markDirty method
      
void
markDirty()
      
     
    
    
Implementation
void markDirty() {
  _markDirty = true;
  // this is done instead of just notifyListeners() so as to differentiate
  // betweena adhoc calls to widgetsWithScreenPositions
  // if you need to call notifyListeners() from within this class,
  // it should always be with markDirty()
  notifyListeners();
}