c# - 如何在来自 View 模型的异步 Web 客户端调用中使用进度条

Question

我正在尝试在 WP7 项目中使用性能进度条，但我在异步 webclient 调用方面遇到了问题。我的代码如下:

更新

    public MainPage()
    {
        InitializeComponent();
         ...................

        this.Loaded += new RoutedEventHandler(MainPage_Loaded);}

    private void MainPage_Loaded(object sender, RoutedEventArgs e)
    {
        if (!App.ViewModel.IsDataLoaded)
        {
            App.ViewModel.LoadData();
        }
    }

还有我实现 LoadData 函数的 ViewModel

    private bool _showProgressBar = false;
    public bool ShowProgressBar
    {
        get { return _showProgressBar; }
        set
        {
            if (_showProgressBar != value)
            {
                _showProgressBar = value;
                NotifyPropertyChanged("ShowProgressBar");
            }
        }
    public void LoadData()
    {
        try
        {
            string defaulturl = "http://....";
            WebClient client = new WebClient();
            Uri uri = new Uri(defaulturl);
            client.DownloadStringCompleted += new DownloadStringCompletedEventHandler(client_DownloadStringCompleted);
            ShowProgressBar = true;
            client.DownloadStringAsync(uri);
        }
        catch (Exception ex)
        {
            MessageBox.Show(ex.Message);
        }

        this.IsDataLoaded = true;

    }

    void client_DownloadStringCompleted(object sender, DownloadStringCompletedEventArgs e)
    {
        try
        {
            //fetch the data
           ShowProgressBar = false;
        }

        }
        catch (Exception ex)
        {
            MessageBox.Show(ex.Message);
        }

    }

    public event PropertyChangedEventHandler PropertyChanged;
    private void NotifyPropertyChanged(String propertyName)
    {.....
    } 

主页 Xaml

    <toolkit:PerformanceProgressBar Margin="0,-12,0,0" x:Name="performanceProgressBar" IsIndeterminate="true" Visibility="{Binding ShowProgressBar, Converter={StaticResource BooleanToVisibilityConverter}}"/>

我的问题是，因为WebClient在执行时是一个异步方法，LoadData已经执行了，我不知道把performanceProgressBar.Visibility放在哪里

如有任何帮助，我们将不胜感激。谢谢!

Answer 1

你在评论中解释得更多了，我明白了更多。听起来您想将 bool 属性绑定(bind)到进度条的可见性。您需要一个 bool 值到可见性的转换器(用谷歌搜索，很容易找到)。

然后你可以这样做:

private bool _showProgressBar = false;
public bool ShowProgressBar
{
    get { _return _showProgressBar; }
    set 
    { 
        _return _showProgressBar;
        OnPropertyChanged("ShowProgressBar");
    }
}

public void LoadData()
{
    try
    {
        string defaulturl = "http://....";
        WebClient client = new WebClient();
        Uri uri = new Uri(defaulturl);
        client.DownloadStringCompleted += new DownloadStringCompletedEventHandler(client_DownloadStringCompleted);

        ShowProgressBar = true;

        client.DownloadStringAsync(uri);
    }
    catch (Exception ex)
    {
        MessageBox.Show(ex.Message);
    }

    this.IsDataLoaded = true;

}

void client_DownloadStringCompleted(object sender, DownloadStringCompletedEventArgs e)
{
    try
    {
        //fetch the data
        ShowProgressBar = false;
    }

    }
    catch (Exception ex)
    {
        MessageBox.Show(ex.Message);
    }

}

在 View 上:

<MyProgressBar Visibility={Binding Path=ShowProgressBar, Converter={StaticResource MyBoolToVisibleConverter}>

显然，MSFT 已经为您提供了一个转换器...这对我来说是个新闻:http://msdn.microsoft.com/en-us/library/system.windows.controls.booleantovisibilityconverter.aspx