【Flutter】SJISでファイルを出力する

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

import 'dart:async';

import 'package:flutter/services.dart';

class FlutterSjis {
  static const MethodChannel _channel = MethodChannel('flutter_sjis');

  /// 文字列を受け取りSJISに変換
  static Future<List<int>> encode(String value) async {
    return await _channel.invokeMethod('encode', value);
  }

  /// SJISのバイト配列を受け取り文字列に変換
  static Future<String> decode(List<int> source) async {
    return await _channel.invokeMethod('decode', source);
  }
}

package com.github.kiyosuke.flutter_sjis

import io.flutter.embedding.engine.plugins.FlutterPlugin
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 java.nio.charset.Charset

class FlutterSjisPlugin : FlutterPlugin, MethodCallHandler {

    private lateinit var channel: MethodChannel

    override fun onAttachedToEngine(flutterPluginBinding: FlutterPlugin.FlutterPluginBinding) {
        channel = MethodChannel(flutterPluginBinding.binaryMessenger, "flutter_sjis")
        channel.setMethodCallHandler(this)
    }

    override fun onMethodCall(call: MethodCall, result: Result) {
        when (call.method) {
            "encode" -> encode(call, result)
            "decode" -> decode(call, result)
            else -> result.notImplemented()
        }
    }

    private fun encode(call: MethodCall, result: Result) {
        val value = call.arguments as String
        val encoded = value.toByteArray(charset = Charset.forName("SJIS"))
        result.success(encoded)
    }

    private fun decode(call: MethodCall, result: Result) {
        val source = call.arguments as ByteArray
        val decoded = String(source, charset = Charset.forName("SJIS"))
        result.success(decoded)
    }

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

import Flutter
import UIKit

public class SwiftFlutterSjisPlugin: NSObject, FlutterPlugin {
    public static func register(with registrar: FlutterPluginRegistrar) {
        let channel = FlutterMethodChannel(name: "flutter_sjis", binaryMessenger: registrar.messenger())
        let instance = SwiftFlutterSjisPlugin()
        registrar.addMethodCallDelegate(instance, channel: channel)
    }
    
    public func handle(_ call: FlutterMethodCall, result: @escaping FlutterResult) {
        switch call.method {
        case "encode":
            encode(call, result: result)
            break
        case "decode":
            decode(call, result: result)
            break
        default:
            result(FlutterMethodNotImplemented)
            break
        }
    }
    
    private func encode(_ call: FlutterMethodCall, result: @escaping FlutterResult) {
        let value = call.arguments as! String
        let encoded = value.data(using: String.Encoding.shiftJIS, allowLossyConversion: true)!
        let data = FlutterStandardTypedData.init(bytes: encoded)
        result(data)
    }
    
    private func decode(_ call: FlutterMethodCall, result: @escaping FlutterResult) {
        let source = call.arguments as! FlutterStandardTypedData
        let decoded = String.init(bytes: source.data, encoding: String.Encoding.shiftJIS)!
        result(decoded)
    }
}

dependencies:
  flutter:
    sdk: flutter

  # パス指定でローカルのプラグインを利用できる
  flutter_sjis:
    path: ./flutter_sjis

import 'dart:io';

import 'package:flutter/material.dart';
import 'package:flutter_sjis/flutter_sjis.dart';
import 'package:path_provider/path_provider.dart';

class MyHomePage extends StatefulWidget {
  const MyHomePage({
    Key? key,
  }) : super(key: key);

  @override
  State<MyHomePage> createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  bool isLoading = false;
  File? savedFile;
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: const Text('FlutterSjis'),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            ElevatedButton.icon(
              onPressed: () async {
                setState(() => isLoading = true);
                try {
                  savedFile = await saveToFile(testData);
                  setState(() {});
                } finally {
                  setState(() => isLoading = false);
                }
              },
              icon: const Icon(Icons.save),
              label: const Text('保存'),
            ),
          ],
        ),
      ),
    );
  }

  Future<File> saveToFile(String data) async {
    final filesdir = await getApplicationSupportDirectory();
    final timestamp = DateTime.now().millisecondsSinceEpoch;
    final filename = '$timestamp.csv';
    final file = File('${filesdir.path}/$filename');
    final encodedData = await FlutterSjis.encode(data);
    return await file.writeAsBytes(encodedData);
  }
}

const testData = """
id,name,birthday,age,gender
1,佐藤,2000-01-01,22,男
2,鈴木,2002-01-01,20,男
3,亀田,2004-01-01,18,女
""";

static Future<List<int>> encode(String value) async {
  return await _channel.invokeMethod('encode', value);
}