How do I change the start/end locations of gradient fills in Rive/Flare programatically in Flutter?

I have a simple ball that uses a gradient fill to make it look like it's lit from one side:

I'd like to modify the gradient fill at runtime in Flutter (for example to make it seem like the light is moving around relative to the ball). I was able to find the coordinates like this:

final fill = (artboard.getNode("Ellipse") as FlutterActorShape).fill as FlutterRadialFill;
print(fill.renderStart);
print(fill.renderEnd);

However, I can't find a way to modify these values. I tried using Vec2D to overwrite the values, however it doesn't change the rendering (perhaps because something was computed from these values that needs invalidating?):

Vec2D.copy(
          _fill.renderStart,
          Vec2D.fromValues(
              200 - _component.x, 200 - _component.y));

Solution

There isn't a nice helpful setter for this property (keyframes manipulate it and invalidate the internal state), but with a little bit of extra verbose (we should really make this friendlier) code you can do it:


class MutateGradient extends FlareController {
  // Store the fill so you don't need to look it up each frame.
  FlutterRadialFill _radialFill;

  // I chose to drive the end position of the gradient with a sin wave so i use
  // a field to accumulate the phase.
  double _phase = 0;

  @override
  bool advance(FlutterActorArtboard artboard, double elapsed) {
    if (_radialFill == null) {
      // Didn't find the fill during init, early out with a false meaning we're
      // done.
      return false;
    }

    _phase += elapsed;

    // No nice setter on the end so we have to set the values manually and then
    // mark the paint dirty so the update loop updates the actual paint and
    // points used to render. Note that the position here can be either in the
    // path's local transform or world (artboard) transform depending on whether
    // transformAffectsStroke was selected in Rive.
    Vec2D.copy(
        _radialFill.end, Vec2D.fromValues(-100, 100 + sin(_phase*2) * 50));
    _radialFill.markPaintDirty();

    // Return true to get another call the next frame...
    return true;
  }

  @override
  void initialize(FlutterActorArtboard artboard) {
    // Find the fill we want to manipulate.
    var node = artboard.getNode("Ellipse");
    if (node is FlutterActorShape) {
      var fill = node.fill;
      if (fill is FlutterRadialFill) {
        _radialFill = fill;
      }
    }
  }

  @override
  void setViewTransform(Mat2D viewTransform) {
    // Inentionally empty, we don't need to convert from artboard (world) to
    // view space in this example.
  }
}

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