Question

我有一个执行以下操作的应用程序：

初始化连接的网络摄像头

private async Task<MediaCapture> InitialiseWebCam()
{
    MediaCapture webCam = null;
    webCam = new MediaCapture();

    try
    {
        await webCam.InitializeAsync();
    }
    catch (Exception ex)
    {
        //Log error
    }

    return webCam;
}

拍摄图像

private async Task<StorageFile> TakePicture(MediaCapture webCam)
{
    try
    {
        var image = await KnownFolders.PicturesLibrary.CreateFileAsync("TestImage.jpg", CreationCollisionOption.GenerateUniqueName);

        var imageEncodingProperties = ImageEncodingProperties.CreatePng();
        await webCam.CapturePhotoToStorageFileAsync(imageEncodingProperties, image);

        return image;
    }
    catch (Exception ex)
    {
        //Log error
    }
    return null;
}

将图像上传到Azure Blob存储

private async Task<StorageFile> UploadPictureToAzure(StorageFile image)
{
string Azure_StorageAccountName = "MYACCOUNTNAME";
string Azure_ContainerName = "MYCONTAINERNAME";
string Azure_AccessKey = "MYACCESSKEY";

    try
    {
        StorageCredentials creds = new StorageCredentials(Azure_StorageAccountName, Azure_AccessKey);
        CloudStorageAccount account = new CloudStorageAccount(creds, useHttps: true);
        CloudBlobClient client = account.CreateCloudBlobClient();
        CloudBlobContainer sampleContainer = client.GetContainerReference(Azure_ContainerName);

        CloudBlockBlob blob = sampleContainer.GetBlockBlobReference(image.Name);

        await blob.UploadFromFileAsync(image);

        return image;
    }
    catch (Exception ex)
    {
        //Log
    }

    return image;
}

调用这三种方法如下：

public void Run(IBackgroundTaskInstance taskInstance)
{
    Monitor();
}

private void Monitor()
{
    var webCam = InitialiseWebCam().Result;
    var image = TakePicture(webCam).Result;

    var output = UploadPictureToAzure(image).Result;
}

此时一切正常，图像出现在我的blob存储中。我希望此行为作为Windows IoT上的无头后台应用程序运行，因此我进行了以下调整：

public void Run(IBackgroundTaskInstance taskInstance)
{
    taskInstance.GetDeferral();

    timer = ThreadPoolTimer.CreatePeriodicTimer(Timer_Tick, TimeSpan.FromSeconds(60));
}

private void Timer_Tick(ThreadPoolTimer threadPoolTimer)
{
    Monitor();
}

根据这些更改，Monitor功能会按预期触发，但UploadPictureToAzure(image).Result的输出为空，即使 webCam 和图像按预期工作，应用程序突然退出。

上述行为的原因是什么？我期待上传图像，整个过程继续在创建的定时器循环中发生。

修改

我试图通过调整我的UploadPictureToAzure来强制同步行为：

private void UploadPictureToAzure(StorageFile image)
{
    string Azure_StorageAccountName = "ACCOUNTNAME";
    string Azure_ContainerName = "CONTAINERNAME";
    string Azure_AccessKey = "ACCESSKEY";

    try
    {
         StorageCredentials creds = new StorageCredentials(Azure_StorageAccountName, Azure_AccessKey);
         CloudStorageAccount account = new CloudStorageAccount(creds, useHttps: true);
         CloudBlobClient client = account.CreateCloudBlobClient();
         CloudBlobContainer sampleContainer = client.GetContainerReference(Azure_ContainerName);

         CloudBlockBlob blob = sampleContainer.GetBlockBlobReference(image.Name);

         var response = blob.UploadFromFileAsync(image);
         response.AsTask().Wait();
     }
     catch (Exception ex)
     {
         //Log
     }
}

现在即使调用 .Wait（），执行也会立即返回并退出应用程序。

如果我删除 ThreadPoolTimer.CreatePeriodicTimer 的使用情况，并将其替换为while循环，则图片上传时不会出现问题。

Answer 1

使用异步方法的方法是，他们倾向于抓取您的代码并将其全部入侵：）...您必须使用await关键字而不是调用Result属性，否则在尝试获取结果之前，您无法确定该函数是否已完成执行。这适用于代码中的所有异步方法。

将您的Monitor方法更改为异步并返回Task，如下所示：

private async Task Monitor()
{
    var webCam = await InitialiseWebCam();
    var image = await TakePicture(webCam);
    var output = await UploadPictureToAzure(image);
}

我猜测Windows手机在顶层有一个异步API，所以你可以等到主线程，但如果没有，here are a couple of workarounds to call async methods synchronously

编辑：

每Quickstart: Create and register a background task (XAML)您可以在Run方法上执行以下操作：

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

    ThreadPoolTimer.CreatePeriodicTimer(async(timer) => { 
        await Monitor();
        _deferral.Complete();
    }, TimeSpan.FromSeconds(60));


}

Answer 2

最后，我能够使用以下代码实现我的需求：

private readonly TimeSpan _delayDuration = TimeSpan.FromMinutes(20);

public void Run(IBackgroundTaskInstance taskInstance)
{
    taskInstance.GetDeferral();

    do
    {
        try
        {
            Monitor().Wait();
        }
        /* SNIP */
    }
    while (true);

private async Task Monitor()
{
    var webCam = await InitialiseWebCam();
    var image = await TakePicture(webCam);
    await UploadPictureToAzure(image);
    await Task.Delay(_delayDuration);
}


private async Task UploadPictureToAzure(StorageFile image)
    {
        var Azure_StorageAccountName = "accountname";
        var Azure_ContainerName = "container";
        var Azure_AccessKey = "key";
        try
        {
            StorageCredentials creds = new StorageCredentials(Azure_StorageAccountName, Azure_AccessKey);
            CloudStorageAccount account = new CloudStorageAccount(creds, true);
            CloudBlobClient client = account.CreateCloudBlobClient();
            CloudBlobContainer sampleContainer = client.GetContainerReference(Azure_ContainerName);
        CloudBlockBlob blob = sampleContainer.GetBlockBlobReference(image.Name);

        await blob.UploadFromFileAsync(image);
    }
    catch (Exception ex)
    {
        /* Logging */
        throw ex;
    }
}

private async Task<MediaCapture> InitialiseWebCam()
{
    var webCam = new MediaCapture();

    try
    {
        await webCam.InitializeAsync();
    }
    catch (Exception ex)
    {
       /* Logging */
       throw ex;
    }

    return webCam;
}

private async Task<StorageFile> TakePicture(MediaCapture webCam)
{
    try
    {
        var image = await KnownFolders.PicturesLibrary.CreateFileAsync("Image.png", CreationCollisionOption.ReplaceExisting);
        var imageEncodingProperties = ImageEncodingProperties.CreatePng();

        await webCam.CapturePhotoToStorageFileAsync(imageEncodingProperties, image);

        return image;
    }
    catch (Exception ex)
    {
        /* Logging */
        throw ex;
    }
}

Async / Await和CreatePeriodicTimer失败

2 个答案: