如何将MenuItem.Header绑定到Window/UserControl依赖项属性？

Question

我想知道如何将MenuItem.Header绑定到父Window/UserControl依赖项属性？这是一个简单的例子:

Window1.xaml:

<Window x:Class="WpfApplication1.Window1"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="Window1" Height="300" Width="300" x:Name="self">
    <Grid>
        <Grid.ContextMenu>
            <ContextMenu>
                <MenuItem Header="{Binding Path=MenuText, ElementName=self}" />
            </ContextMenu>
        </Grid.ContextMenu>
        <TextBlock Text="{Binding Path=MenuText, ElementName=self}"/>
    </Grid>
</Window>

Window1.xaml.cs:

public partial class Window1 : Window {
    public static readonly DependencyProperty MenuTextProperty = DependencyProperty.Register(
        "MenuText", typeof (string), typeof (Window1), new PropertyMetadata("Item 1"));

    public Window1()
    {
        InitializeComponent();
    }

    public string MenuText {
        get { return (string)this.GetValue(MenuTextProperty); }
        set { this.SetValue(MenuTextProperty, value); }
    }
}

在我的例子中,textblock显示"Item 1",上下文菜单显示空项.我做错了什么？在我看来,我遇到了严重的WPF数据绑定原则的误解.

Answer 1

您应该在Visual Studio的"输出"窗口中看到:

System.Windows.Data错误:4:无法找到引用'ElementName = self'的绑定源.BindingExpression:路径= MenuText; 的DataItem = NULL; target元素是'MenuItem'(Name =''); target属性是'Header'(类型'Object')

这是因为ContextMenu与VisualTree断开连接,您需要以不同方式执行此绑定.

一种方法是via ContextMenu.PlacementTarget(应该是Grid),你可以使用它的DataContext来建立绑定,例如:

<MenuItem Header="{Binding RelativeSource={RelativeSource AncestorType=ContextMenu}, Path=PlacementTarget.DataContext.MenuText}"/>

或者在ContextMenu中设置DataContext:

<ContextMenu DataContext="{Binding RelativeSource={RelativeSource Self}, Path=PlacementTarget.DataContext}">
    <MenuItem Header="{Binding Path=MenuText}"/>
</ContextMenu>

如果这不是一个选项(因为Grid的DataContext不能是Window/UserControl),您可以尝试通过TagGrid的方式将引用传递给Window/UserControl .

<Grid ...
      Tag="{x:Reference self}">
    <Grid.ContextMenu>
        <!-- The DataContext is now bound to PlacementTarget.Tag -->
        <ContextMenu DataContext="{Binding RelativeSource={RelativeSource Self}, Path=PlacementTarget.Tag}">
            <MenuItem Header="{Binding Path=MenuText}"/>
        </ContextMenu>
    ...

作为旁注:由于这种行为,我倾向于定义一个帮助器样式,App.xaml以使所有ContextMenus从其父级"伪继承"DataContext:

    <!-- Context Menu Helper -->
    <Style TargetType="{x:Type ContextMenu}">
        <Setter Property="DataContext" Value="{Binding PlacementTarget.DataContext, RelativeSource={RelativeSource Self}}"/>
    </Style>