WPF MenuItem Command.Executed 未触发，但 Command.CanExecute 触发

Question

这几天让我发疯了！

我无法让命令的“已执行”部分在菜单“1 级”以下的任何内容上触发。我可以看到始终在所有菜单项上调用“CanExecute”回调，并且回调始终返回 true。

请有人指出我正确的方向吗？我知道我可以在 MyTypes:MyMenuDescriptorType 中添加 ICommand 的实现，但这不适合我想要实现的模型。

在此先感谢

private void CreateActionCommand_CanExecute(object sender, CanExecuteRoutedEventArgs e)
{
    e.CanExecute = true;
    e.Handled = true;
}
private void CreateActionCommand_Executed(object sender, ExecutedRoutedEventArgs e)
{
    Debug.WriteLine("This only happens on Level 1");
}

<UserControl.Resources>
    <ResourceDictionary>
        <RoutedUICommand x:Key="CreateAction" Text="CreateAction"/>
    </ResourceDictionary>
</UserControl.Resources>    
<Grid x:Name="LayoutRoot">
    <Menu>
        <Menu.CommandBindings>
            <CommandBinding Command="{StaticResource CreateAction}" Executed="CreateActionCommand_Executed" CanExecute="CreateActionCommand_CanExecute"/>               
        </Menu.CommandBindings>         
        <Menu.Resources>
            <Style x:Key="MyMenuStyle" TargetType="{x:Type MenuItem}">
                <Setter Property="Header" Value="{Binding Description}"/>
                <Setter Property="Command" Value="{StaticResource CreateAction}" />
                <Setter Property="CommandParameter" Value="{Binding}"/>
                <Setter Property="CommandTarget" Value="{Binding Path=., RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type Menu}}}"/>                   
            </Style>                                        
        </Menu.Resources>           
        <MenuItem Header="Root Level">
            <MenuItem Header="Level 1" ItemsSource="{Binding MyListOfThings}" ItemContainerStyle="{StaticResource MyMenuStyle}">
                <MenuItem.Resources>
                    <HierarchicalDataTemplate 
                        DataType="{x:Type MyTypes:MyMenuDescriptorType}" 
                        ItemsSource="{Binding Path=Children}" 
                        ItemContainerStyle="{StaticResource MyMenuStyle}"/>
                </MenuItem.Resources>
            </MenuItem>
        </MenuItem>
    </Menu>
</Grid>

更新：我已经弄清楚出了什么问题。我的简化示例没有更新属性“MyListOfThings”。如果我在菜单的 PreviewMouseLeftButtonDown 事件中更新“MyListOfThings”，则控件会执行奇怪的操作并且命令不会触发。但是，如果我在 MouseOver 事件中更新属性，那么一切都会按预期工作！

我不确定这是为什么。也许有人可以解释一下？

Answer 1

你可以尝试实现一个DelegateCommand，你可以找到snippet code here。

在您的 MenuItem 之后，将如下所示：

<MenuItem Header="DoSomething" Command="{Binding DoSomethingCommand}"/>

在您的 ViewModel 中：

public MainViewModel()
    {
        DoSomethingCommand= new DelegateCommand(DoSomethingCommandExec, DoSomethingCommandCanExec);
    }

    public ICommand DoSomethingCommand{ get; private set; }

    private bool DoSomethingCommandCanExec()
    {
        return true;
    }

    private void DoSomethingCommandExec()
    {
        // DoSomethingCommand
    }

我认为这是实现它的好方法。每个 MenuItem 都会有一个命令。您可以将 Menuitems 放入可以绑定到与此处相同的命令的 MenuItem 中。

喜欢：

<MenuItem Header="DoSomething" Command="{Binding DoSomethingCommand}">
    <MenuItem Header="DoSomething" Command="{Binding DoSomethingCommand}"/>
</MenuItem>

希望有所帮助。

您好！

Answer 2

基于this 问题。它对我有用：

哈姆：

<Window x:Class="WpfApplication3.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    xmlns:local="clr-namespace:WpfApplication3"
    mc:Ignorable="d"
    Title="MainWindow" Height="350" Width="525">
<Window.Resources>
    <ResourceDictionary>
        <ContextMenu x:Key="ContextMenu1">
            <MenuItem Header="Menu1" Command="{Binding PlacementTarget.Tag.Menu1Executed, RelativeSource={RelativeSource AncestorType=ContextMenu}}" />
        </ContextMenu>
    </ResourceDictionary>
</Window.Resources>

<Grid ShowGridLines="True" ContextMenu="{DynamicResource ContextMenu1}" Tag="{Binding}">
    <Grid.RowDefinitions>
        <RowDefinition Height="100*"/>
    </Grid.RowDefinitions>

    <Grid.ColumnDefinitions>
        <ColumnDefinition Width="100*"/>
    </Grid.ColumnDefinitions>

    <Label Grid.Column="1" Content="Content" Margin="2"/>
</Grid>

cs：

public partial class MainWindow : Window
{
    public MainWindow()
    {
        InitializeComponent();
        DataContext = new MainWindowViewModel();
    }
}

public class MainWindowViewModel
{
    public MainWindowViewModel()
    {
        Menu1Executed = new DelegateCommand(ShowMenu1);
    }

    public ICommand Menu1Executed { get; set; }

    private void ShowMenu1(object obj)
    {
        // Yay!
    }
}

public class DelegateCommand : ICommand
{
    private readonly Action<object> execute;
    private readonly Predicate<object> canExecute;

    public DelegateCommand(Action<object> execute)
        : this(execute, null)
    { }

    public DelegateCommand(Action<object> execute, Predicate<object> canExecute)
    {
        if (execute == null)
            throw new ArgumentNullException("execute");

        this.execute = execute;
        this.canExecute = canExecute;
    }

    #region ICommand Members

    [DebuggerStepThrough]
    public bool CanExecute(object parameter)
    {
        return canExecute == null ? true : canExecute(parameter);
    }

    public event EventHandler CanExecuteChanged
    {
        add { CommandManager.RequerySuggested += value; }
        remove { CommandManager.RequerySuggested -= value; }
    }

    public void Execute(object parameter)
    {
        execute(parameter);
    }

    #endregion
}