如何在 DropNet 中实现 CancellationToken 支持？

Question

我想在 MonoTouch 应用程序中异步访问 DropBox API。
我认为使用 DropNet 会很方便，它本身依赖于 RestSharp。

这两个库都运行良好，但返回 Tasks 的 DropNet 重载无法让您将请求与取消令牌相关联。

这是他们的实现方式：

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 的实现是基于 TaskCompletionSource 和 was 最初的 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;
    }
}

如何更改或扩展此代码以支持使用 CancellationToken 取消？
我想这样称呼它：

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

Answer 1

因为CancellationToken是一个值类型，我们可以将它作为可选参数添加到具有默认值的API中，避免空检查，这很甜蜜。

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);
}

现在，RestSharp ExecuteAsync 方法返回封装了底层HttpWebRequest 的RestRequestAsyncHandle，以及Abort 方法。这就是我们取消事物的方式。

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;
}

最后，Lauren's implementation 将请求放入线程池，但我看不出这样做的理由——ExecuteAsync 本身是异步的。所以我不这样做。

这就是取消 DropNet 操作。

我还做了一些可能对您有用的调整。

因为我无法安排 DropBox Tasks 而不求助于将 ExecuteTask 调用包装到另一个 Tasks 中，所以我决定为请求找到最佳并发级别，结果证明是 4 用于我，并明确设置：

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

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

我也满足于让未处理的任务异常在地狱中腐烂，所以我就是这样做的：

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

最后一个观察是您不应该在DropNet 返回的任务上调用Start()，因为该任务会立即开始。如果您不喜欢这样，则需要将ExecuteTask 包装在另一个不受TaskCompletionSource 支持的“真实”任务中。