How to draw a route in google maps with flutter

Question

I have added a google map to my flutter application, But when I use a flutter_polyline_points to show the road nothing is gone to show

I haven see any tutorial to fix this problem but then I knew that to Decode an encoded google polyline string e.g _piFps|U_ulLnnqC_mqNvxq`@

How to get the google polyline string to use in my app

that is my code

  _addPolyLine() {
    PolylineId id = PolylineId("poly");
    Polyline polyline = Polyline(
        polylineId: id, color: Colors.red, points: polylineCoordinates);
    polylines[id] = polyline;
    setState(() {});
  }

  _getPolyline() async {
    PolylineResult result = await polylinePoints.getRouteBetweenCoordinates(
      googleAPiKey,
      PointLatLng(36.3416751898107, 43.6087730170814),
      PointLatLng(36.18702370508709, 43.979696040252136),
      travelMode: TravelMode.driving,
    );
    if (result.points.isNotEmpty) {
      result.points.forEach((PointLatLng point) {
        polylineCoordinates.add(LatLng(point.latitude, point.longitude));
      });
    }
    _addPolyLine();
  }

 GoogleMap(
        initialCameraPosition: CameraPosition(
            target: LatLng(_originLatitude, _originLongitude), zoom: 15),
        myLocationEnabled: true,
        tiltGesturesEnabled: true,
        compassEnabled: true,
        scrollGesturesEnabled: true,
        zoomGesturesEnabled: true,
        onMapCreated: _onMapCreated,
        markers: Set<Marker>.of(markers.values),
        polylines: Set<Polyline>.of(polylines.values),
      ),