一个函数。在运行完全相同的函数的

MainPage.xaml.cs

中，它不会跳过此操作

ScheduleAgent.cs

        static ScheduledAgent()
    {
       ....<irrelevant code>....

    protected override void OnInvoke(ScheduledTask task)
    {

        .... < Code went here that shouldn't matter > ....
        }
        Debug.WriteLine("Imagename = " + imageName);


            DownloadImagefromServer("https://s3-us-west-2.amazonaws.com/qoutescreen/0" + imageName + ".jpg");

---

我建议使用“”关键字。这将导致代码暂停并等待操作完成，而无需担心互斥锁或其他线程阻塞策略

不幸的是，Windows Phone缺少WebClient上的非异步方法，因此有一点设置，显然是从中窃取的

要将async与WebClient一起使用，请安装NuGet软件包，然后可以使用OpenReadTaskAsync

---

因此，执行过程会等待“OpenRead”调用完成，然后再处理结果，然后返回控制。

问题在于

DownloadImagefromServer

是异步的，因此该方法会在图像下载之前返回。然后调用

NotifyComplete

，这将停止后台代理的执行。在文件下载完成之前，使用互斥锁防止

OnInvoke

方法返回。但它会完全跳过函数调用。所以对我来说，似乎它甚至没有尝试，并不是说它没有足够的时间。我的假设是错误的吗？当然，设置一个断点，但我不明白为什么它会跳过调用我就是这么做的。我在所有地方设置了断点，它似乎只是跳过了它。该函数从未被命中，我也不知道为什么。@JcKelley在计划代理中实例化新的BitmapImage对象时遇到了一些问题。当我调用构造函数时，它总是抛出异常。我试图将来自OnInvoke（）方法的所有代码放入Deployment.Current.Dispatcher.BeginInvoke（）调用中，但随后所有WeClient下载都停止工作。您是否遇到过类似的问题？它无法将

OpenRead

识别为webClient对象的有效方法？你认为这是一个单独的问题还是相关的？哦，你是对的。Windows Phone只有异步方法。我已经编辑了答案。等待应该仍然阻止线程，直到打开完成。似乎它仍然没有咬。“不能等待无效”，我需要返回任务对象吗？啊，是的，你需要。对所有的错误感到抱歉。我没有在Visual Studio中使用我的计算机，因此无法编译。我尝试了这一点，但似乎遇到了相同的错误。有什么想法吗？

        // If debugging is enabled, launch the agent again in one minute.
        // debug, so run in every 30 secs
        #if(DEBUG_AGENT)
        ScheduledActionService.LaunchForTest(task.Name, TimeSpan.FromSeconds(30));
        System.Diagnostics.Debug.WriteLine("Periodic task is started again: " + task.Name);
        #endif

        // Call NotifyComplete to let the system know the agent is done working.
        NotifyComplete();
    }
    //===============================================================================
    private async void LockScreenChange(string filePathOfTheImage, bool isAppResource)
    {
        if (!LockScreenManager.IsProvidedByCurrentApplication)
        {
            // If you're not the provider, this call will prompt the user for permission.
            // Calling RequestAccessAsync from a background agent is not allowed.
            await LockScreenManager.RequestAccessAsync();
        }

        // Only do further work if the access is granted.
        if (LockScreenManager.IsProvidedByCurrentApplication)
        {
            // At this stage, the app is the active lock screen background provider.
            // The following code example shows the new URI schema.
            // ms-appdata points to the root of the local app data folder.
            // ms-appx points to the Local app install folder, to reference resources bundled in the XAP package
            var schema = isAppResource ? "ms-appx:///" : "ms-appdata:///Local/";
            var uri = new Uri(schema + filePathOfTheImage, UriKind.Absolute);

            // Set the lock screen background image.
            LockScreen.SetImageUri(uri);

            // Get the URI of the lock screen background image.
            var currentImage = LockScreen.GetImageUri();
            System.Diagnostics.Debug.WriteLine("The new lock screen background image is set to {0}", currentImage.ToString());                
        }
    }
            private async Task DownloadImagefromServer(string imgUrl)
    {
        Debug.WriteLine("Attempting to Get Image from Server...");
        WebClient client = new WebClient();

        try
        {
            // I GET A "Cannot await Void" problem here?!?!?
            //=================================================================
            var result = await client.OpenReadAsync(new Uri(imgUrl, UriKind.Absolute));
            BitmapImage bitmap = new BitmapImage();
            bitmap.SetSource(result);
            return result;
        }
        catch (Exception e)
        {
            Debug.WriteLine(e);

        }

        client.OpenReadCompleted += new OpenReadCompletedEventHandler(client_OpenReadCompleted);
        //client.OpenReadAsync(new Uri(imgUrl, UriKind.Absolute));
    }

    //------------------------------------------------------------------
    //THIS FUNCTION IS NEVER HIT WHEN RUN IN THE ScheduleAgent.cs CODE, HOWEVER
    // WHEN IT IS RUN INT HE MainPage.xaml.cs CODE (Which is a copy/paste) IT RUNS FINE
    // WHY IS IT NOT HITTING THIS? HOW CAN I FIX IT?
    //------------------------------------------------------------------
    void client_OpenReadCompleted(object sender, OpenReadCompletedEventArgs e)
    {
        BitmapImage bitmap = new BitmapImage();
        bitmap.SetSource(e.Result);
        //img.Source = bitmap;

        // Create a filename for JPEG file in isolated storage.
        String tempJPEG = "DownloadedWalleper.jpg";

        // Create virtual store and file stream. Check for duplicate tempJPEG files.
        using (IsolatedStorageFile myIsolatedStorage = IsolatedStorageFile.GetUserStoreForApplication())
        {
            if (myIsolatedStorage.FileExists(tempJPEG))
            {
                myIsolatedStorage.DeleteFile(tempJPEG);
            }

            IsolatedStorageFileStream fileStream = myIsolatedStorage.CreateFile(tempJPEG);

            StreamResourceInfo sri = null;
            Uri uri = new Uri(tempJPEG, UriKind.Relative);
            sri = Application.GetResourceStream(uri);

            //BitmapImage bitmap = new BitmapImage();
            //bitmap.SetSource(sri.Stream);
            WriteableBitmap wb = new WriteableBitmap(bitmap);

            // Encode WriteableBitmap object to a JPEG stream.
            Extensions.SaveJpeg(wb, fileStream, wb.PixelWidth, wb.PixelHeight, 0, 85);

            //wb.SaveJpeg(fileStream, wb.PixelWidth, wb.PixelHeight, 0, 85);
            fileStream.Close();
        }
        LockScreenChange("DownloadedWalleper.jpg", false);
    }
}

private async void DownloadImagefromServer(string imgUrl)
{
    Debug.WriteLine("Attempting to Get Image from Server...");
    WebClient client = new WebClient();

    var result = await client.OpenReadTaskAsync(new Uri(imgUrl, UriKind.Absolute));

    BitmapImage bitmap = new BitmapImage();
    bitmap.SetSource(result);
    //img.Source = bitmap;

    ... rest of your code from OpenReadCompleted goes here
}