Question

我正在学习wpf mvvm并且一直在努力解决我觉得可能很简单但我自己无法解决的问题。

我想要的是能够在填充的组合框中选择一个项目，然后根据该选择填充另一个组合框。我似乎无法在选择时加载第二个组合框。

我添加了一个包含2个组合框和一个文本块的示例。当我运行应用程序并在第一个组合中选择一个项目时，文本块会根据绑定进行更新，但我不知道在哪里调用代码来更新第二个组合框。

我尝试将调用添加到SelectedString属性setter，但似乎从未调用过。我敢肯定我错过了一些简单的东西，但我需要有人帮助揭开面纱！

我已尝试过其他帖子的建议，但到目前为止，我都没有成功。

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>

以下是视图模型：

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;
    }
}

Answer 1

SelectedString依赖项属性实现是错误的：您应该注册PropertyChangedCallback以在直接访问DP时通知，而不是使用CLR属性集（请参阅http://msdn.microsoft.com/en-us/library/system.windows.propertychangedcallback.aspx）;这样，即使直接使用DP，也可以更改相关集合。

绑定依赖项属性（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，我会考虑在View Models上使用POCO方法，实现INotifyPropertyChanged：DP很难写入并为VM添加大量噪声（除了对System.Windows的恶意依赖）。

查看此博客文章，进行广泛比较：http://kentb.blogspot.com/2009/03/view-models-pocos-versus.html

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;
}

根据另一个组合框的选择填充一个组合框

2 个答案: