reading-writing-files.md

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This guide describes how to read and write files in Flutter by using the PathProvider class.

• TOC {:toc}

Introduction

The PathProvider class provides a platform-agnostic way to access commonly used locations on the device's filesystem. The class currently supports access to two filesystem locations:

• Temporary directory: A temporary directory (cache) that the system can clear at any time. On iOS, this corresponds to the value that NSTemporaryDirectory() returns. On Android, this is the value that getCacheDir() returns.
• Documents directory: A directory for the app to store files that only it can access. The system clears the directory only when the app is deleted. On iOS, this corresponds to NSDocumentsDirectory. On Android, this is the AppData directory.

Once your Flutter app has a reference to a file location, you can use the dart:io APIs to perform read/write operations to the filesystem. For more information about working on files and directories using Dart, see this overview and these examples.

Example of reading and writing to a file

The following example shows how to modify the default app created by flutter create, so that the app persists the number of button presses across relaunches.

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

void main() {
  runApp(
    new MaterialApp(
      title: 'Flutter Demo',
      theme: new ThemeData(primarySwatch: Colors.blue),
      home: new FlutterDemo(),
    ),
  );
}

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

  @override
  _FlutterDemoState createState() => new _FlutterDemoState();
}

class _FlutterDemoState extends State<FlutterDemo> {
  int _counter;

  @override
  void initState() {
    super.initState();
    _readCounter().then((int value) {
      setState(() {
        _counter = value;
      });
    });
  }

  Future<File> _getLocalFile() async {
    // get the path to the document directory.
    String dir = (await PathProvider.getApplicationDocumentsDirectory()).path;
    return new File('$dir/counter.txt');
  }

  Future<int> _readCounter() async {
    try {
      File file = await _getLocalFile();
      // read the variable as a string from the file.
      String contents = await file.readAsString();
      return int.parse(contents);
    } on FileSystemException {
      return 0;
    }
  }

  Future<Null> _incrementCounter() async {
    setState(() {
      _counter++;
    });
    // write the variable as a string to the file
    await (await _getLocalFile()).writeAsString('$_counter');
  }

  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      appBar: new AppBar(title: new Text('Flutter Demo')),
      body: new Center(
        child: new Text('Button tapped $_counter time${
          _counter == 1 ? '' : 's'
        }.'),
      ),
      floatingActionButton: new FloatingActionButton(
        onPressed: _incrementCounter,
        tooltip: 'Increment',
        child: new Icon(Icons.add),
      ),
    );
  }
}