FlutterでAndroid/iOSのネイティブのAPIを使う

package com.example.flash_light

import android.content.Context
import android.hardware.camera2.CameraManager
import android.os.Build
import android.os.Bundle

import io.flutter.app.FlutterActivity
import io.flutter.plugin.common.MethodChannel
import io.flutter.plugins.GeneratedPluginRegistrant

class MainActivity : FlutterActivity() {
    companion object {
        private const val CHANNEL_NAME = "com.example/flash_light"
    }

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        GeneratedPluginRegistrant.registerWith(this)


        MethodChannel(flutterView, CHANNEL_NAME).setMethodCallHandler { methodCall, result ->
            when (methodCall.method) {
                "setTorchMode" -> (methodCall.arguments as HashMap<String, *>)["enabled"]?.let {
                    setTorchMode(it as Boolean, result)
                }
                else -> result.notImplemented()
            }
        }
    }

    private fun setTorchMode(enabled: Boolean, result: MethodChannel.Result) {
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.M) {
            val cameraManager = getSystemService(Context.CAMERA_SERVICE) as CameraManager
            val cameraId = cameraManager.cameraIdList[0]
            cameraManager.setTorchMode(cameraId, enabled)
            result.success(enabled)
        } else {
            result.error("UNAVAILABLE", "Flash not available", null)
        }
    }
}

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

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

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

class _MyAppState extends State<MyApp> {
  static const channel = MethodChannel('com.example/flash_light');

  bool _isFlashLightOn = false;

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      theme: ThemeData(primarySwatch: Colors.blue),
      home: Scaffold(
        appBar: AppBar(title: Text('Flash Light Sample')),
        body: Center(
          child: RaisedButton(
            onPressed: _setTorchMode,
            child: Text('Flash Light: ${_isFlashLightOn ? "ON" : "OFF"}'),
          ),
        ),
      ),
    );
  }

  void _setTorchMode() async {
    final isEnabled = await channel.invokeMethod<bool>(
      "setTorchMode",
      {"enabled": !_isFlashLightOn},
    );
    setState(() => _isFlashLightOn = isEnabled);
  }
}