异步等待 web api 文件 io

Question

我在 Web api 控制器中使用以下异步代码来处理 XML 文件。一切都按预期工作，但是这是 async/await 方法的正确使用。我基本上是从 XML 文件中提取所有图像，然后将它们保存到磁盘。我想尝试最小化文件 io 的影响。

public async Task<HttpResponseMessage> PostFile()
{
    await Task.WhenAll(this.ProcessProofOfPressenceImages(address, images, surveyReference), this.ProcessSketchImages(propertyPlans, images, surveyReference), this.ProcessExteriorImages(exteriorSketch, images, surveyReference));
    //db code
}

private async Task ProcessProofOfPressenceImages(Dictionary<object, object> container, List<Dictionary<string, string>> images, string surveyReference)
{
    if(images != null)
    {
        await Task.WhenAll(this.ProcessImagesHelper(container, images, surveyReference, "propertyImage"));
    }
}

private async Task ProcessSketchImages(Dictionary<object, object> container, List<Dictionary<string, string>> images, string surveyReference)
{
    if(images != null)
    {
        await Task.WhenAll(this.ProcessImagesHelper(container, images, surveyReference, "sketchPlanImage"), this.ProcessImagesHelper(container, images, surveyReference, "sketchFrontImage"), this.ProcessImagesHelper(container, images, surveyReference, "sketchRearImage"), this.ProcessImagesHelper(container, images, surveyReference, "sketchLeftSideImage"), this.ProcessImagesHelper(container, images, surveyReference, "sketchRightSideImage"));
    }
}

private async Task ProcessExteriorImages(Dictionary<object, object> container, List<Dictionary<string, string>> images, string surveyReference)
{
    List<Task> tasks = new List<Task>();

    if(images != null)
    {
        await Task.WhenAll(this.ProcessImagesHelper(container, images, surveyReference, "image1"), this.ProcessImagesHelper(container, images, surveyReference, "image2"), this.ProcessImagesHelper(container, images, surveyReference, "image3"), this.ProcessImagesHelper(container, images, surveyReference, "image4"), this.ProcessImagesHelper(container, images, surveyReference, "image5"), this.ProcessImagesHelper(container, images, surveyReference, "image6"));
    }
}

private async Task ProcessImagesHelper(Dictionary<object, object> container, List<Dictionary<string, string>> images, string surveyReference, string image)
{
    if(container.ContainsKey(image) && !String.IsNullOrEmpty(container[image].ToString()))
    {
        using(MemoryStream memoryStream = new MemoryStream((byte[])container[image]))
        {
            string url = String.Format(@"{0}{1}{2}_{3}.jpg", EcoConfiguration.Instance.RootUrl, EcoConfiguration.Instance.SurveyImageRootUrl, surveyReference, image.SplitOnCapital("_"));

            using(FileStream fileStream = new FileStream(url, FileMode.Create, FileAccess.Write))
            {
                Dictionary<string, string> imageDetails = new Dictionary<string, string>();
                imageDetails.Add("TypeId", ((int)SurveyImageType.Exterior).ToString());
                imageDetails.Add("ImageUrl", url);
                if(container.ContainsKey(image + "Description"))
                {
                    imageDetails.Add("Description", container[image + "Description"].ToSafeString());
                }
                images.Add(imageDetails);
                await memoryStream.CopyToAsync(fileStream);
            }
        }
    }
}

非常欢迎任何 cmets/建议。

Answer 1

文件流的棘手之处在于您需要将isAsync: true 或FileOptions.Asynchronous 传递给构造函数/工厂方法才能获得真正的异步流。如果你不这样做，那么底层文件流实际上是阻塞的，异步方法只是使用线程池来伪造异步操作。

在您的代码中让我印象深刻的另一件事是您对async 进行了一些不必要的使用。 async 应该仅在您需要时使用。例如，这种方法：

private async Task ProcessProofOfPressenceImages(Dictionary<object, object> container, List<Dictionary<string, string>> images, string surveyReference)
{
  if(images != null)
  {
    await Task.WhenAll(this.ProcessImagesHelper(container, images, surveyReference, "propertyImage"));
  }
}

可以写成：

private Task ProcessProofOfPressenceImages(Dictionary<object, object> container, List<Dictionary<string, string>> images, string surveyReference)
{
  if(images != null)
  {
    return Task.WhenAll(this.ProcessImagesHelper(container, images, surveyReference, "propertyImage"));
  }

  return Task.FromResult<object>(null);
}

这为您节省了不必要的状态机。同样的建议也适用于ProcessSketchImages 和ProcessExteriorImages。

关于ProcessImagesHelper，它看起来不错，虽然我不确定你为什么需要MemoryStream。 （异步）将字节数组写入磁盘同样容易。

如果您对async 性能提示感兴趣，请联系Stephen Toub has an excellent video。