B4J Question TextChanged Event - When?

Sub Process_Globals
 Private fx As JFX
 Private MainForm As Form
 Dim tf As TextField
 Dim b1, b2 As Button
 Dim flag As Boolean = False
End Sub
Sub AppStart (Form1 As Form, Args() As String)
 MainForm = Form1
 'MainForm.RootPane.LoadLayout("Layout1") 'Load the layout file.
 MainForm.Show
 tf.Initialize("tf")
 b1.Initialize("b1")
 b1.Text = "b1"
 b2.Initialize("b2")
 b2.Text = "b2"
 b2.Enabled = False
 MainForm.RootPane.AddNode(tf,10,10,120,30)
 MainForm.RootPane.AddNode(b1,10,50,100,20)
 MainForm.RootPane.AddNode(b2,10,80,100,20)
End Sub
Sub tf_TextChanged (Old As String, New As String)
 If flag = True Then b2.Enabled = True
End Sub
Sub b1_Click
 Log("format text in tf")
 flag = True
 tf.Text = tf.Text.ToUpperCase
 b1.Enabled=False
End Sub
Sub b2_Click
 Log("TextArea now reads : "&tf.text)
End Sub