命令绑定到ContextMenu（在ListBox中的ListBoxItem上）不起作用

Question

在WPF中，使用MVVM指示灯，有一个Class（由一些学生组成），而Class包含一些Student。

右键单击一个学生的姓名，然后显示MessageBox，这样就可以了：

ClassDetailView.xaml

<UserControl DataContext="{Binding ClassDetail, Source={StaticResource Locator}}">
    <DockPanel>
        <ListBox 
            ItemsSource="{Binding Students}" 
            DisplayMemberPath="Name">
            <ListBox.ContextMenu>
                <ContextMenu DataContext="{Binding Path=PlacementTarget, RelativeSource={RelativeSource Self}}">
                    <MenuItem 
                        Header="Show Selected" 
                        Command="{Binding Path=DataContext.RemoveStudentCommand}"
                        CommandParameter="{Binding Path=SelectedItem}"/>
                </ContextMenu>
            </ListBox.ContextMenu>
        </ListBox>
    </DockPanel>
</UserControl>

但是，它不能以这种方式工作（使用ListBox.ItemContainerStyle）：

<ListBox.ItemContainerStyle>
    <Style TargetType="ListBoxItem">
        <Setter Property="ContextMenu">
            <Setter.Value>
                <ContextMenu DataContext="{Binding Path=PlacementTarget, RelativeSource={RelativeSource Self}}">
                    <MenuItem Header="Show Selected" 
                            Command="{Binding Path=DataContext.RemoveStudentCommand}"
                            CommandParameter="{Binding Path=SelectedItem}"/>
                 </ContextMenu>
             </Setter.Value>
         </Setter>
     </Style>
 </ListBox.ItemContainerStyle>

而不是

<ListBox.ContextMenu>
    <ContextMenu ...>
        ...
    <ContextMenu />
</ListBox.ContextMenu>

ClassDetailViewModel.cs

namespace ContextMenu.ViewModel
{
    public class ClassDetailViewModel : ViewModelBase
    {
        public ClassDetailViewModel()
        {
            CreateData();
        }

        public void CreateData()
        {
            students.Add(new StudentViewModel() { Name = "QQ" });
            students.Add(new StudentViewModel() { Name = "WW" });
            students.Add(new StudentViewModel() { Name = "EE" });
            students.Add(new StudentViewModel() { Name = "RR" });
            students.Add(new StudentViewModel() { Name = "AA" });
            students.Add(new StudentViewModel() { Name = "SS" });
            students.Add(new StudentViewModel() { Name = "DD" });
            students.Add(new StudentViewModel() { Name = "FF" });
            students.Add(new StudentViewModel() { Name = "ZZ" });
            students.Add(new StudentViewModel() { Name = "XX" });
        }

        public const string StudentsPropertyName = "Students";
        private ObservableCollection<StudentViewModel> students = 
            new ObservableCollection<StudentViewModel>();
        public ObservableCollection<StudentViewModel> Students
        {
            get { return students; }
            set
            {
                if (students == value) { return; }
                students = value;
                RaisePropertyChanged(StudentsPropertyName);
            }
        }

        private RelayCommand<StudentViewModel> removeStudentCommand;
        public RelayCommand<StudentViewModel> RemoveStudentCommand
        {
            get
            {
                return removeStudentCommand
                    ?? (removeStudentCommand =
                        new RelayCommand<StudentViewModel>(ExecuteRemoveStudentCommand));
            }
        }
        private void ExecuteRemoveStudentCommand(StudentViewModel student)
        {
            if (null == student) { return; }
            MessageBox.Show(string.Format("RemoveStudent:{0}", student.Name));
        }
    }
}

StudentViewModel.cs

namespace ContextMenu.ViewModel
{
    public class StudentViewModel : ViewModelBase
    {
        public const string NamePropertyName = "Name";
        private string name = "";
        public string Name
        {
            get { return name; }
            set
            {
                if (name == value) { return; }
                name = value;
                RaisePropertyChanged(NamePropertyName);
            }
        }
    }
}

Answer 1

您可以将contextmenu用作资源吗？

类似的东西：

<UserControl.Resources>

<ContextMenu x:name="contextMenuExample" DataContext="{Binding Path=PlacementTarget, RelativeSource={RelativeSource Self}}">
                    <MenuItem Header="Show Selected" 
                            Command="{Binding Path=DataContext.RemoveStudentCommand}"
                            CommandParameter="{Binding Path=SelectedItem}"/>
                 </ContextMenu>
</UserControl.Resources>

然后在列表中，执行以下操作：

<Listbox ContextMenu = {StaticResource contextMenuExample} ... />

或者你真的想使用ItemContainerStyle吗？

from - ＆gt; how to right click on item from Listbox and open menu on WPF

<ListBox Name="someListBox" MouseDown="someListBox_MouseDown">
    <ListBox.Resources>

        <!--Defines a context menu-->
        <ContextMenu x:Key="MyElementMenu">
            <MenuItem Header="Delete" Click="MenuItemDelete_Click"/>
        </ContextMenu>

        <!--Sets a context menu for each ListBoxItem in the current ListBox-->
        <Style TargetType="{x:Type ListBoxItem}">
             <Setter Property="ContextMenu" Value="{StaticResource MyElementMenu}"/>
        </Style>

    </ListBox.Resources>
    <ListBoxItem>...</ListBoxItem>
    <ListBoxItem>...</ListBoxItem>
    <ListBoxItem>...</ListBoxItem>
</ListBox>

Answer 2

您需要一个代理才能将命令绑定到listboxitem的上下文菜单。请参阅答案：

http://www.thomaslevesque.com/2011/03/21/wpf-how-to-bind-to-data-when-the-datacontext-is-not-inherited/

Answer 3

通过将ContextMenu移动到ListBoxItem，您已将DataContext从ClassDetailsViewModel（ListBox的DataContext）更改为StudentViewModel（ListBoxItem的DataContext）。因此，您需要更改路径以访问父ListBox的DataContext以访问RelayCommand。

<ListBox.ItemContainerStyle>
    <Style TargetType="ListBoxItem">
        <Setter Property="ContextMenu">
            <Setter.Value>
                <ContextMenu DataContext="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type ListBox}}, Path=DataContext}">
                    <MenuItem Header="Show Selected" 
                            Command="{Binding Path=RemoveStudentCommand}"
                            CommandParameter="{Binding Path=SelectedItem}"/>
                 </ContextMenu>
             </Setter.Value>
         </Setter>
     </Style>
 </ListBox.ItemContainerStyle>