1. override
bool hitTest(HitTestResult result, { Point position })

Determines the set of render objects located at the given position.

Returns true if the given point is contained in this render object or one of its descendants. Adds any render objects that contain the point to the given hit test result.

The caller is responsible for transforming position into the local coordinate space of the callee. The callee is responsible for checking whether the given position is within its bounds.

Hit testing requires layout to be up-to-date but does not require painting to be up-to-date. That means a render object can rely upon performLayout having been called in hitTest but cannot rely upon paint having been called. For example, a render object might be a child of a RenderOpacity object, which calls hitTest on its children when its opacity is zero even through it does not paint its children.

Source 

@override
bool hitTest(HitTestResult result, { Point position }) {
  _updateClip();
  assert(_clip != null);
  Point center = _clip.center;
  // convert the position to an offset from the center of the unit circle
  Offset offset = new Offset((position.x - center.x) / _clip.width,
                             (position.y - center.y) / _clip.height);
  // check if the point is outside the unit circle
  if (offset.distanceSquared > 0.25) // x^2 + y^2 > r^2
    return false;
  return super.hitTest(result, position: position);
}