Question

This is a weird one and I don’t really even know what to search for, but trust me I have.

I have a text box and bound to its OnTextChanged event is the below method.

The purpose here is to give the text box focus, move the cursor to the end of the TextBox and return focus back to whatever was actually focused (usually a button). The problem is that it seems the TextBox is not “redrawn” (for lack of a better word?) before I send the focus back to the originally focused element so the cursor position does not update on screen (though all properties think it has).

Currently, I have brutally hacked this together that basically delays the refocus of the previous focused item by 10 ms and runs it in a different thread so the UI has time to update. Now, this is obviously an arbitrary amount of time and works fine on my machine but someone running this app on an older machine may have problems.

Is there a proper way to do this? I can’t figure it out.

private void TextBoxBase_OnTextChanged(object sender, TextChangedEventArgs e)
{
    if (sender == null) return;
    var box = sender as TextBox;

    if (!box.IsFocused)
    {

        var oldFocus = FocusManager.GetFocusedElement(FocusManager.GetFocusScope(this));
        box.Select(box.Text.Length, 0);
        Keyboard.Focus(box); // or box.Focus(); both have the same results

        var thread = new Thread(new ThreadStart(delegate
                                                    {
                                                        Thread.Sleep(10);
                                                        Dispatcher.Invoke(new Action(() => oldFocus.Focus()));
                                                    }));
        thread.Start();
    }
}

EDIT

A new idea I had was to run the oldFocus.Focus() method once the UI is done updating so I tried the following but I get the same result 🙁

var oldFocus = FocusManager.GetFocusedElement(FocusManager.GetFocusScope(this));

Dispatcher.Invoke(DispatcherPriority.Send, new Action(delegate
 {
   box.Select(box.Text.Length, 0);
   box.Focus();
 }));

Dispatcher.Invoke(DispatcherPriority.SystemIdle, new Action(() => oldFocus.Focus()));

Answer 1

After many days, I was finally able to get it to work. It required the Dispatcher to check if the textbox has both focus AND keyboardfocus and lots of loops.

Here’s the code for reference. There’s some comments in it but if anyone hits this page looking for an answer, you’ll have to read through it yourself. A reminder, this is on text change.

protected void TextBox_ShowEndOfLine(object sender, TextChangedEventArgs e)
    {
        if (sender == null) return;
        var box = sender as TextBox;

        if (!box.IsFocused && box.IsVisible)
        {
            IInputElement oldFocus = FocusManager.GetFocusedElement(FocusManager.GetFocusScope(this));
            box.Focus();
            box.Select(box.Text.Length, 0);
            box.Focus();

            // We wait for keyboard focus and regular focus before returning focus to the button
            var thread = new Thread((ThreadStart)delegate
                                        {
                                            // wait till focused
                                            while (true)
                                            {
                                                var focused = (bool)Dispatcher.Invoke(new Func<bool>(() => box.IsKeyboardFocusWithin && box.IsFocused && box.IsInputMethodEnabled), DispatcherPriority.Send);
                                                if (!focused)
                                                    Thread.Sleep(1);
                                                else
                                                    break;
                                            }

                                            // Focus the old element
                                            Dispatcher.Invoke(new Action(() => oldFocus.Focus()), DispatcherPriority.SystemIdle);
                                        });
            thread.Start();
        }
        else if (!box.IsVisible)
        {
            // If the textbox is not visible, the cursor will not be moved to the end. Wait till it's visible.
            var thread = new Thread((ThreadStart)delegate
                                        {
                                            while (true)
                                            {
                                                Thread.Sleep(10);
                                                if (box.IsVisible)
                                                {
                                                    Dispatcher.Invoke(new Action(delegate
                                                                                     {
                                                                                         box.Focus();
                                                                                         box.Select(box.Text.Length, 0);
                                                                                         box.Focus();

                                                                                     }), DispatcherPriority.ApplicationIdle);
                                                    return;
                                                }
                                            }
                                        });
            thread.Start();
        }
    }