【Flutter】ファイルアクセス

import 'package:flutter/material.dart';
import 'dart:ui' as ui;
import 'dart:io';
import 'package:path_provider/path_provider.dart';

void main() {
  runApp(new MyApp());
}
class MyApp extends StatelessWidget {

  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      title: 'Generated App',
      theme: new ThemeData(
        primarySwatch: Colors.blue,
        primaryColor: const Color(0xFF2196f3),
        canvasColor: const Color(0xFFfafafa),
      ),
      home: MyHomePage(),
    );
  }
}

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

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  final _controller = TextEditingController();
  final _fname = 'flutter_sampledata.txt';

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Home'),
      ),
      body: Padding(
        padding: EdgeInsets.all(20),
        child:Column(
          children:<Widget> [
               Text('FILE ACESS.',
               style: TextStyle(fontSize: 32,
               fontWeight: ui.FontWeight.w500),
               ),
              Padding(padding: EdgeInsets.all(10.0)),
              TextField(
                controller: _controller,
                style: TextStyle(fontSize: 24),
                minLines: 1,
                maxLines: 5,
              )
              ],
            ),
          ),
          bottomNavigationBar: BottomNavigationBar(
            backgroundColor: Colors.blue,
            selectedItemColor: Colors.white,
            unselectedItemColor: Colors.white,
            currentIndex: 0,
            items: <BottomNavigationBarItem>[
              BottomNavigationBarItem(
                label: 'Save',
                icon: Icon(Icons.save, color: Colors.white, size: 32),
              ),
              BottomNavigationBarItem(
                label: 'Load',
                icon: Icon(Icons.open_in_new, color: Colors.white, size:32)
              ),
            ],
            onTap: (int value) async {
              switch (value) {
                case 0:
                  saveIt(_controller.text);
                  setState(() {
                    _controller.text = '';
                  });
                  showDialog(
                    context: context,
                    builder: (BuildContext context) => AlertDialog(
                      title: Text("saved!"),
                      content: Text("save message to file."),
                    )
                  );
                  break;
                case 1:
                  String value = await loadIt();
                  setState((){
                    _controller.text = value;
                  });
                  showDialog(
                    context: context,
                    builder: (BuildContext context) => AlertDialog(
                      title: Text("loaded!"),
                      content: Text("load message from file."),
                    )
                  );
                  break;
                default:
                  print('no default.');
              }
            },
          ),
    );
  }

  Future<File> getDataFile(String filename) async {
    final directory = await getApplicationDocumentsDirectory();
    return File(directory.path + '/' + filename);
  }

  void saveIt(String value) async {
    final file = await getDataFile(_fname);
    file.writeAsString(value);
  }

  Future<String> loadIt() async {
    try {
      final file = await getDataFile(_fname);
      return file.readAsString();
    } catch (e) {
      return '*** no data ***';
    }
  }
}

リソースファイルの読み込み
L assetsフォルダ内にdocumentsというフォルダを用意し、その中にdata.txtファイルを作成する
pubspec.yamlに追記

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

void main() {
  runApp(new MyApp());
}
class MyApp extends StatelessWidget {

  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      title: 'Generated App',
      theme: new ThemeData(
        primarySwatch: Colors.blue,
        primaryColor: const Color(0xFF2196f3),
        canvasColor: const Color(0xFFfafafa),
      ),
      home: MyHomePage(),
    );
  }
}

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

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  final _controller = TextEditingController();
  final _fname = 'documents/data.txt';

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Home'),
      ),
      body: Padding(
        padding: EdgeInsets.all(20),
        child:Column(
          children:<Widget> [
               Text('RESOURCE ACCESS',
               style: TextStyle(fontSize: 32,
               fontWeight: ui.FontWeight.w500),
               ),
              Padding(padding: EdgeInsets.all(10.0)),
              TextField(
                controller: _controller,
                style: TextStyle(fontSize: 24),
                minLines: 1,
                maxLines: 5,
              )
              ],
            ),
          ),
          floatingActionButton: FloatingActionButton(
            child: Icon(Icons.open_in_new),
            onPressed: () async {
              final value = await loadIt();
              setState(() {
                _controller.text = value;
              });
              showDialog(
                context: context,
                builder: (BuildContext context) =>
                    AlertDialog(
                      title: Text("loaded!"),
                      content: Text("load message from Asset."),
                    )
              );
            }
          )
    );
  }

  Future<String> getDataAsset(String path) async {
    return await rootBundle.loadString(path);
  }

  Future<String> loadIt() async {
    try {
      final res = await getDataAsset(_fname);
      return res;
    } catch (e) {
      return '*** no data ***';
    }
  }
}