title: Update widget from other widget if they are in 2 different dart file
body: <p>Creating a sample calculator app in flutter. But no idea how to communicate each widgets, which are different dart files.</p>
  
  <ul>
  <li>Main.dart  ---------- ( starting point of the application)</li>
  <li>CalculatorFrame.dart  --------- ( importing numberboards , entry textbox and 
   forms calculator view)</li>
  <li>NumberButton.dart -------- ( represent the button widget)</li>
  <li>NumberBoards.dart  ---------- ( import numberbutton and creates the buttons ( 0-9) portion of the calculator)</li>
  <li>EntryTextbox.dart  ----------- ( represent the textbox entry for button pressed action and calculated values)</li>
  </ul>
  
  <p>I have a difficulty to implement the interaction with these widgets. On button pressed, need to update the EntryTextbox widget. How can it be done if the widgets are in different dart files.</p>
  
  <p>Please suggest.</p>