void update(InputValue newInput)

Updates the overlay after the selection has changed.

If this method is called while the SchedulerBinding.schedulerPhase is SchedulerBinding.persistentCallbacks, i.e. during the build, layout, or paint phases (see WidgetsBinding.beginFrame), then the update is delayed until the post-frame callbacks phase. Otherwise the update is done synchronously. This means that it is safe to call during builds, but also that if you do call this during a build, the UI will not update until the next frame (i.e. many milliseconds later).

Source 

void update(InputValue newInput) {
  if (_input == newInput)
    return;
  _input = newInput;
  if (SchedulerBinding.instance.schedulerPhase == SchedulerPhase.persistentCallbacks) {
    SchedulerBinding.instance.addPostFrameCallback(_markNeedsBuild);
  } else {
    _markNeedsBuild();
  }
}