WPF 简单异步模式

以WeatherForecast为例. 需求: 用户在窗体上点击一个按钮, 程序去网络上查询天气情况, 并把结果显示在窗体上. 网络查询是一个耗时任务, 在等待结果的同时, 用户将看到一个旋转的时钟动画表示程序正在查询.

模式为:

1. 窗口类MainWindow中有耗时函数: string FetchWeatherFromInternet().
2. 窗口类MainWindow中包含一个函数 UpdateUIWhenWeatherFetched(string result), 用于把任务结果显示在界面上.
3. 当用户点击按钮时, 在 btnFetchWeather_Click() 中,
   
   如果是同步调用, 代码很简单, 如下:
   string result = this.FetchWeatherFromInternet();
   this.UpdateUserInterface( result );
   
   现在需要异步执行, 稍微麻烦点, 需要用到3个delegate, 其中一个代表要执行的任务, 另一个代表任务结束后的callback, 还有一个代表交给UI执行的任务, 上述代码被替换成如下:
   
   // 代表要执行的异步任务
   Func<string> asyncAction = this.FetchWeatherFromInternet();
   
   // 处理异步任务的结果
   Action<IAsyncResult> resultHandler = delegate( IAsyncResult asyncResult )
   {
            //获得异步任务的返回值, 这段代码必须在UI线程中执行
            string weather = asyncAction.EndInvoke( asyncResult );
            this.UpdateUIWhenWeatherFetched( weather );
   };
   
   //代表异步任务完成后的callback
   AsyncCallback asyncActionCallback = delegate( IAsyncResult asyncResult )
   {
           this.Dispatcher.BeginInvoke( DispatcherPriority.Background, resultHandler, asyncResult );
   };
   
   //这是才开始执行异步任务
   asyncAction.BeginInvoke( asyncActionCallback, null );

=======================================================================

private void ForecastButtonHandler(object sender, RoutedEventArgs e)
{
    this.UpdateUIWhenStartFetchingWeather();

    //异步任务封装在一个delegate中, 此delegate将运行在后台线程 
    Func<string> asyncAction = this.FetchWeatherFromInternet;

//在UI线程中得到异步任务的返回值，并更新UI
//必须在UI线程中执行

    Action<IAsyncResult> resultHandler = 

delegate(IAsyncResult asyncResult)
{
string weather = asyncAction.EndInvoke(asyncResult);
this.UpdateUIWhenWeatherFetched(weather);
};

    //异步任务执行完毕后的callback, 此callback运行在后台线程上. 
    //此callback会异步调用resultHandler来处理异步任务的返回值.
    AsyncCallback asyncActionCallback = delegate(IAsyncResult asyncResult)
    {
        this.Dispatcher.BeginInvoke(DispatcherPriority.Background, resultHandler, asyncResult);
    };

    //在UI线程中开始异步任务, 
    //asyncAction(后台线程), asyncActionCallback(后台线程)和resultHandler(UI线程)
    //将被依次执行
    asyncAction.BeginInvoke(asyncActionCallback, null);
}

private string FetchWeatherFromInternet()
{
    // Simulate the delay from network access.
    Thread.Sleep(4000);
    String weather = "rainy";
    return weather;
}

private void UpdateUIWhenStartFetchingWeather()
{
    // Change the status
    this.fetchButton.IsEnabled = false;
    this.weatherText.Text = "";
}

private void UpdateUIWhenWeatherFetched(string weather)
{
    //Update UI text
    this.fetchButton.IsEnabled = true;
    this.weatherText.Text = weather;
}