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绑定到依赖项属性时为什么不设置它?

dependency-properties wpf .net

stiank81 · 技术社区 · 14 年前

我的WPF应用程序中显示了两个集合,我希望其中一个的元素在另一个中被禁用。执行此操作时,我将创建一个继承列表框的自定义控件filteringlistbox,并希望在其中添加一些处理,以禁用通过filteringlistbox上的属性在集合集中设置的元素。现在,我的问题是,没有设置从中筛选元素的ObservableCollection的依赖属性-即使我在XAML中绑定到它。

我创建了一个简化的应用程序,在这里我重现了这个问题。这是我的XAML:

<StackPanel>
    <StackPanel Orientation="Horizontal">
        <StackPanel Orientation="Vertical">
            <TextBlock>Included</TextBlock>
            <ListBox x:Name="IncludedFooList" ItemsSource="{Binding IncludedFoos}"></ListBox>
        </StackPanel>
        <Button Margin="10" Click="Button_Click">Add selected</Button>
        <StackPanel Orientation="Vertical">
            <TextBlock>Available</TextBlock>
            <Listbox:FilteringListBox x:Name="AvailableFooList" ItemsSource="{Binding AvailableFoos}" FilteringCollection="{Binding IncludedFoos}"></Listbox:FilteringListBox>
        </StackPanel>                
    </StackPanel>            
</StackPanel>

这是我的自定义组件-当前仅保留依赖项属性:

public class FilteringListBox : ListBox
{
    public static readonly DependencyProperty FilteringCollectionProperty =
        DependencyProperty.Register("FilteringCollection", typeof(ObservableCollection<Foo>), typeof(FilteringListBox));                                                 

    public ObservableCollection<Foo> FilteringCollection
    {
        get
        {
            return (ObservableCollection<Foo>)GetValue(FilteringCollectionProperty);
        }
        set
        {
            SetValue(FilteringCollectionProperty, value);
        }
    }
}

对于完整的代码,代码隐藏和类定义如下:

public partial class MainWindow : Window
{
    private MainViewModel _vm;

    public MainWindow()
    {
        InitializeComponent();
        _vm = new MainViewModel();
        DataContext = _vm;
    }

    private void Button_Click(object sender, RoutedEventArgs e)
    {
        if (AvailableFooList.SelectedItem == null)
            return;
        var selectedFoo = AvailableFooList.SelectedItem as Foo;
        _vm.IncludedFoos.Add(selectedFoo);
    }
}

public class MainViewModel
{
    public MainViewModel()
    {
        IncludedFoos = new ObservableCollection<Foo>();
        AvailableFoos = new ObservableCollection<Foo>();
        GenerateAvailableFoos(); 
    }

    private void GenerateAvailableFoos()
    {
        AvailableFoos.Add(new Foo { Text = "Number1" });
        AvailableFoos.Add(new Foo { Text = "Number2" });
        AvailableFoos.Add(new Foo { Text = "Number3" });
        AvailableFoos.Add(new Foo { Text = "Number4" });
    }

    public ObservableCollection<Foo> IncludedFoos { get; set; }
    public ObservableCollection<Foo> AvailableFoos { get; set; }
}

public class Foo
{
    public string Text { get; set; }
    public override string ToString()
    {
        return Text;
    }
}

我将断点添加到FilteringListBox中DependencyProperty FilteringCollection的setter和getter,但它从未被触发。为什么?我怎么修?

3 回复 | 直到 14 年前

Bubblewrap 14 年前

绑定系统绕过依赖属性的set和get访问器。如果要在依赖项属性更改时执行代码,则应添加 PropertyChangedCallback 到 DependencyProperty 定义。

Filip Ekberg 14 年前

msdn有一个关于 Dependency Property Callbacks and Validation ,您需要注册 PropertyChangedCallback

来自msdn的示例

public static readonly DependencyProperty AquariumGraphicProperty 
= DependencyProperty.Register(
  "AquariumGraphic",
  typeof(Uri),
  typeof(AquariumObject),
  new FrameworkPropertyMetadata(null,
      FrameworkPropertyMetadataOptions.AffectsRender, 
      new PropertyChangedCallback(OnUriChanged)
  )
);

private static void OnUriChanged(DependencyObject d, 
                                 DependencyPropertyChangedEventArgs e) {
  Shape sh = (Shape) d;
  sh.Fill = new ImageBrush(new BitmapImage((Uri)e.NewValue));
}

Jakob Christensen 14 年前

WPF框架从不直接使用get和set属性。你只提供它们作为你自己的方便。相反,您需要向依赖项属性注册添加回调。当值绑定到依赖项属性时,将调用回调。因此,您的代码 FilteredListBox 应更改为类似于以下内容的内容:

public partial class FilteringListBox : ListBox
{
    public static readonly DependencyProperty FilteringCollectionProperty =
        DependencyProperty.Register("FilteringCollection", typeof(ObservableCollection<Foo>), typeof(FilteringListBox), 
        new PropertyMetadata(null, FilteringCollectionPropertyCallback));

    static void FilteringCollectionPropertyCallback(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        FilteringListBox listbox = d as FilteringListBox;
        // Do some work here
    }

    public ObservableCollection<Foo> FilteringCollection
    {
        get
        {
            return (ObservableCollection<Foo>) GetValue(FilteringCollectionProperty);
        }
        set
        {
            SetValue(FilteringCollectionProperty, value);
        }
    }
}