flutter create --template=plugin --platforms=android,ios flutter_battery_plugin

flutter_battery_plugin/
├── lib/flutter_battery_plugin.dart  // API Dart
├── android/src/.../FlutterBatteryPlugin.kt
├── ios/Classes/FlutterBatteryPlugin.swift

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

class FlutterBatteryPlugin {
  static const MethodChannel _channel = MethodChannel('flutter_battery_plugin');

  static Future<int?> getBatteryLevel() async {
    final int? level = await _channel.invokeMethod('getBatteryLevel');
    return level;
  }
}

class FlutterBatteryPlugin: FlutterPlugin, MethodCallHandler {
  override fun onAttachedToEngine(binding: FlutterPlugin.FlutterPluginBinding) {
    val channel = MethodChannel(binding.binaryMessenger, "flutter_battery_plugin")
    channel.setMethodCallHandler(this)
  }

  override fun onMethodCall(call: MethodCall, result: Result) {
    if (call.method == "getBatteryLevel") {
      val batteryLevel = ... // เหมือนบทที่ 16
      result.success(batteryLevel)
    } else {
      result.notImplemented()
    }
  }

  override fun onDetachedFromEngine(binding: FlutterPlugin.FlutterPluginBinding) {}
}

public class FlutterBatteryPlugin: NSObject, FlutterPlugin {
  public static func register(with registrar: FlutterPluginRegistrar) {
    let channel = FlutterMethodChannel(name: "flutter_battery_plugin", binaryMessenger: registrar.messenger())
    let instance = FlutterBatteryPlugin()
    registrar.addMethodCallDelegate(instance, channel: channel)
  }

  public func handle(_ call: FlutterMethodCall, result: @escaping FlutterResult) {
    if call.method == "getBatteryLevel" {
      // ดึงค่า battery แล้วส่งกลับ
    } else {
      result(FlutterMethodNotImplemented)
    }
  }
}