使用 Rx 简化异步 Silverlight Web 服务请求

Question

我使用 Rx 为我的 WCF Web 服务编写了一个简化的 Silverlight 客户端库，但是我注意到有时我会丢失已完成的事件。

public IObservable<XElement> GetReport(string reportName)
{
    return from client in Observable.Return(new WebServiceClient())
           from request in Observable.ToAsync<string>(client.GetReportDataAsync)(reportName)
           from result in Observable.FromEvent<GetReportDataCompletedEventArgs>(client, "GetReportDataCompleted").Take(1)
           from close in this.CloseClient(client)
           select result.EventArgs.Result;
}

我认为问题是由于 Web 服务在订阅完成事件之前被调用并返回造成的。我不知道如何让 Rx 在 Async 调用之前订阅事件。我试过 StartWith 但这要求输入和输出类型相同，有什么想法吗？

Answer 1

似乎最好的答案是使用 Observable.CreateWithDisposable()

例如

public IObservable<XElement> GetReport(string reportName)
{
    return from client in Observable.Return(new WebServiceClient())
            from completed in Observable.CreateWithDisposable<GetReportDataCompletedEventArgs>(observer =>
                {
                    var subscription = Observable.FromEvent<GetReportDataCompletedEventArgs>(client, "GetReportDataCompleted")
                        .Take(1)
                        .Select(e => e.EventArgs)
                        .Subscribe(observer);
                    client.GetReportDataAsync(reportName);
                    return subscription;
                })
            from close in this.CloseClient(client)
            select completed.Result;
}

为了使它更易于使用，我将 CreateWithDisposable 重构为一个通用函数，可用于我的所有 Web 服务调用，包括根据事件 args 类型自动确定事件名称：

private IObservable<T> CallService<T>(ICommunicationObject serviceClient, Action start) where T : AsyncCompletedEventArgs
{
    if (typeof(T) == typeof(AsyncCompletedEventArgs))
    {
        throw new InvalidOperationException("Event arguments type cannot be used to determine event name, use event name overload instead.");
    }

    string completedEventName = typeof(T).Name.TrimEnd("EventArgs");
    return CallService<T>(serviceClient, start, completedEventName);
}

private IObservable<T> CallService<T>(ICommunicationObject serviceClient, Action start, string completedEventName) where T : AsyncCompletedEventArgs
{
    return Observable.CreateWithDisposable<T>(observer =>
    {
        var subscription = Observable.FromEvent<T>(serviceClient, completedEventName).Take(1).Select(e => e.EventArgs).Subscribe(observer);
        start();
        return subscription;
    });
}

// Example usage:
public IObservable<XElement> GetReport(string reportName)
{
    return from client in Observable.Return(new WebServiceClient())
            from completed in this.CallService<GetReportDataCompletedEventArgs>(client, () => client.GetReportDataAsync(reportName))
            from close in this.CloseClient(client)
            select completed.Result;
}

/// <summary>
/// Asynchronously closes the web service client
/// </summary>
/// <param name="client">The web service client to be closed.</param>
/// <returns>Returns a cold observable sequence of a single success Unit.</returns>
private IObservable<AsyncCompletedEventArgs> CloseClient(WebServiceClient client)
{
    return this.CallService<AsyncCompletedEventArgs>(client, client.CloseAsync, "CloseCompleted");
}

希望这对其他人有帮助！

Answer 2

我需要使用一般的WebClient.DownloadStringAsync 所以这里是我的版本。

首先，包装事件：

public static IObservable<IEvent<DownloadStringCompletedEventArgs>>
    GetDownloadStringObservableEvent(this WebClient wc)
{
    return Observable.FromEvent<DownloadStringCompletedEventArgs>(
        wc, "DownloadStringCompleted");
}

然后创建扩展方法：

public static IObservable<string> GetDownloadString(this WebClient wc, Uri uri)
{
    return Observable.CreateWithDisposable<string>(
        observer => {
            // Several downloads may be going on simultaneously. The token allows
            // us to establish that we're retrieving the right one.
            Guid token = Guid.NewGuid();
            var stringDownloaded = wc.GetDownloadStringObservableEvent()
                    .Where(evt => ((Guid)evt.EventArgs.UserState) == token)
                    .Take(1);        //implicitly unhooks handler after event is received
            bool errorOccurred = false;
            IDisposable unsubscribe =
                stringDownloaded.Subscribe(
                    // OnNext action
                    ev => {
                        // Propagate the exception if one is reported.
                        if (ev.EventArgs.Error != null) {
                            errorOccurred = true;
                            observer.OnError(ev.EventArgs.Error);
                        } else if (!ev.EventArgs.Cancelled) {
                            observer.OnNext(ev.EventArgs.Result);
                        }
                    },
                    // OnError action (propagate exception)
                    ex => observer.OnError(ex),
                    // OnCompleted action
                    () => {
                        if (!errorOccurred) {
                            observer.OnCompleted();
                        }
                    });
            try {
                wc.DownloadStringAsync(uri, token);
            } catch (Exception ex) {
                observer.OnError(ex);
            }
            return unsubscribe;
        }
    );
}

用法很简单：

wc.GetDownloadString(new Uri("http://myservice"))
    .Subscribe(resultCallback , errorCallback);