问flutter:为什么我无法获得电池电量？

EN

import 'dart:async';

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

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'battery level',
      home: MyHomePage(title: 'battery level'),
    );
  }
}

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

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

class _MyHomePageState extends State<MyHomePage> {
  static const platform = const MethodChannel('samples.fltter.io/battery');
  String _batteryLevel = 'Unknow battery level';

  Future<Null> getBatteryLevel() async {
    String batteryLevel = '';
    try {
      int res = await platform.invokeMethod('getBatteryLevel');
      batteryLevel = 'battery level is: $res';
    } on PlatformException catch(e) {
      batteryLevel = 'caught an error';
    }
    setState(() {
      _batteryLevel = batteryLevel;
    });
    print(_batteryLevel);
  }
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Center(
        child: Column(
          children: <Widget>[
            RaisedButton(
              child: Text('click'),
              onPressed: (){getBatteryLevel();},
            ),
            Text(_batteryLevel),
          ],
        ),
      )
    );
  }
}

E/flutter (11202): [ERROR:flutter/lib/ui/ui_dart_state.cc(157)] Unhandled Exception: MissingPluginException(No implementation found for method getBatteryLevel on channel samples.fltter.io/battery)
E/flutter (11202): #0      MethodChannel._invokeMethod (package:flutter/src/services/platform_channel.dart:154:7)
E/flutter (11202): <asynchronous suspension>
E/flutter (11202): #1      MethodChannel.invokeMethod (package:flutter/src/services/platform_channel.dart:329:12)
E/flutter (11202): #2      _MyHomePageState._getBatteryLevel (package:batterylevel/main.dart:54:41)
E/flutter (11202): #3      _MyHomePageState.build.<anonymous closure> (package:batterylevel/main.dart:75:29)
E/flutter (11202): #4      _InkResponseState._handleTap (package:flutter/src/material/ink_well.dart:779:19)
E/flutter (11202): #5      _InkResponseState.build.<anonymous closure> (package:flutter/src/material/ink_well.dart:862:36)
E/flutter (11202): #6      GestureRecognizer.invokeCallback (package:flutter/src/gestures/recognizer.dart:182:24)
E/flutter (11202): #7      TapGestureRecognizer.handleTapUp (package:flutter/src/gestures/tap.dart:504:11)
E/flutter (11202): #8      BaseTapGestureRecognizer._checkUp (package:flutter/src/gestures/tap.dart:282:5)
E/flutter (11202): #9      BaseTapGestureRecognizer.handlePrimaryPointer (package:flutter/src/gestures/tap.dart:217:7)
E/flutter (11202): #10     PrimaryPointerGestureRecognizer.handleEvent (package:flutter/src/gestures/recognizer.dart:475:9)
E/flutter (11202): #11     PointerRouter._dispatch (package:flutter/src/gestures/pointer_router.dart:76:12)
E/flutter (11202): #12     PointerRouter._dispatchEventToRoutes.<anonymous closure> (package:flutter/src/gestures/pointer_router.dart:122:9)
E/flutter (11202): #13     _LinkedHashMapMixin.forEach (dart:collection-patch/compact_hash.dart:379:8)
E/flutter (11202): #14     PointerRouter._dispatchEventToRoutes (package:flutter/src/gestures/pointer_router.dart:120:18)
E/flutter (11202): #15     PointerRouter.route (package:flutter/src/gestures/pointer_router.dart:106:7)
E/flutter (11202): #16     GestureBinding.handleEvent (package:flutter/src/gestures/binding.dart:218:19)
E/flutter (11202): #17     GestureBinding.dispatchEvent (package:flutter/src/gestures/binding.dart:198:22)
E/flutter (11202): #18     GestureBinding._handlePointerEvent (package:flutter/src/gestures/binding.dart:156:7)
E/flutter (11202): #19     GestureBinding._flushPointerEventQueue (package:flutter/src/gestures/binding.dart:102:7)
E/flutter (11202): #20     GestureBinding._handlePointerDataPacket (package:flutter/src/gestures/binding.dart:86:7)
E/flutter (11202): #21     _rootRunUnary (dart:async/zone.dart:1196:13)
E/flutter (11202): #22     _CustomZone.runUnary (dart:async/zone.dart:1085:19)
E/flutter (11202): #23     _CustomZone.runUnaryGuarded (dart:async/zone.dart:987:7)
E/flutter (11202): #24     _invoke1 (dart:ui/hooks.dart:275:10)
E/flutter (11202): #25     _dispatchPointerDataPacket (dart:ui/hooks.dart:184:5)