在initState中更新your _controller如下：

void initState() {
    super.initState();

    // Setup the listener.
    controller.addListener(() {
      if (_controller.position.atEdge) {
        bool atBegin = _controller.position.pixels == 0;
        if (atBegin) {
          /// here you are at the top
          print('here you are at the top');
        } else {
          /// here you are at the bottom
          print('here you are at the bottom');
        }
      }
    });
  }