vtk SliderWidget in Qt environment

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I am learning vtk example in Qt environment. I tested this example SliderWidget.

I just change the code as below:

sliderWidget->SetInteractor(this->ui->qvtkWidget->GetInteractor());

However, the SliderWidget doesn’t show in the screen. Do I need set something?

Here is the whole code.

vtkSmartPointer<vtkSliderRepresentation2D> sliderRep =
	  vtkSmartPointer<vtkSliderRepresentation2D>::New();

  sliderRep->SetMinimumValue(3.0);
  sliderRep->SetMaximumValue(20.0);
  sliderRep->SetValue(5.0);
  sliderRep->SetTitleText("Sphere Resolution");

  // Set color properties:
  sliderRep->GetSliderProperty()->SetColor(1, 0, 0);//red
  sliderRep->GetTitleProperty()->SetColor(1, 0, 0);//red
  sliderRep->GetLabelProperty()->SetColor(1, 0, 0);//red
  sliderRep->GetSelectedProperty()->SetColor(0, 1, 0);//green
  sliderRep->GetTubeProperty()->SetColor(1, 1, 0);//yellow
  sliderRep->GetCapProperty()->SetColor(1, 1, 0);//yellow

  sliderRep->GetPoint1Coordinate()->SetCoordinateSystemToDisplay();
  sliderRep->GetPoint1Coordinate()->SetValue(40, 40);
  sliderRep->GetPoint2Coordinate()->SetCoordinateSystemToDisplay();
  sliderRep->GetPoint2Coordinate()->SetValue(100, 40);

  vtkSmartPointer<vtkSliderWidget> sliderWidget =
	  vtkSmartPointer<vtkSliderWidget>::New();
  sliderWidget->SetInteractor(this->ui->qvtkWidget->GetInteractor());
  sliderWidget->SetRepresentation(sliderRep);
  sliderWidget->SetAnimationModeToAnimate();
  sliderWidget->EnabledOn();
  sliderWidget->On();