我该如何处理这个windowsphone网络客户端异步问题

本文关键字：windowsphone 网络 客户端 问题 异步 何处理 处理 | 更新日期: 2024-10-19 00:15:39

我有一个方法如下：

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

另一类如下

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总是为零，所以我相信函数调用"GetRateForCurrency"会在调用异步回调之前返回。我如何确保这种情况不会发生，因为我需要在返回变量ExchangeRate之前进行设置。谢谢此外，我注意到回调永远不会被调用，因为我在其中有一个断点，还有一个没有被调用的异常。所以我不知道问题出在哪里。任何帮助都很感激。

我该如何处理这个windowsphone网络客户端异步问题

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;
    }
}

EDIT:使用异步模式的同一类

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;
    }
}

消耗异步类：

// 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.
            });
    }
}

也许您将不得不将UI更改发送到UI线程。我这里没有WP框架来证实这一点，但我认为确实如此。

当您运行异步方法时，您将在完成的方法中得到结果，即

StringDownloadCompleted

因此，在您的代码中，您调用async方法并立即返回ExchangeRate，它将始终为0。

您必须在已完成的方法StringDownloadCompleted 中获取ExchangeRate

如果你想在GetRateForCurrency中获得汇率，请同步调用

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

以下是您需要做的操作。

您在类中创建了一个事件。在您的代码中，您启动异步web客户端调用。一旦调用完成，就可以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。

一旦设置了事件，调用者就会收到通知。

如果你想要一个例子，可以在我的帖子中查看来源http://invokeit.wordpress.com/2012/06/30/bing-mapcontrol-offline-tiles-solution-wpdev-wp7dev/

它是bing-maps示例的扩展，并且包含地址查找器类。看看它是如何被激发的，以及客户端是如何得到通知的

调用uri 后可以等待

公共十进制GetRateForCurrency（字符串fromCurrency，字符串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webcontrol的标签并进行

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

您唯一的选择是使GetRateForCurrency也异步（这意味着引发自己的Completed事件）。

如果支持TPL，您可以使用Task<T>作为一种很好的方式来将异步封装到链中，但不幸的是，WP7不支持它。

作为一种替代方案，以及我所做的，是使用反应扩展（Microsoft.Phone.Reactive）并将IObservable传递到链的下游——然而，如果你只想在这一种情况下使用Rx，那么它还有很多需要学习的地方。

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