' Initialize a text changed listener.
' An instance of TextPatternRange will become invalid if 
' one of the following occurs:
' 1) The text in the provider changes via some user activity.
' 2) ValuePattern.SetValue is used to programatically change 
' the value of the text in the provider.
' The only way the client application can detect if the text 
' has changed (to ensure that the ranges are still valid), 
' is by setting a listener for the TextChanged event of 
' the TextPattern. If this event is raised, the client needs 
' to update the targetDocumentRange member data to ensure the 
' user is working with the updated text. 
' Clients must always anticipate the possibility that the text 
' can change underneath them.
Dim onTextChanged As AutomationEventHandler = _
New AutomationEventHandler(AddressOf TextChanged)
Automation.AddAutomationEventHandler( _
TextPattern.TextChangedEvent, targetDocument, TreeScope.Element, onTextChanged)