how to implement Libgdx touchDragged?

how to implement Libgdx touchDragged?

I'm beginner in Java and in game development too, I was following some tutorials but now I'm stuck on implementing touchDragged listener using libgdx,
here is my code and can anyone suggest how to drag image when user touches it and than moves finger?
I use stage and actor and I want to catch touchDragged event on actor.
Thanks.
public void create () {
    Texture.setEnforcePotImages(false);

    stage = new Stage();
    Gdx.input.setInputProcessor(stage);
    // create a SpriteBatch with which to render the sprite
    batch = new SpriteBatch();

    // load the sprite's texture. note: usually you have more than
    // one sprite in a texture, see {@see TextureAtlas} and {@see TextureRegion}.
    texture = new Texture(Gdx.files.internal("ball3.png"));
    Skin skin = new Skin();
    skin.add("default", new Label.LabelStyle(new BitmapFont(), Color.WHITE));
    skin.add("ball", texture);
    Image sourceImage = new Image(skin, "ball");
    sourceImage.setBounds(50, 125, 100, 100);
    stage.addActor(sourceImage);

    // create an {@link OrthographicCamera} which is used to transform
    // touch coordinates to world coordinates.
    camera = new OrthographicCamera();

    // we want the camera to setup a viewport with pixels as units, with the
    // y-axis pointing upwards. The origin will be in the lower left corner
    // of the screen.
    camera.setToOrtho(false);
}

public void render () {
    Gdx.gl.glClear(GL10.GL_COLOR_BUFFER_BIT);
    stage.act(Gdx.graphics.getDeltaTime());
    stage.draw();
    Table.drawDebug(stage);



    // if a finger is down, set the sprite's x/y coordinate.
    if (Gdx.input.isTouched()) {
        // the unproject method takes a Vector3 in window coordinates (origin in
        // upper left corner, y-axis pointing down) and transforms it to world
        // coordinates.
        camera.unproject(spritePosition.set(Gdx.input.getX(), Gdx.input.getY(), 0));
    }
}