Question

I am using an asynchronous operation that completes in three steps, returning partial results before completion. Originally it’s using events to signal progress, and I want to wrap it properly as an awaitable operation (or a chain of them).

To be exact, it’s the PhotoCamera.CaptureImage method in Windows Phone 7, which fires events in the following order (with relevant result types):

1. CaptureStarted, not that interesting to me
2. CaptureThumbnailAvailable (Stream)
3. CaptureImageAvailable (Stream)
4. CaptureCompleted (bool)

How to wrap this? Currently I’m using a TaskCompletionSource and calling SetResult on CaptureCompleted. It does not address steps 2 and 3, the thumbnail and image becoming available. I decided to use callbacks for those instead.

My extension method (leaving out the code for brevity):

public static Task<bool> CaptureImageAsync(this PhotoCamera photoCamera,
                                           Action<Stream> thumbnailAvailableCallback,
                                           Action<Stream> imageAvailableCallback)

This doesn’t feel entirely clean, a mixture of Tasks and callbacks. I know of IProgress<T>. However, the progress values aren’t of uniform type (Stream, Stream, bool)… and wouldn’t that stretch the IProgress<T> pattern too far?

Using multiple Task<T> that are awaited in order is an option, but that supposes a specific order that I’d also have to enforce somehow. Maybe each task result could contain the “next” task as a member:

ThumbnailResult thumbnailResult = await photoCamera.CaptureImageAsync();
...
ImageResult imageResult = await thumbnailResult.NextTask;
...
bool finalResult = await imageResult.NextTask;

I still think that I’m somewhat off the mark here. Ideas?

Answer 1

Personally i’ll create a class for it with 3 methods returning Task<T>. The caller will be able to call the 3 methods in any order he wants and it’ll always work. if GetFinalResultAsync is called first and then GetThumbnailAsync then only consequence is that the second Task<T> will aready have a result when await is used and it will return synchronously.

var photoCapture = photoCamera.BeginCapture();

var thumbnailResult = await photoCapture.GetThumbnailAsync();
var imageResult = await photoCapture.GetImageAsync();
var finalResult = await photoCapture.GetFinalResultAsync();

To wait for everything :

var thumbnailTask = photoCapture.GetThumbnailAsync();
var imageTask = photoCapture.GetImageAsync();
var finalTask = photoCapture.GetFinalResultAsync();

await Task.WhenAll(thumbnailTask, imageTask, finalTask);
DoWork(thumbnailTask.Result, imageTask.Result, finalTask.Result);