WPF KeyBinding swallowing keys, preventing TextBox use

Question

Problem Overview:

Any

KeyBinding

's defined at a level higher than a

TextBox

(with no modifier keys assigned), prevents the user from typing those keys inside the

TextBox

.

Minimal XAML Hierarchy:

<Window>

  <UserControl>

    <Border>

      <UserControl>

        <TextBox>

Minimal Command/KeyBinding:

<UserControl.Resources>

    <RoutedUICommand x:Key="Commands.SomeCommand" />

</UserControl.Resources>

<UserControl.InputBindings>

    <KeyBinding Key="A" Command="{StaticResource Commands.SomeCommand} />

</UserControl.InputBindings>

<UserControl.CommandBindings>

    <CommandBinding Command="{StaticResource Commands.SomeCommand}" Executed="..." />

</UserControl.CommandBindings>

The

Command

and

KeyBinding

, are defined at the first

UserControl

level. So in this example, in the textbox, the user can type freely until they press the

A

key, and then it just does not insert the letter into the textbox. I can clearly see that the

TextBox.KeyDown

and

TextBox.PreviewKeyDown

are firing when you press the

A

key (and

Handled = false

) , but the letter will not get added to the text of the textbox and

TextBox.PreviewTextInput

does not fire.

I'm looking for any suggestions that may indicate what is swallowing the keypress and stopping it from getting processed by the TextBox, or anything related to how I can debug this issue.

EDIT:
Thanks to Snoop, I was able to clearly see the problem.

TextBox.PreviewKeyDown
tunnels down and fires through the visual tree, starting at the Window, and ending at the TextBox

TextBox.KeyDown
bubbles back up starting at the TextBox and heading towards the window

TextBox.KeyDown
gets Handled set to true by the first UserControl that has the KeyBinding set.

TextBox.PreviewTextInput
never fires, nor does the textbox process the input, because the KeyDown event was set as handled.

This still leaves the problem, how do you prevent the UserControl from handling the input if a textbox has focus? Within the Command execution, I can check if a textbox has keyboard focus, but by this time it's too late.

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Answer 1

TextInput and PreviewTextInput only fires when the Text actually changes / might change.

As you updated your question to reflect, the Command intercepts the event and the (Preview)TextInput events are never raised.

The nicest solution would be to add a modifier key to your KeyBinding, but I suspect that is not your preferred way to go.

Another option would be to e.Handle the PreviewKeyDown event on the TextBox and raise the TextComposition events yourself, using something like:

target.RaiseEvent(new TextCompositionEventArgs(InputManager.Current.PrimaryKeyboardDevice, 
new TextComposition(InputManager.Current, target, "A")) 
{ 
    RoutedEvent = TextCompositionManager.TextInputEvent 
});

(Alternatively, insert into textBox.Text at the correct CaretIndex)

Truth be told, it would still be a hack.

WPF KeyBinding swallowing keys, preventing TextBox use

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