Is it possible to detect Keyboard focus events globally? - wpf

Question

The following events can be used, but, they must be attach for each element:
GotKeyboardFocus, LostKeyboardFocus
Is there a way in .NET WPF to globally detect if the focused element changed ? without having to add event listeners for all possible elements ?

Answer 1

You can do this in any class with this:
//In the constructor
EventManager.RegisterClassHandler(
typeof(UIElement),
Keyboard.PreviewGotKeyboardFocusEvent,
(KeyboardFocusChangedEventHandler)OnPreviewGotKeyboardFocus);
...
private void OnPreviewGotKeyboardFocus(object sender,
KeyboardFocusChangedEventArgs e)
{
// Your code here
}

Answer 2

You can hook to the tunneling preview events:
<Window x:Class="WpfApplication1.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Height="350" Width="525"
PreviewGotKeyboardFocus="Window_PreviewGotKeyboardFocus"
PreviewLostKeyboardFocus="Window_PreviewLostKeyboardFocus">
....
This way, as shown above, the window would be notified before all descendants when any of the descendants gets or loses the keyboard focus.
Read this for more information.

Answer 3

You can add a routed event handler to your main window and specify you're interested in handled events.
mainWindow.AddHandler(
UIElement.GotKeyboardFocusEvent,
OnElementGotKeyboardFocus,
true
);

Answer 4

Have a look at how Microsoft trigger CommandManager.RequerySuggested event when focus changes: they subscribe to InputManager.PostProcessInput event.
ReferenceSource
Simple example:
static KeyboardControl()
{
InputManager.Current.PostProcessInput += InputManager_PostProcessInput;
}
static void InputManager_PostProcessInput(object sender, ProcessInputEventArgs e)
{
if (e.StagingItem.Input.RoutedEvent == Keyboard.GotKeyboardFocusEvent ||
e.StagingItem.Input.RoutedEvent == Keyboard.LostKeyboardFocusEvent)
{
KeyboardFocusChangedEventArgs focusArgs = (KeyboardFocusChangedEventArgs)e.StagingItem.Input;
KeyboardControl.IsOpen = focusArgs.NewFocus is TextBoxBase;
}
}
This also works in multi-window applications.