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Flutter Native 方法调用

Flutter Native 方法调用

作者: 初见破晓 | 来源:发表于2019-02-14 18:16 被阅读20次

在混合开发中，无论哪种技术手段都避免不了native与框架的互相调用。Flutter基于以下方法实现了与native的互相调用。

Flutter称这种操作叫platform-specific

接下来要实现一个栗子、Flutter端获取Native的电量，并且当电量改变时会通知Flutter端。

一、Flutter调用Native

使用android端实现

1、创建一个项目

项目创建

2、在Flutter端封装一个获取手机电量的方法

import 'package:flutter/services.dart';

class BatteryManager {
  /// MethodChannel name是一个唯一标记，不重复就好
  static const platform =
      const MethodChannel('org.tiny.platformspecific/battery');

  /// 远程调用需要异步操作, 把这个方法生命成 async
  static Future<int> getBattery() async {
    // 处理异常
    try {
      // getBatteryLevel 是远程方法的名字
      final int result = await platform.invokeMethod('getBatteryLevel');
      return Future.value(result);
    } on PlatformException catch (e) {
      return Future.value(-1);
    }
  }
}

3、Android端具体实现

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、注册

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、总结

以上完成了Flutter到Android端的调用、套路还是很简单的，总结一下：
1、Flutter端创建 MethodChannel，使用 invokeMethod 调用远程方法
2、Native端创建 MethodChannel，实现 MethodCallHandler
3、Native完成注册

二、Flutter对Native设置监听

因为电量是一个不断变化的值，所以就需要Flutter对Native的监听这时候使用MethodChannel是不行的，所以有请下一个EventChannel，还是先上代码。

1、Flutter端创建

参照之前的Flutter代码进行了修改，使电量管理成为一个单例

import 'dart:async';

import 'package:flutter/services.dart';

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

  /// _私有构造方法
  BatteryManager._(this._methodChannel, this._eventChannel);

  /// 创建单例
  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;
  }

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

  /// 监听电量的变化
  StreamSubscription setOnBatteryListener(void onEvent(int battery)) {
    if (_eventStreamSubscription == null) {
      _eventStreamSubscription = _eventChannel
          .receiveBroadcastStream()
          .listen((data) => onEvent(data));
    }
    return _eventStreamSubscription;
  }
}

2、Android端代码

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、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、android studio 代码离奇报错

我在android studio中编辑代码的时候，出现了各种莫名其妙的错误，比如



显示我现在最小sdk版本是1？wtf。所以我直接用as打开了flutter中的android项目， 啥毛病也没有了

2、取消广播

就android端实现而言，获取电量是通过广播获取的， 所以要在生命周期中加入取消的动作，flutter页面被销毁时调用了dispose方法，结果我得到的是一堆报错，说明没有调用unregisterReceiver，天地良心，我确实写了，但flutter并没有为我调用dispose。
原因是这样的， 我在flutter使用hot reload，直接在首页调用了（整个应用只有一个页面）直接按back键退出时，可能是reload的问题，导致了dispose不调用。我把代码修改成了这个样子。

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();
  }
}

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