Question about Data Exchange

["chao yeaj" <yeajchao gmail com>]

Hello everyone
  You know ,in an application
   one widget may associated with one data member

  For example,there is a entry widget ,the user can write some data
into the entry widget;In the application, there must be a data
variable named str_widget,associated to the entry widget.

   When the user changed the entry's text ,the str_widget variable's
value  should be update;and, when the program changed the str_widget's
value somewhere, we need update the entry widget's display too!
(Update the UI )

   The problem is: i did not know how and when update the data
variable and the widget ?

   I need your help !
Any comments would be much appreciated ,and thanks in advance !