How to implement CancellationToken support in DropNet?

Question

I want to asynchronously access DropBox API in a MonoTouch app.
I thought it would be convenient to use DropNet which itself relies on RestSharp.

Both libraries work well but DropNet overloads that return Tasks don't give you a way to associate requests with cancellation tokens.

This is how their implementation looks:

public Task<IRestResponse> GetThumbnailTask(string path, ThumbnailSize size)
{
    if (!path.StartsWith("/")) path = "/" + path;
    var request = _requestHelper.CreateThumbnailRequest(path, size, Root);
    return ExecuteTask(ApiType.Content, request, cancel);
}

ExecuteTask implementation is based on TaskCompletionSource and was originally written by Laurent Kempé:

public static class RestClientExtensions
{
    public static Task<TResult> ExecuteTask<TResult>(
        this IRestClient client, IRestRequest request
        ) where TResult : new()
    {
        var tcs = new TaskCompletionSource<TResult>();

        WaitCallback asyncWork = _ => {
            try {
                client.ExecuteAsync<TResult>(request,
                    (response, asynchandle) => {
                        if (response.StatusCode != HttpStatusCode.OK) {
                            tcs.SetException(new DropboxException(response));
                        } else {
                            tcs.SetResult(response.Data);
                        }
                    }
                );
            } catch (Exception exc) {
                    tcs.SetException(exc);
            }
        };

        return ExecuteTask(asyncWork, tcs);
    }


    public static Task<IRestResponse> ExecuteTask(
        this IRestClient client, IRestRequest request
        )
    {
        var tcs = new TaskCompletionSource<IRestResponse>();

        WaitCallback asyncWork = _ => {
            try {
                client.ExecuteAsync(request,
                    (response, asynchandle) => {
                        if (response.StatusCode != HttpStatusCode.OK) {
                            tcs.SetException(new DropboxException(response));
                        } else {
                            tcs.SetResult(response);
                        }
                    }
                );
            } catch (Exception exc) {
                    tcs.SetException(exc);
            }
        };

        return ExecuteTask(asyncWork, tcs);
   }

    private static Task<TResult> ExecuteTask<TResult>(
        WaitCallback asyncWork, TaskCompletionSource<TResult> tcs
        )
    {
        ThreadPool.QueueUserWorkItem(asyncWork);
        return tcs.Task;
    }
}

How do I change or extend this code to support cancellation with CancellationToken?
I'd like to call it like this:

var task = dropbox.GetThumbnailTask(
    "/test.jpg", ThumbnailSize.ExtraLarge2, _token
);

score 1 · Accepted Answer · answered Nov 12 '12 at 15:52

Because CancellationToken is a value type, we can add it as an optional parameter to the APIs with default value and avoid null checks, which is sweet.

public Task<IRestResponse> GetThumbnailTask(
    string path, ThumbnailSize size, CancellationToken cancel = default(CancellationToken)
) {
    if (!path.StartsWith("/")) path = "/" + path;
    var request = _requestHelper.CreateThumbnailRequest(path, size, Root);
    return ExecuteTask(ApiType.Content, request, cancel);
}

Now, RestSharp ExecuteAsync method returns RestRequestAsyncHandle that encapsulates underlying HttpWebRequest, along with Abort method. This is how we cancel things.

public static Task<TResult> ExecuteTask<TResult>(
    this IRestClient client, IRestRequest request, CancellationToken cancel = default(CancellationToken)
    ) where TResult : new()
{
    var tcs = new TaskCompletionSource<TResult>();
    try {
        var async = client.ExecuteAsync<TResult>(request, (response, _) => {
            if (cancel.IsCancellationRequested || response == null)
                return;

            if (response.StatusCode != HttpStatusCode.OK) {
                tcs.TrySetException(new DropboxException(response));
            } else {
                tcs.TrySetResult(response.Data);
            }
        });

        cancel.Register(() => {
            async.Abort();
            tcs.TrySetCanceled();
        });
    } catch (Exception ex) {
        tcs.TrySetException(ex);
    }

    return tcs.Task;
}

public static Task<IRestResponse> ExecuteTask(this IRestClient client, IRestRequest request, CancellationToken cancel = default(CancellationToken))
{
    var tcs = new TaskCompletionSource<IRestResponse>();
    try {
        var async = client.ExecuteAsync<IRestResponse>(request, (response, _) => {
            if (cancel.IsCancellationRequested || response == null)
                return;

            if (response.StatusCode != HttpStatusCode.OK) {
                tcs.TrySetException(new DropboxException(response));
            } else {
                tcs.TrySetResult(response);
            }
        });

        cancel.Register(() => {
            async.Abort();
            tcs.TrySetCanceled();
        });
    } catch (Exception ex) {
        tcs.TrySetException(ex);
    }

    return tcs.Task;
}

Finally, Lauren's implementation puts requests in thread pool but I don't see a reason to do so—ExecuteAsync is asynchronous itself. So I don't do that.

And that's it for cancelling DropNet operations.

I also made a few tweaks which may be useful to you.

Because I had no way of scheduling DropBox Tasks without resorting to wrapping ExecuteTask calls into another Tasks, I decided to find an optimal concurrency level for requests, which turned out to be 4 for me, and set it explicitly:

static readonly Uri DropboxContentHost = new Uri("https://api-content.dropbox.com");

static DropboxService()
{
    var point = ServicePointManager.FindServicePoint(DropboxContentHost);
    point.ConnectionLimit = 4;
}

I was also content with letting unhandled task exceptions rot in hell, so that's what I did:

TaskScheduler.UnobservedTaskException += (sender, e) => {
    e.SetObserved();
};

One last observation is you shouldn't call Start() on a task returned by DropNet because the task starts right away. If you don't like that, you'll need to wrap ExecuteTask in yet another “real” task not backed by TaskCompletionSource.

How to implement CancellationToken support in DropNet?

1 Answers1