flutter学习笔记（二）

本地访问-MethodChannel

1. 本地访问是指跨平台的解决方案，有访问本地android或者ios资源的能力,主要是MethodChannel和MessageChannel两种，第一种是调用方法，第二种是传递信息。通信的双方是Flutter和本地操作系统或者应用，而且方法的调用和消息的方法可以从任何一方发起，类似远程过程调用。

• Flutter是一个比较独立的环境，要想访问本地的资源，Flutter提供了Channel机制，类似Cilent-Server模式或者RPC
• 通过Channel的名称打通Channel，隧道
• Flutter和本地是对称的，请求可以从任何一发发起，这篇文章主要是记录从Flutter给本地发送请求

---

• 首先我们创建Flutte工程,ide随你自己喜欢，修改UI代码如下:

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

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 {
  final String title;
  @override
  _MyHomePageState createState() => new _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  int _counter = 0;
  static const platformMethodChannel = const MethodChannel("com.xyjy.flutterchannel/test");
  num nativeMessage = 0;

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

  Future<Null> getData() async{
    num _message;
      try{
        final num result = await platformMethodChannel.invokeMethod("getTime");
        _message = result;
      }
      on PlatformException catch(e){
        print(e.message);
      }
      setState(() {
        nativeMessage = _message;
      });
  }

  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      appBar: new AppBar(
        // Here we take the value from the MyHomePage object that was created by
        // the App.build method, and use it to set our appbar title.
        title: new Text(widget.title),
      ),
      body: new Center(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            new Text(
              "$nativeMessage",
            ),
            new Text(
              '$nativeMessage',
              style: Theme.of(context).textTheme.display1,
            ),
          ],
        ),
      ),
      floatingActionButton: new FloatingActionButton(
        onPressed: ()=>getData(),
        tooltip: 'Increment',
        child: new Icon(Icons.add),
      ), // This trailing comma makes auto-formatting nicer for build methods.
    );
  }
}

• 很简单我只是将_counter显示的替换成了自己定义的nativeMessage，接着往下

1. 首先我们要引入service包和async包。

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

2. 声明一个static const变量platformMethodChannel ，赋值MethodChannel类型的对象。

 class _MyHomePageState extends State<MyHomePage> {
  int _counter = 0;
  static const platformMethodChannel = const MethodChannel("com.xyjy.flutterchannel/test");
  num nativeMessage = 0;

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

3. 在上面我们要注意，给MethodChannel一个名字com.xyjy.flutterchannel/test,这个名字和本地（这里代表android部分）那边的要一样
   我们创建了MethodChannel，我们准备一个用来发送请求的函数。我们创建一个叫getData的异步函数
   await platformMethodChannel.invokeMethod('getTime') 是调用本地的getTime方法，并且这个方法是异步的，await表示阻塞执行。
   nativeMessage = _message; 表示通知Flutter状态改变，刷新界面。

 Future<Null> getData() async{
    // num是数字类型
    num _message;
      try{
        final num result = await platformMethodChannel.invokeMethod("getTime");
        _message = result;
      }
      on PlatformException catch(e){
        print(e.message);
      }
      setState(() {
        nativeMessage = _message;
      });
  }

4. 修改build里面的显示，将_counter 替换为nativeMessage ，并将floatingActionButton的onTap事件替换为我们定义好的getData方法

  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      appBar: new AppBar(
        // Here we take the value from the MyHomePage object that was created by
        // the App.build method, and use it to set our appbar title.
        title: new Text(widget.title),
      ),
      body: new Center(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            new Text(
              "$nativeMessage",
            ),
            new Text(
              '$nativeMessage',
              style: Theme.of(context).textTheme.display1,
            ),
          ],
        ),
      ),
      floatingActionButton: new FloatingActionButton(
        onPressed: ()=>getData(),
        tooltip: 'Increment',
        child: new Icon(Icons.add),
      ), // This trailing comma makes auto-formatting nicer for build methods.
    );
  }

5. 接下来本地方法如何编写，打开MainActivity

 public class MainActivity extends FlutterActivity {
  private static final  String CHANNAL = "com.xyjy.flutterchannel/test";
  @Override
  protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    GeneratedPluginRegistrant.registerWith(this);
    new MethodChannel(getFlutterView() , CHANNAL).setMethodCallHandler(new MethodChannel.MethodCallHandler()
    {
      @Override
      public void onMethodCall(MethodCall methodCall, MethodChannel.Result result)
      {
          if (methodCall.method.equals("getTime"))
          {
              long time = getTime();
              result.success(time);
          }
          else
          {
            result.notImplemented();
          }
      }
    });
  }

  private long getTime()
  {
    return System.currentTimeMillis();
  }

}

• CHANNAL 里面的字符串一定是与Flutter定义一致，在MethodChannel的中调用.setMethodCallHandler() 方法，需要一个MethodCallHandler 对象，是一个匿名内部类，有一个方法onMethodCall,在Flutter发送请求时，onMethodCall方法会执行， result.success(time);返回信息,这样就可以进行本地调用，点击悬浮按钮得到时间(此时的时间是通过本地返回的时间)

  
  QQ图片20180709174145.png