flutter/examples/platform_channel_swift/lib/main.dart

// Copyright 2017 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

import 'dart:async';

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

class PlatformChannel extends StatefulWidget {
  @override
  _PlatformChannelState createState() => new _PlatformChannelState();
}

class _PlatformChannelState extends State<PlatformChannel> {
  static const PlatformMethodChannel platform = const PlatformMethodChannel('samples.flutter.io/battery');
  String _batteryLevel = '';

  Future<Null> _getBatteryLevel() async {
    String batteryLevel;
    try {
      final int result = await platform.invokeMethod('getBatteryLevel');
      batteryLevel = 'Battery level at $result % .';
    } on PlatformException catch (e) {
      batteryLevel = "Failed to get battery level: '${e.message}'.";
    } 
    setState(() {
      _batteryLevel = batteryLevel;
    });
  }

  @override
  Widget build(BuildContext context) {
    return new Material(
      child: new Center(
        child: new Column(
          mainAxisAlignment: MainAxisAlignment.spaceEvenly,
          children: <Widget>[
            new RaisedButton(
              child: new Text('Get Battery Level'),
              onPressed: _getBatteryLevel,
            ),
            new Text(_batteryLevel, key: new Key('Battery level label')),
          ],
        ),
      ),
    );
  }
}

void main() {
  runApp(new PlatformChannel());
}