如何使这个Observable更可重用?

时间:2014-07-11 00:12:18

标签: c# system.reactive

这是一个可观察的序列,用于从Web服务检索分页数据。每个Web服务响应都包含一个nextRecordsUrl,指示从哪里获取下一组记录。将此Observable转换为更可重用的东西的最佳方法是什么?

网络服务设置:

var auth = new AuthenticationClient ();
await auth.UsernamePasswordAsync (consumerKey, consumerSecret, userName, password + passwordSecurityToken);
var forceClient = new ForceClient (auth.InstanceUrl, auth.AccessToken, auth.ApiVersion);

The Observable:

var observable = Observable.Create<QueryResult<Account>> (async (IObserver<QueryResult<Account>> o) =>
{
    try
    {
        var queryResult = await forceClient.QueryAsync<Account> ("SELECT Id, Name from Account");
        if (queryResult != null)
        {
            o.OnNext (queryResult);

            while (!string.IsNullOrEmpty (queryResult.nextRecordsUrl))
            {
                queryResult = await forceClient.QueryContinuationAsync<Account> (queryResult.nextRecordsUrl);
                if (queryResult != null)
                {
                    o.OnNext (queryResult);
                }
            }
        }
        o.OnCompleted ();
    }
    catch (Exception ex)
    {
        o.OnError (ex);
    }

    return () => {};
});

订阅观察结果并收集结果:

   var accounts = new List<Account> ();
    observable.Subscribe (
        observer => accounts.AddRange (observer.records),
        ex => Console.WriteLine (ex.Message),
        () => {});

编辑:使用Brandon的解决方案我现在可以使用Aggregate生成结果列表

List<Account> accounts = await forceClient.QueryPages<Account> ("SELECT Id, Name from Account")
                .Aggregate (new List<Account> (), (list, value) =>
                {
                    list.AddRange (value.records);
                    return list;
                });

2 个答案:

答案 0 :(得分:2)

这是你想要的东西吗?

public static IObservable<QueryResult<TResult>> QueryPages<TResult>(this ForceClient forceClient, string query)
{
    return Observable.Create<QueryResult<T>> (async (observer, token) =>
    {
        // No need for try/catch.  Create() will call OnError if your task fails.
        // Also no need for OnCompleted().  Create() calls it when your task completes
        var queryResult = await forceClient.QueryAsync<TResult> (query);
        while (queryResult != null)
        {
            observer.OnNext (queryResult);

            // check the token *after* we call OnNext
            // because if an observer unsubscribes
            // it typically occurs during the notification
            // e.g. they are using .Take(..) or
            // something.
            if (string.IsNullOrEmpty(queryResult.nextRecordsUrl) ||
                token.IsCancellationRequested)
            {
                break;
            }

            queryResult = await forceClient.QueryContinuationAsync<TResult> (queryResult.nextRecordsUrl);
        }
        // No need to return anything.  Just the Task itself is all that Create() wants.
    }
});

// Usage:
var forceClient = // ...
var foos = forceClient.QueryPages<Foo>("SELECT A, B, C FROM Foo");

注意我将其切换到提供取消令牌的重载,以便在观察者取消订阅时您可以停止抓取页面(即使观察者已经停止侦听,您的原始版本也会继续抓取页面)。另请注意,异步Create等待您的Task并为您拨打OnErrorOnCompleted,因此您无需担心大部分时间。

答案 1 :(得分:2)

信不信由你,Rx-Expiremental库(也由MS维护)有一个名为Expand的运算符。 Expand用于从一个observable中获取每个元素,并通过一个函数运行它,该函数生成另一个相同类型的observable。那个可观察的东西然后被压平成原始的,并且每个项目都经历了相同的过程。

想象一下,给定一个带有可观察子节点的树节点。您可以使用expand来轻松遍历此树。由于链表只是树的约束版本,并且由于您拥有的实际上是每个节点都是可观察的链表,因此您可以使用expand。

public static IObservable<QueryResult<TResult>> QueryPages<TResult>(this ForceClient forceClient, string query)
{
    return Observable.FromAsync(() => forceClient.QueryAsync<TResult>(query))
        .Where(QueryResultIsValid)
        .Expand(result =>
            Observable.FromAsync(() => forceClient.QueryContinuationAsync<TResult>(queryResult.nextRecordsUrl))
                .Where(QueryResultIsValid)
        );
}

public static bool QueryResultIsValid(QueryResult<TResult> result)
{
    return result != null;
}