transform method

@override Matrix4? transform(Rect bounds, { TextDirection? textDirection, })

When a Gradient creates its Shader, it will call this method to determine what transform to apply to the shader for the given Rect and TextDirection.

Implementers may return null from this method, which achieves the same final effect as returning Matrix4.identity.

Implementation

@override
Matrix4? transform(Rect bounds, {TextDirection? textDirection}) {
  double w = bounds.width, h = bounds.height, m = w / h, mx = 1.0, my = 1.0;

  // Adjust for the aspect ratio of the widget:
  if (m > 1) {
    mx *= m;
  } else {
    my /= m;
  }

  // Convert from [-1 - +1] to [0 - 1], & find the pixel location of the gradient center:
  double cx = (center.x + 1.0) / 2.0, cy = (center.y + 1.0) / 2.0;
  Offset pt = Offset(
      bounds.left + bounds.width * cx, bounds.top + bounds.height * cy);

  Matrix4 transform = Matrix4(
    a * mx, b * my, 0.0, 0.0, //
    c * mx, d * my, 0.0, 0.0, //
    0.0, 0.0, 1.0, 0.0, //
    0.0, 0.0, 0.0, 1.0,
  );

  // This offsets the draw position to account for the widget's position being
  // multiplied against the transformation:
  List<double> loc = transform.applyToVector3Array([pt.dx, pt.dy, 0.0]);
  double dx = pt.dx - loc[0], dy = pt.dy - loc[1];

  return Matrix4.identity()
    ..translate(dx, dy, 0.0)
    ..multiply(transform);
}