如何在带有flutter的google地图上长按drop pins？

import 'package:flutter/material.dart';
import 'package:google_maps_flutter/google_maps_flutter.dart';

class MapScreen extends StatefulWidget {
  @override
  _MapScreenState createState() => _MapScreenState();
}

class _MapScreenState extends State<MapScreen> {
  GoogleMapController _mapController;
  Set<Marker> _markers = {};

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Google Map with Flutter'),
      ),
      body: GoogleMap(
        initialCameraPosition: CameraPosition(
          target: LatLng(37.7749, -122.4194), // 初始位置（旧金山）
          zoom: 12.0, // 初始缩放级别
        ),
        markers: _markers,
        onMapCreated: (GoogleMapController controller) {
          _mapController = controller;
        },
        gestureRecognizers: Set()
          ..add(Factory<EagerGestureRecognizer>(() => EagerGestureRecognizer())),
      ),
    );
  }

  void _addMarker(LatLng location) {
    setState(() {
      _markers.add(
        Marker(
          markerId: MarkerId(location.toString()),
          position: location,
          infoWindow: InfoWindow(
            title: 'Marker Title',
            snippet: 'Marker Snippet',
          ),
          icon: BitmapDescriptor.defaultMarker,
        ),
      );
    });
  }
}