Using OnSelectionChange method

I am tring to register a function with the OnSelectionChange method , but it never gets called.

oEditor.AttachToOnSelectionChange( function(e){  alert( e.GetHTML() )} )

It seems to only register when place in the On_Complete method

function FCKeditor_OnComplete( editorInstance ){
   editorInstance.Events.AttachEvent( 'OnSelectionChange', function(e) {  alert(e.GetHTML()) } ) ;
}

The on_complete method does not help me very much because I need to add the functions dynamically
Can someone provide a sample of it usage

Thanks