B4J Question Inhibit exit from Textfield

Sub Process_Globals
   Private fx As JFX
   Private MainForm As Form
   Private tf1 As TextField
   Private tf2 As TextField
End Sub

Sub AppStart (Form1 As Form, Args() As String)
   MainForm = Form1
   MainForm.RootPane.LoadLayout("1") 'Load the layout file.
   MainForm.Show
   tf1.Id="1"
   tf2.Id="2"
End Sub

Sub tf1_FocusChanged (HasFocus As Boolean)
   If HasFocus = False Then
       If tf1.Text.ToUpperCase="NO" Then
           Sleep(100)
           tf1.RequestFocus
       End If
   End If
End Sub