当你在开发flutter应用的时候，有时会需要调用native的api，往往遇到flutter并没有相应的package， 这时候flutter plugin就开始发挥作用了，这篇文章将会讲解开发一个简单flutter plugin的步骤和方法，好了，让我们开始动手吧。

1.在Android Studio 中创建一个Flutter Plugin 项目，如下图

上图中你能看到项目描述中写到，如果需要暴露Andorid或iOS的API给开发者时，选择”Plugin”项目类型。
这个项目我们命名为:flutter_native_log_plugin, 当我们完成创建项目后，有两个文件我们需要看一看， 一个是位于android/src下的FlutterNativeLogPlugin.java， 这段代码是用来和本地设备交互，然后将交互结果返回供flutter前端调用， 如下所示：

package com.cube8.flutter_native_log_plugin;

import io.flutter.plugin.common.MethodCall;
import io.flutter.plugin.common.MethodChannel;
import io.flutter.plugin.common.MethodChannel.MethodCallHandler;
import io.flutter.plugin.common.MethodChannel.Result;
import io.flutter.plugin.common.PluginRegistry.Registrar;

/** FlutterNativeLogPlugin */
public class FlutterNativeLogPlugin implements MethodCallHandler {
  /** Plugin registration. */
  public static void registerWith(Registrar registrar) {
    final MethodChannel channel = new MethodChannel(registrar.messenger(), 
        "flutter_native_log_plugin");
    channel.setMethodCallHandler(new FlutterNativeLogPlugin());
  }

  @Override
  public void onMethodCall(MethodCall call, Result result) {
    if (call.method.equals("getPlatformVersion")) {
      result.success("Android " + android.os.Build.VERSION.RELEASE);
    } else {
      result.notImplemented();
    }
  }
}

另一个 /lib/mian.dart文件，这段代码是主要用来和native代码交互， 如下所示：

import 'dart:async';

import 'package:flutter/services.dart';

class FlutterNativeLogPlugin {
  static const MethodChannel _channel =
      const MethodChannel('flutter_native_log_plugin');

  static Future<String> get platformVersion async {
    final String version = await _channel.invokeMethod('getPlatformVersion');
    return version;
  }
}

2.现在我们开始编写我们的Plugin.

在lib/flutter_native_log_plugin.dart 文件中，我们先创建一个新的方法，代码如下：

import 'dart:async';

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

enum Log { DEBUG, WARNING, ERROR }

class FlutterNativeLogPlugin {
  static const MethodChannel _channel =
      const MethodChannel('flutter_native_log_plugin');

  static Future<String> printLog(
      {Log logType, @required String tag, @required String msg}) async {
    String log = "debug";
    if (logType == Log.WARNING) {
      log = "warning";
    } else if (logType == Log.ERROR) {
      log = "error";
    } else {
      log = "debug";
    }

    final Map<String, dynamic> params = <String, dynamic>{
      'tag': tag,
      'msg': msg,
      'logType': log
    };

    final String result = await _channel.invokeMethod('printLog', params);

    return result;
  }
}

在Android端，我们将android/src下的FlutterNativePlugin.java改写如下：

package com.cube8.flutter_native_log_plugin;

import android.util.Log;

import io.flutter.plugin.common.MethodCall;
import io.flutter.plugin.common.MethodChannel;
import io.flutter.plugin.common.MethodChannel.MethodCallHandler;
import io.flutter.plugin.common.MethodChannel.Result;
import io.flutter.plugin.common.PluginRegistry.Registrar;

/**
 * FlutterNativeLogPlugin
 */
public class FlutterNativeLogPlugin implements MethodCallHandler {
    /**
     * Plugin registration.
     */
    public static void registerWith(Registrar registrar) {
        final MethodChannel channel = new MethodChannel(registrar.messenger(), "flutter_native_log_plugin");
        channel.setMethodCallHandler(new FlutterNativeLogPlugin());
    }

    @Override
    public void onMethodCall(MethodCall call, Result result) {
        if (call.method.equals("printLog")) {
            String msg = call.argument("msg");
            String tag = call.argument("tag");
            String logType = call.argument("logType");

            if (logType.equals("warning")) {
                Log.w(tag, msg);
            } else if (logType.equals("error")) {
                Log.e(tag, msg);
            } else {
                Log.d(tag, msg);
            }

            result.success("Logged Successfully!");
        } else {
            result.notImplemented();
        }
    }
}

3.测试plugin。当开发完了我们的plugin之后，我们需要测试这个新plugin是否可用，于是对example/lib的main.dart文件作如下修改：

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

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

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

class _MyAppState extends State<MyApp> {

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

  void printLogs() async {
    print(await FlutterNativeLogPlugin.printLog(
        tag: "Debug", msg: "This is ordinary Log")); // default logType
    print(await FlutterNativeLogPlugin.printLog(
        tag: "Debug",
        msg: "This is warning Log",
        logType: Log.WARNING)); // logType = warning
    print(await FlutterNativeLogPlugin.printLog(
        tag: "Debug",
        msg: "This is error Log",
        logType: Log.ERROR)); // logType = error
    print(await FlutterNativeLogPlugin.printLog(
        tag: "Debug",
        msg: "This is debug Log",
        logType: Log.DEBUG)); // logType = debug
  }

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(
          title: const Text('Plugin example app'),
        ),
        body: Center(
          child: RaisedButton(
            child: Text("PrintLogs"),
            onPressed: printLogs,
          ),
        ),
      ),
    );
  }
}

点击app中的按钮，控制台将看到如下输出，说明plugin可以顺利运行了。

4.最后一步就是将我们开发的plugin发布到dart pub供以后直接调用。打开控制台，需要确认定位到plugin项目的根目录，然后输入如下命令：

flutter packages pub publish --dry-run

这段命令会做一个程序相关文件和信息的检查，确保待发布的plugin信息完整，根据控制台的提示完善信息后，与下图相似：

接着输入如下命令，正式将plugin发布到dart pub中：

flutter packages pub publish

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