根据另一个下拉框的选择填充一个下拉框

Question

我正在学习WPF MVVM，并且一直在努力解决我认为很简单的问题，但是自己无法解决。

我的目标是能够在已填充的ComboBox中选择一个项目，然后根据该选择填充另一个ComboBox。我似乎无法在选择后加载第二个ComboBox。

我已经包含了一个具有2个ComboBox和1个TextBlock的示例。当我运行应用程序并在第一个ComboBox中选择一个项目时，基于绑定，TextBlock会更新，但是我不知道在哪里调用代码以更新第二个ComboBox。

我尝试将调用添加到SelectedString属性setter中，但是那似乎从未被调用过。我肯定错过了什么简单的东西，但我需要有人帮助我揭开谜团！

我已经尝试了其他帖子上的建议，但到目前为止还没有成功。

以下是视图的XAML：

<Window x:Class="ThrowAwayMVVMApp.Views.MainView"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Title="Main Window" Height="400" Width="800">

    <DockPanel>
        <Grid>
            <Grid.RowDefinitions>
                <RowDefinition Height="60*" />
                <RowDefinition Height="282*" />
            </Grid.RowDefinitions>
            <!-- Add additional content here -->
            <ComboBox ItemsSource="{Binding MyStrings}" SelectedItem="{Binding Path=SelectedString, Mode=TwoWay}" Height="23" HorizontalAlignment="Left" Margin="18,24,0,0" Name="comboBox1" VerticalAlignment="Top" Width="204" />
            <TextBlock Text="{Binding SelectedString}" Height="23" HorizontalAlignment="Left" Margin="276,24,0,0" Name="textBlock1" VerticalAlignment="Top" Width="227" Background="#FFFAE7E7" />
            <ComboBox ItemsSource="{Binding ResultStrings}"  Height="23" HorizontalAlignment="Left" Margin="543,25,0,0" Name="comboBox2" VerticalAlignment="Top" Width="189" />
        </Grid>
    </DockPanel>
</Window>

这是视图模型:

public class MainViewModel : ViewModelBase
{
    public MainViewModel()
    {
        this.MyStrings = new ObservableCollection<string>
            {
                "One",
                "Two",
                "Three",
                "Four",
                "Five"
            };
    }

    public ObservableCollection<string> MyStrings { get; set; }
    public ObservableCollection<string> ResultStrings { get; set; }

    public string SelectedString
    {
        get { return (string)GetValue(SelectedStringProperty); }
        set 
        { 
            SetValue(SelectedStringProperty, value);
            this.ResultStrings = getResultStrings(value);
        }
    }

    // Using a DependencyProperty as the backing store for SelectedString.  This enables animation, styling, binding, etc...
    public static readonly DependencyProperty SelectedStringProperty =
        DependencyProperty.Register("SelectedString", typeof(string), typeof(MainViewModel), new UIPropertyMetadata(""));


    private ObservableCollection<string> getResultStrings(string input)
    {
        ObservableCollection<string> result = null;

        if (input == "Three")
        {
            result = new ObservableCollection<string> { "Six", "Seven", "Eight" };
        }
        else
        {
            result = new ObservableCollection<string> { "Nine", "Ten", "Eleven" };
        }

        return result;
    }
}

- Chris Dunaway

---

为什么你在ViewModel中声明了一个DP，通常它应该放在你绑定的控件中？ - Agies

Answer 1

所选字符串依赖属性的实现有误：您应该注册PropertyChangedCallback以在直接访问DP而非CLR属性设置时得到通知（请参见http://msdn.microsoft.com/en-us/library/system.windows.propertychangedcallback.aspx）；这样，即使直接使用DP，您也可以更改相关集合。

当绑定依赖属性（如SelectedString）时，WPF绑定不使用CLR属性设置器，因此永远不会调用getResultStrings。

顺便说一下，我认为在视图模型上使用POCO方法，实现INotifyPropertyChanged：DP很难编写，会给VM添加大量噪音（除了对System.Windows的恶意依赖）。

请查看此博客文章进行详细比较：http://kentb.blogspot.com/2009/03/view-models-pocos-versus.html

- Marco Amendola

Answer 2

尝试

private string selectedString;
public string SelectedString
{
    get { return selectedString; }
    set 
    { 
        if (selectedString == value) return;
        selectedString = value;
        // Required for the UI to know the change was successful
        RaisePropertyChanged("SelectedString");
        LoadStringResults(value);
    }
}

private ObservableCollection<string> resultStrings;
public ObservableCollection<string> ResultStrings
{
    get { return resultStrings; }
    set 
    { 
        if (resultStrings== value) return;
        resultStrings= value;
        // Required for databinding to know that ResultStrings changed
        // Previously you changed this property without updating the UI
        RaisePropertyChanged("ResultStrings");
    }
}

private void LoadStringResults(string input)
{
    ObservableCollection<string> result = null;

    if (input == "Three")
    {
        result = new ObservableCollection<string> { "Six", "Seven", "Eight" };
    }
    else
    {
        result = new ObservableCollection<string> { "Nine", "Ten", "Eleven" };
    }

    ResultStrings = result;
}

- Agies