ScrollPane scrolling velocity

Question

I'm working with a custom ScrollPane, but I can't find how to change the velocity of scrolling (not on fling action). Can anybody tell me how reduce scrolling velocity on calling setScrollY() or setScrollX()?

Answer 1

I had a look at the ScrollPane source code on GitHub. That's quite a difficult problem.

The animation is done in the ScrollPane's act method. If you set the scroll value manually, the visual scroll value used for rendering will align on the scroll value over time through an interpolation process (usually in a few frame).

Here is the code responsible for this behaviour in the ScrollPane's act method :

            // ...

            if (visualAmountX != amountX) {
                if (visualAmountX < amountX)
                    visualScrollX(Math.min(amountX, visualAmountX + Math.max(200 * delta, (amountX - visualAmountX) * 7 * delta)));
                else
                    visualScrollX(Math.max(amountX, visualAmountX - Math.max(200 * delta, (visualAmountX - amountX) * 7 * delta)));
                animating = true;
            }
            if (visualAmountY != amountY) {
                if (visualAmountY < amountY)
                    visualScrollY(Math.min(amountY, visualAmountY + Math.max(200 * delta, (amountY - visualAmountY) * 7 * delta)));
                else
                    visualScrollY(Math.max(amountY, visualAmountY - Math.max(200 * delta, (visualAmountY - amountY) * 7 * delta)));
                animating = true;
            }

            // ... 

As you can see, the speed depends mostly on the distance between the current visual value and the setted "official" value. Some values that limit and defines the speed are hardcoded ("200", "7"), so there is currently no way to setup the speed for this animation cleanly.

Here are some ideas/solution :

• Override the act method of the Scrollpane. Just copy the source from the full act method and tune the hardcoded values (but that's dirty)

• Create customs actions to update the visual scroll value and the scroll value manually.

• Modify the Scrollpane class to make the animation speed use the velocity attributes and make a pull request on the libgdx Github repository, so that everybody will maybe benefit of this fix one day.

Answer 2

You could try creating your own version of the ScrollPane by inheriting from ScrollPane and then overriding visualScrollX(float pixelsX) and visualScrollY(float pixelsY), and multiply the parameter by some factor before calling the base class implementation;

class MyScrollPane extends ScrollPane {
  @Override
  protected void visualScrollX(float pixelsX) {
     super.visualScrollX(pixelsX * 0.5f);
  }

  @Override
  protected void visualScrollY(float pixelsY) {
     super.visualScrollY(pixelsY * 2.0f);
  }
}