当TextBlock是UserControl的一部分时，如何处理TextBlock.KeyDown事件？

Question

I have this simple UserControl :

<UserControl x:Class="WPFTreeViewEditing.UserControl1"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
             mc:Ignorable="d" 
             d:DesignHeight="300" d:DesignWidth="300">
    <Grid>
        <TextBlock Text="Hello, world!" KeyDown="TextBlock_KeyDown" />
    </Grid>
</UserControl>

I want to handle TextBlock.KeyDown event. So, I've added an event handler to the code-behind:

public partial class UserControl1 : UserControl
{
    public UserControl1()
    {
        InitializeComponent();
    }

    private void TextBlock_KeyDown(object sender, KeyEventArgs e)
    {
        MessageBox.Show("Key up!");
    }
}

but it doesn't fire. What's wrong?

UPDATE.

PreviewKeyDown doesn't fire too.

This UserControl is used in HierarchicalDataTemplate then:

<Window x:Class="WPFTreeViewEditing.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:local="clr-namespace:WPFTreeViewEditing"
        Title="MainWindow" Height="265" Width="419">
    <Grid>
        <TreeView ItemsSource="{Binding}">
            <TreeView.Resources>
                <HierarchicalDataTemplate DataType="{x:Type local:ViewModel}" ItemsSource="{Binding Items}">
                    <local:UserControl1 />
                </HierarchicalDataTemplate>
            </TreeView.Resources>
        </TreeView>
    </Grid>
</Window>

Answer 1

From the documentation for UIElement.KeyDown :

Occurs when a key is pressed while focus is on this element.

You're using TextBlock which doesn't have the focus, so your KeyDown event will be handled by another control.

You can switch to TextBox and appy some styles so it'll look and behave like TextBlock, but you'll be able to get the focus and handle the event.

Answer 2

您应该使用PreviewKeyDown事件而不是KeyDown事件。

Answer 3

Ok, even though this question was posted a long time ago I had the same problem and found a way to get KeyDown events working, though it might not be what you're looking for I'll post the code to help future people with the same problem.

First thing first a KeyDown event handler in an Xaml Object will only fire off if that object has focus. Therefore you need a CoreWindow event handler, it's kind off the same thing but it will always run no matter what object or thing has focus. The following will be the code.

//CLASS.xaml.h
ref class CLASS{
private:
    Platform::Agile<Windows::UI::Core::CoreWindow> window;

public:
    void KeyPressed(Windows::UI::Core::CoreWindow^ Window, Windows::UI::Core::KeyEventArgs^ Args);


//CLASS.xaml.cpp
CLASS::CLASS(){
    InitializeComponent();
    window = Window::Current->CoreWindow;

    window->KeyDown += ref new TypedEventHandler
    <Windows::UI::Core::CoreWindow^, Windows::UI::Core::KeyEventArgs^>(this, &CLASS::KeyPressed);
};

void CLASS::KeyPressed(Windows::UI::Core::CoreWindow^ Window, Windows::UI::Core::KeyEventArgs^ Args){
SimpleTextBox->Text = Args->VirtualKey.ToString();
};

Basically you want a value to hold your window and use that to create a new TypedEventHandler. For safety you'll generally want to do this in your class' constructor a function that's only called once the moment the class starts (I still prefer the constructor though).

You can use this method to create an event handler for any event. Just change the "KeyDown" for another attribute like KeyUp, PointerMoved, PointerPressed and change the "&CLASS::KeyPressed" to the name of the function you want to be fired the moment you get an event of a corresponding type.