sclass ScreenCamStreamWithMeta extends SourceTriggeredStream<ScreenShotMeta> is Steppable {
  settable volatile Rect area = screenBounds(0);

  public bool step() {
    if (ended()) false;
    catchError(lambda0 shootImpl);
    true;
  }
  
  long frameCount() { ret elementCount(); }
  
  void shootImpl {
    newElement(shootScreenWithMeta(area));
  }
  
  void useScreen(int iScreen) {
    area = screenBounds_safe(iScreen);
  }
}

Began life as a copy of #1033670