在混合開發(fā)中，無(wú)論哪種技術(shù)手段都避免不了native與框架的互相調(diào)用牢贸。Flutter基于以下方法實(shí)現(xiàn)了與native的互相調(diào)用忙厌。

Flutter稱這種操作叫platform-specific

接下來(lái)要實(shí)現(xiàn)一個(gè)栗子、Flutter端獲取Native的電量娶视，并且當(dāng)電量改變時(shí)會(huì)通知Flutter端隘蝎。

一购啄、Flutter調(diào)用Native

使用android端實(shí)現(xiàn)

1、創(chuàng)建一個(gè)項(xiàng)目

項(xiàng)目創(chuàng)建

2嘱么、在Flutter端封裝一個(gè)獲取手機(jī)電量的方法

import 'package:flutter/services.dart';

class BatteryManager {
  /// MethodChannel name是一個(gè)唯一標(biāo)記狮含，不重復(fù)就好
  static const platform =
      const MethodChannel('org.tiny.platformspecific/battery');

  /// 遠(yuǎn)程調(diào)用需要異步操作, 把這個(gè)方法生命成 async
  static Future<int> getBattery() async {
    // 處理異常
    try {
      // getBatteryLevel 是遠(yuǎn)程方法的名字
      final int result = await platform.invokeMethod('getBatteryLevel');
      return Future.value(result);
    } on PlatformException catch (e) {
      return Future.value(-1);
    }
  }
}

3、Android端具體實(shí)現(xiàn)

package org.tiny.platformspecific;

import android.content.Context;
import android.content.ContextWrapper;
import android.content.Intent;
import android.content.IntentFilter;

import io.flutter.app.FlutterActivity;
import io.flutter.plugin.common.MethodCall;
import io.flutter.plugin.common.MethodChannel;
import io.flutter.view.FlutterView;

/**
 * Created by tiny on 2/21/2019.
 */
public class BatteryManager implements MethodChannel.MethodCallHandler {
    private static final String CHANNEL = "org.tiny.platformspecific/battery";
    private Context mContext;

    static void registerWith(FlutterActivity activity) {
        new BatteryManager(activity.getFlutterView());
    }

    private BatteryManager(FlutterView view) {
        mContext = view.getContext();
        new MethodChannel(view, CHANNEL).setMethodCallHandler(this);
    }

    @Override
    public void onMethodCall(MethodCall methodCall, MethodChannel.Result result) {
        String method = methodCall.method;
        switch (method) {
            case "getBatteryLevel":
                result.success(getBatteryLevel());
                break;
            default:
                result.notImplemented();
                break;
        }
    }

    private int getBatteryLevel() {
        Intent intent = new ContextWrapper(mContext.getApplicationContext()).
                registerReceiver(null, new IntentFilter(Intent.ACTION_BATTERY_CHANGED));
        if (intent == null) return -1;
        return intent.getIntExtra(android.os.BatteryManager.EXTRA_LEVEL, -1);
    }
}

4曼振、注冊(cè)

package org.tiny.platformspecific;

import android.os.Bundle;

import io.flutter.app.FlutterActivity;
import io.flutter.plugins.GeneratedPluginRegistrant;

public class MainActivity extends FlutterActivity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        GeneratedPluginRegistrant.registerWith(this);
        BatteryManager.registerWith(this);
    }
}

5几迄、總結(jié)

以上完成了Flutter到Android端的調(diào)用、套路還是很簡(jiǎn)單的冰评，總結(jié)一下：
1映胁、Flutter端創(chuàng)建 MethodChannel，使用 invokeMethod 調(diào)用遠(yuǎn)程方法
2甲雅、Native端創(chuàng)建 MethodChannel解孙，實(shí)現(xiàn) MethodCallHandler
3、Native完成注冊(cè)

二抛人、Flutter對(duì)Native設(shè)置監(jiān)聽

因?yàn)殡娏渴且粋€(gè)不斷變化的值弛姜，所以就需要Flutter對(duì)Native的監(jiān)聽這時(shí)候使用MethodChannel是不行的，所以有請(qǐng)下一個(gè)EventChannel妖枚，還是先上代碼廷臼。

1、Flutter端創(chuàng)建

參照之前的Flutter代碼進(jìn)行了修改，使電量管理成為一個(gè)單例

import 'dart:async';

import 'package:flutter/services.dart';

class BatteryManager {
  static BatteryManager _instance;
  final MethodChannel _methodChannel;
  final EventChannel _eventChannel;
  StreamSubscription _eventStreamSubscription;

  /// _私有構(gòu)造方法
  BatteryManager._(this._methodChannel, this._eventChannel);

  /// 創(chuàng)建單例
  static BatteryManager getInstance() {
    if (_instance == null) {
      final MethodChannel methodChannel =
          const MethodChannel('org.tiny.platformspecific/battery');
      final EventChannel eventChannel =
          const EventChannel('org.tiny.platformspecific/charging');
      _instance = BatteryManager._(methodChannel, eventChannel);
    }
    return _instance;
  }

  /// 得到電量
  Future<int> getBatteryLevel() async {
    try {
      final int level = await _methodChannel.invokeMethod('getBatteryLevel');
      return Future.value(level);
    } on PlatformException catch (e) {
      return Future.value(-1);
    }
  }

  /// 監(jiān)聽電量的變化
  StreamSubscription setOnBatteryListener(void onEvent(int battery)) {
    if (_eventStreamSubscription == null) {
      _eventStreamSubscription = _eventChannel
          .receiveBroadcastStream()
          .listen((data) => onEvent(data));
    }
    return _eventStreamSubscription;
  }
}

2荠商、Android端代碼

package org.tiny.platformspecific;

import android.content.BroadcastReceiver;
import android.content.Context;
import android.content.ContextWrapper;
import android.content.Intent;
import android.content.IntentFilter;

import io.flutter.app.FlutterActivity;
import io.flutter.plugin.common.EventChannel;
import io.flutter.plugin.common.MethodCall;
import io.flutter.plugin.common.MethodChannel;

/**
 * Created by tiny on 2/21/2019.
 */
public class BatteryManager implements MethodChannel.MethodCallHandler, EventChannel.StreamHandler {
    private static final String METHOD = "org.tiny.platformspecific/battery";
    private static final String EVENT = "org.tiny.platformspecific/charging";
    private FlutterActivity mFlutterActivity;
    private BroadcastReceiver mChargingStateChangeReceiver;

    static void registerWith(FlutterActivity activity) {
        new BatteryManager(activity);
    }

    private BatteryManager(FlutterActivity activity) {
        mFlutterActivity = activity;
        new MethodChannel(mFlutterActivity.getFlutterView(), METHOD).setMethodCallHandler(this);
        new EventChannel(mFlutterActivity.getFlutterView(), EVENT).setStreamHandler(this);
    }

    @Override
    public void onMethodCall(MethodCall methodCall, MethodChannel.Result result) {
        String method = methodCall.method;
        switch (method) {
            case "getBatteryLevel":
                result.success(getBatteryLevel());
                break;
            default:
                result.notImplemented();
                break;
        }
    }

    private int getBatteryLevel() {
        Intent intent = new ContextWrapper(mFlutterActivity.getApplicationContext()).
                registerReceiver(null, new IntentFilter(Intent.ACTION_BATTERY_CHANGED));
        if (intent == null) return -1;
        return intent.getIntExtra(android.os.BatteryManager.EXTRA_LEVEL, -1);
    }

    @Override
    public void onListen(Object arguments, final EventChannel.EventSink eventSink) {
        mChargingStateChangeReceiver = new BroadcastReceiver() {
            @Override
            public void onReceive(Context context, Intent intent) {
                int battery = intent.getIntExtra(android.os.BatteryManager.EXTRA_LEVEL, -1);
                eventSink.success(battery);
            }
        };
        IntentFilter filter = new IntentFilter(Intent.ACTION_BATTERY_CHANGED);
        mFlutterActivity.registerReceiver(mChargingStateChangeReceiver, filter);
    }

    @Override
    public void onCancel(Object arguments) {
        if (mChargingStateChangeReceiver != null) {
            mFlutterActivity.unregisterReceiver(mChargingStateChangeReceiver);
        }
    }
}

3寂恬、使用

class MyApp extends StatefulWidget {
  @override
  MyAppState createState() => MyAppState();
}

class MyAppState extends State<MyApp> {
  int _battery = 0;
  StreamSubscription _streamSubscription;

  @override
  void initState() {
    _getBattery();
    super.initState();
  }

  void _getBattery() async {
    int battery = await BatteryManager.getInstance().getBatteryLevel();
    _setBattery(battery);
    _streamSubscription =
        BatteryManager.getInstance().setOnBatteryListener(_setBattery);
  }

  void _setBattery(int battery) {
    setState(() {
      _battery = battery;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Center(
        child: Text('Battery: $_battery'),
      ),
    );
  }

  @override
  void dispose() {
    _streamSubscription?.cancel();
    super.dispose();
  }
}

三、遇到的一些坑

1结啼、android studio 代碼離奇報(bào)錯(cuò)

我在android studio中編輯代碼的時(shí)候掠剑，出現(xiàn)了各種莫名其妙的錯(cuò)誤，比如

顯示我現(xiàn)在最小sdk版本是1郊愧？wtf。所以我直接用as打開了flutter中的android項(xiàng)目井佑， 啥毛病也沒(méi)有了

2属铁、取消廣播

就android端實(shí)現(xiàn)而言，獲取電量是通過(guò)廣播獲取的躬翁， 所以要在生命周期中加入取消的動(dòng)作焦蘑，flutter頁(yè)面被銷毀時(shí)調(diào)用了dispose方法，結(jié)果我得到的是一堆報(bào)錯(cuò)盒发，說(shuō)明沒(méi)有調(diào)用unregisterReceiver例嘱，天地良心，我確實(shí)寫了宁舰，但flutter并沒(méi)有為我調(diào)用dispose拼卵。
原因是這樣的， 我在flutter使用hot reload蛮艰，直接在首頁(yè)調(diào)用了（整個(gè)應(yīng)用只有一個(gè)頁(yè)面）直接按back鍵退出時(shí)腋腮，可能是reload的問(wèn)題，導(dǎo)致了dispose不調(diào)用壤蚜。我把代碼修改成了這個(gè)樣子即寡。

import 'dart:async';

import 'package:flutter/material.dart';
import 'package:platform_specific/battery_manager.dart';

void main() => runApp(Main());

class Main extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: Hello(),
    );
  }
}

class Hello extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
        home: Scaffold(
      appBar: AppBar(),
      body: Center(
        child: RaisedButton(
          child: Text('Hello'),
          onPressed: () {
            Navigator.of(context).push(MaterialPageRoute(builder: (context) {
              return MyApp();
            }));
          },
        ),
      ),
    ));
  }
}

class MyApp extends StatefulWidget {
  @override
  MyAppState createState() => MyAppState();
}

class MyAppState extends State<MyApp> {
  int _battery = 0;
  StreamSubscription _streamSubscription;

  @override
  void initState() {
    _getBattery();
    super.initState();
  }

  void _getBattery() async {
    int battery = await BatteryManager.getInstance().getBatteryLevel();
    _setBattery(battery);
    _streamSubscription =
        BatteryManager.getInstance().setOnBatteryListener(_setBattery);
  }

  void _setBattery(int battery) {
    setState(() {
      _battery = battery;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Center(
        child: Text('Battery: $_battery'),
      ),
    );
  }

  @override
  void dispose() {
    _streamSubscription?.cancel();
    super.dispose();
  }
}