Question

我有一个方法如下：

      public decimal GetExchangeRate(string fromCurrency, string toCurrency)
      {
            GoogleCurrencyService googleCurrencyService = new GoogleCurrencyService();
            return googleCurrencyService.GetRateForCurrency(fromCurrency, toCurrency);

      }

和另一个课程如下

public class GoogleCurrencyService
{
    public decimal GetRateForCurrency(string fromCurrency, string toCurrency)
    {

        try
        {
            WebClient client = new WebClient();
            client.DownloadStringCompleted += new DownloadStringCompletedEventHandler(StringDownloadCompleted);
            client.DownloadStringAsync(new Uri(_requestUri + fromCurrency + "=?" + toCurrency));

        }
        catch (Exception)
        {
            ExchangeRate = 0;
        }

        return ExchangeRate;
    }

    private void StringDownloadCompleted(object sender, DownloadStringCompletedEventArgs e)
    {
        _response = e.Result;
        ExchangeRate = ParseResponseAndGetExchangeRate();
    }

}//class GoogleCurrencyService

变量ExchangeRate总是为0，所以我相信在调用异步回调之前函数调用“GetRateForCurrency”会返回。我如何确保不会发生这种情况，因为我需要在返回之前设置变量ExchangeRate。谢谢。此外，我注意到回调永远不会被调用，因为我有一个断点，并且异常也没有被调用。所以我不知道问题出在哪里。任何帮助都表示赞赏。

Answer 1

您可以使用事件等待句柄来阻止当前线程并等待异步调用...

public class GoogleCurrencyService
{
    private const string RequestUri = "http://www.google.com/ig/calculator?hl=en&q=1{0}%3D%3F{1}";

    public decimal ExchangeRate { get; private set; }

    public decimal GetRateForCurrency(string fromCurrency, string toCurrency)
    {
        ExchangeRate = 0;
        // use a signaler to block this thread and wait for the async call.
        var signaler = new ManualResetEvent(false);
        try
        {
            var client = new WebClient();
            client.DownloadStringCompleted += StringDownloadCompleted;
            // pass the signaler as user token
            client.DownloadStringAsync(new Uri(String.Format(RequestUri, fromCurrency, toCurrency)), signaler);

            // wait for signal, it will be set by StringDownloadCompleted
            signaler.WaitOne();
        }
        finally
        {
            signaler.Dispose();
        }

        return ExchangeRate;
    }

    private void StringDownloadCompleted(object sender, DownloadStringCompletedEventArgs e)
    {
        try
        {
            ExchangeRate = ParseResponseAndGetExchangeRate(e.Result);
        }
        finally
        {
            // set signal
            ((ManualResetEvent)e.UserState).Set();
        }
    }

    private decimal ParseResponseAndGetExchangeRate(string result)
    {
        return 123;
    }
}

编辑：使用异步模式的同一个类

public class GoogleCurrencyService
{
    private const string RequestUri = "http://www.google.com/ig/calculator?hl=en&q=1{0}%3D%3F{1}";

    public void GetRateForCurrency(string fromCurrency, string toCurrency, Action<decimal> callback)
    {
        var client = new WebClient();
        client.DownloadStringCompleted += StringDownloadCompleted;
        // pass the callback as user token
        client.DownloadStringAsync(new Uri(String.Format(RequestUri, fromCurrency, toCurrency)), callback);
    }

    private void StringDownloadCompleted(object sender, DownloadStringCompletedEventArgs e)
    {
        // parse response to get the rate value
        var rate = ParseResponseAndGetExchangeRate(e.Result);

        // if a callback was specified, call it passing the rate.
        var callback = (Action<decimal>)e.UserState;
        if (callback != null)
            callback(rate);
    }

    private decimal ParseResponseAndGetExchangeRate(string result)
    {
        return 123;
    }
}

使用异步类：

// this is your UI form/control/whatever
public class MyUI
{
    public void OnButtonToGetRateClick()
    {
        var from = "USD"; // or read from textbox...
        var to = "EUR";

        // call the rate service
        var service = new GoogleCurrencyService();
        service.GetRateForCurrency(from, to, (rate) =>
            {
                // do stuff here to update UI.
                // like update ui.
            });
    }
}

也许您必须将UI更改分发给ui线程。我这里没有WP框架来确认是这种情况，但我认为确实如此。

Answer 2

当您运行异步方法时，您将获得

的已完成方法的结果

StringDownloadCompleted

所以在您的代码中，您调用异步方法并立即返回ExchangeRate，它始终为0。

您必须在已完成的方法StringDownloadCompleted

中获取ExchangeRate

如果你想让你的GetRateForCurrency中的ExchangeRate进行同步调用

client.DownloadString(new Uri(_requestUri + fromCurrency + "=?" + toCurrency));

Answer 3

这是你需要做的。

你在课堂上创建一个活动。在您的代码中，您将激活异步Web客户端调用。调用完成后，您将包装数据并设置事件。我倾向于定义可以保存数据的事件参数。

一旦设置了该事件，呼叫者就会收到通知。

如果你想要一个例子，请看一下我帖子中的来源 http://invokeit.wordpress.com/2012/06/30/bing-mapcontrol-offline-tiles-solution-wpdev-wp7dev/

它是bing maps样本的扩展，它包含地址查找器类。看看如何解雇它以及如何通知客户

Answer 4

ok打电话给uri后等待

public decimal GetRateForCurrency（string fromCurrency，string toCurrency） {

    try
    {
        WebClient client = new WebClient();
        client.DownloadStringCompleted += new DownloadStringCompletedEventHandler(StringDownloadCompleted);
        client.DownloadStringAsync(new Uri(_requestUri + fromCurrency + "=?" + toCurrency));
        Thread.sleep(500000); //waiting
    }
    catch (Exception)
    {
        ExchangeRate = 0;
    }

    return ExchangeRate;
}

所以设置一个类似标签的webcontrol并执行此操作

 private void StringDownloadCompleted(object sender, DownloadStringCompletedEventArgs e)
{
    _response = e.Result;
    yourlabel.Text = _response ;
    ExchangeRate = ParseResponseAndGetExchangeRate();
}

Answer 5

此处您唯一的选择是使GetRateForCurrency异步（这意味着提升自己的Completed事件）。

是否支持TPL，你可以使用Task<T>作为将异步包装在链中的好方法，但不幸的是WP7不支持它。

作为替代方案，以及我所做的，是使用Reactive Extensions（Microsoft.Phone.Reactive）并在链中传递IObservable - 但是，如果你是Rx则需要学习很多东西只会在这一场景中使用它。

我该如何处理这个windows phone webclient异步问题？

5 个答案: