code: func (a *Application) SetBeforeDrawFunc(handler func(screen tcell.Screen) bool) *Application {
  	a.beforeDraw = handler
  	return a
  }
docs: // SetBeforeDrawFunc installs a callback function which is invoked just before
  // the root primitive is drawn during screen updates. If the function returns
  // true, drawing will not continue, i.e. the root primitive will not be drawn
  // (and an after-draw-handler will not be called).
  //
  // Note that the screen is not cleared by the application. To clear the screen,
  // you may call screen.Clear().
  //
  // Provide nil to uninstall the callback function.