如何在一个扑动的应用程序中通过Google Rest Api对步数计数器进行编码？

我想在Flutter应用程序中借助Google Fit REST API编写一个计步器 . 但是无法理解我应该如何在Google Fit官方文档links的帮助下进行编码 . 如何在我的flutter项目中使用带有Google Fit REST API的googleapis 0.51.0 Flutter包？

main.dart

import 'dart:async';
import 'package:flutter/material.dart';
import 'package:googleapis_auth/auth_io.dart';
import 'package:googleapis/fitness/v1.dart' as fitness;

final _credentials = new ServiceAccountCredentials.fromJson(r'''
{
  "private_key_id": ...,
  "private_key": ...,
  "client_email": ...,
  "client_id": ...,
  "type": "service_account"
}
''');

const _SCOPES = const [fitness.FitnessApi.FitnessBodyWriteScope];

void main() => runApp(new MyApp());

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      title: 'Flutter Demo',
      theme: new ThemeData(

        primarySwatch: Colors.blue,
      ),
      home: new MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);

  final String title;

  @override
  _MyHomePageState createState() => new _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  int _counter = 0;

  void _incrementCounter() {
    setState(() {
      _counter++;
    });
  }

  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      appBar: new AppBar(
        title: new Text(widget.title),
      ),
      body: new Center(
        child: new Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            new Text(
              'You have pushed the button this many times:',
            ),
            new Text(
              '$_counter',
              style: Theme.of(context).textTheme.display1,
            ),
          ],
        ),
      ),
      floatingActionButton: new FloatingActionButton(
        onPressed: _incrementCounter,
        tooltip: 'Increment',
        child: new Icon(Icons.add),
      ),
    );
  }
}

pubspec.yaml

dependencies:
  flutter:
    sdk: flutter
  googleapis: "^0.51.0"
  googleapis_auth: "^0.2.5"