为什么当焦点在 WindowsFormsHost 内时 Keyboard.FocusedElement 为空？它破坏了 WPF 命令路由

Question

我有一个自定义的RoutedUICommandMyCommand，它通过ICommand.Execute 执行。顶部窗口有一个绑定来处理它：

<Window.CommandBindings>
    <CommandBinding Command="local:MainWindow.MyCommand" CanExecute="CanExecuteCommmand" Executed="CommandExecuted"/>
</Window.CommandBindings>

这是该命令的唯一处理程序。我还有一个WindowsFormsHost，里面有WinForms的TextBox控件（用于演示目的）。 当焦点在 TextBox 内时，MyCommand 不会到达顶部窗口。当焦点在 WPF 的本机 TextBox 内时，命令处理程序会按预期调用。 p>

我发现 这是因为当焦点位于 WindowsFormsHost 内部时，Keyboard.FocusedElement 是 null。为什么在这种情况下是 null，它是 WPF 错误还是设计功能？我错过了什么吗？

我相信命令应该到达顶部窗口，无论焦点在哪里（当它是可视化树中唯一的处理程序并且FocusManager.IsFocusScope 设置正确时）。我有一个related question about that。

项目源可here。

XAML：

<Window x:Class="WpfCommandTest.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:local="clr-namespace:WpfCommandTest" 
        xmlns:wf="clr-namespace:System.Windows.Forms;assembly=System.Windows.Forms"  
        Title="MainWindow" Height="480" Width="640" Background="Gray">

    <Window.CommandBindings>
        <CommandBinding Command="local:MainWindow.MyCommand" CanExecute="CanExecuteCommmand" Executed="CommandExecuted"/>
    </Window.CommandBindings>

    <StackPanel Margin="20,20,20,20">
        <TextBox Name="textBoxOutput" Focusable="True" IsTabStop="True" Height="150" Text="WPF TextBox&#x0a;"/>
        <WindowsFormsHost Focusable="True" KeyboardNavigation.IsTabStop="True"  Height="150">
            <wf:TextBox x:Name="textBoxWf" Text="WinForms TextBox" />
        </WindowsFormsHost>
        <Button FocusManager.IsFocusScope="True" Name="btnTest" Focusable="False" IsTabStop="False" Content="Test (ICommand.Execute)" Click="btnTest_Click" Width="200"/>
        <Button FocusManager.IsFocusScope="True" Focusable="False" IsTabStop="False" Content="Test (Command property)" Command="local:MainWindow.MyCommand" Width="200"/>
        <Button FocusManager.IsFocusScope="True" Name="btnClearFocus" Focusable="False" IsTabStop="False" Content="Clear Focus" Click="btnClearFocus_Click" Width="200"/>
    </StackPanel>

</Window>

C#：

using System;
using System.Windows;
using System.Windows.Input;

namespace WpfCommandTest
{
    public partial class MainWindow : Window
    {
        public static readonly RoutedUICommand MyCommand = new RoutedUICommand("MyCommand", "MyCommand", typeof(MainWindow));
        const string Null = "null";

        public MainWindow()
        {
            InitializeComponent();
            this.Loaded += (s, e) => textBoxOutput.Focus(); // set focus on the TextBox
        }

        void CanExecuteCommmand(object sender, CanExecuteRoutedEventArgs e)
        {
            e.CanExecute = true;
        }

        void CommandExecuted(object sender, ExecutedRoutedEventArgs e)
        {
            var routedCommand = e.Command as RoutedCommand;
            var commandName = routedCommand != null ? routedCommand.Name : Null;
            Log("*** Executed: {0} ***, {1}", commandName, FormatFocus());
        }

        void btnTest_Click(object sender, RoutedEventArgs e)
        {
            Log("btnTest_Click, {0}", FormatFocus());
            ICommand command = MyCommand;
            if (command.CanExecute(null))
                command.Execute(null);
        }

        void btnClearFocus_Click(object sender, RoutedEventArgs e)
        {
            FocusManager.SetFocusedElement(this, this);
            Keyboard.ClearFocus();
            Log("btnClearFocus_Click, {0}", FormatFocus());
        }

        void Log(string format, params object[] args)
        {
            textBoxOutput.AppendText(String.Format(format, args) + Environment.NewLine);
            textBoxOutput.CaretIndex = textBoxOutput.Text.Length;
            textBoxOutput.ScrollToEnd();
        }

        string FormatType(object obj)
        {
            return obj != null ? obj.GetType().Name : Null;
        }

        string FormatFocus()
        {
            return String.Format("focus: {0}, keyboard focus: {1}",
                FormatType(FocusManager.GetFocusedElement(this)),
                FormatType(Keyboard.FocusedElement));
        }
    }
}

Answer 1

要完成互操作表单-WPF，您需要这样做：

App.xaml.cs：

    public partial class App : Application
      {
      protected override void OnStartup(StartupEventArgs e)
        {
        WindowsFormsHost.EnableWindowsFormsInterop();
        base.OnStartup(e);
        }
      }