How to dynamicly change one widget's size according to another widget's size

["Ye Chao" <yeajchao gmail com>]

Hello everyone

      In my application,user can change the main window's size by mouse
      If  the main window's size changed,the widget in it may change too
      There are many widgets in the main window

      I want one widget's  width always equal to another widget's width

     but i do not know how to achieve that,
     I do not know I must capture which signal

     Can you help me ?