如何在flutter中显示google地图中的旅行时间？

dependencies:
  flutter:
    sdk: flutter
  google_maps_flutter: ^1.2.0

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

class MapPage extends StatefulWidget {
  @override
  _MapPageState createState() => _MapPageState();
}

class _MapPageState extends State<MapPage> {
  GoogleMapController mapController;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Google Maps'),
      ),
      body: GoogleMap(
        onMapCreated: (controller) {
          setState(() {
            mapController = controller;
          });
        },
        initialCameraPosition: CameraPosition(
          target: LatLng(37.7749, -122.4194), // 设置地图初始位置
          zoom: 12.0, // 设置地图初始缩放级别
        ),
      ),
    );
  }
}

import 'package:flutter/material.dart';
import 'package:google_maps_flutter/google_maps_flutter.dart';
import 'package:http/http.dart' as http;
import 'dart:convert';

class MapPage extends StatefulWidget {
  @override
  _MapPageState createState() => _MapPageState();
}

class _MapPageState extends State<MapPage> {
  GoogleMapController mapController;
  String travelTime = '';

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Google Maps'),
      ),
      body: Column(
        children: [
          Expanded(
            child: GoogleMap(
              onMapCreated: (controller) {
                setState(() {
                  mapController = controller;
                });
              },
              initialCameraPosition: CameraPosition(
                target: LatLng(37.7749, -122.4194),
                zoom: 12.0,
              ),
            ),
          ),
          RaisedButton(
            child: Text('获取旅行时间'),
            onPressed: () {
              getTravelTime();
            },
          ),
          Text(travelTime),
        ],
      ),
    );
  }

  void getTravelTime() async {
    final response = await http.get(
        'https://maps.googleapis.com/maps/api/directions/json?origin=37.7749,-122.4194&destination=37.7749,-122.5194&key=YOUR_API_KEY');
    final data = json.decode(response.body);
    final duration = data['routes'][0]['legs'][0]['duration']['text'];
    setState(() {
      travelTime = '旅行时间：$duration';
    });
  }
}