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Reading and Writing Files

In some cases, it can be handy to read and write files to disk. This can be used to persist data across app launches or download data from the internet and save it for later offline use.

In order to save files to disk, we’ll need to combine the path_provider plugin with the dart:io library.

Directions

Find the correct local path
Create a reference to the file location
Write data to the file
Read data from the file

1. Find the correct local path

In this example, we’ll display a counter. When the counter changes, we’ll want to write data on disk so we can read it again when the app loads. Therefore, we need to ask: Where should we store this data?

The path_provider plugin provides a platform-agnostic way to access commonly used locations on the device’s filesystem. The plugin 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 NSDocumentDirectory. On Android, this is the AppData directory.

In our case, we’ll want to store information in the documents directory! We can find the path to the documents directory like so:

Future<String> get _localPath async {
  final directory = await getApplicationDocumentsDirectory();
  
  return directory.path;
}

2. Create a reference to the file location

Once we know where to store the file, we’ll need to create a reference to the file’s full location. We can use the File class from the dart:io library to achieve this.

Future<File> get _localFile async {
  final path = await _localPath;
  return new File('$path/counter.txt');
}

3. Write data to the file

Now that we have a File to work with, we can use it to read and write data! First, we’ll write some data to the file. Since we’re working with a counter, we’ll simply store the integer as a String.

Future<File> writeCounter(int counter) async {
  final file = await _localFile;
  
  // Write the file
  return file.writeAsString('$counter');
}

4. Read data from the file

Now that we have some data on disk, we can read it! Once again, we’ll use the File class to do so.

Future<int> readCounter() async {
  try {
    final file = await _localFile;

    // Read the file
    String contents = await file.readAsString();

    return int.parse(contents);
  } catch (e) {
    // If we encounter an error, return 0
    return 0;
  }
}

Testing

In order to test code that interacts with files, we’ll need to Mock calls to the MethodChannel. The MethodChannel is the class that Flutter uses to communicate with the host platform.

In our tests, we can’t interact with the filesystem on a device. We’ll need to interact with our test environment’s filesystem!

To mock the method call, we can provide a setupAll function in our test file. This function will run before the tests are executed.

setUpAll(() async {
  // Create a temporary directory to work with
  final directory = await Directory.systemTemp.createTemp();
  
  // Mock out the MethodChannel for the path_provider plugin
  const MethodChannel('plugins.flutter.io/path_provider')
      .setMockMethodCallHandler((MethodCall methodCall) async {
    // If we're getting the apps documents directory, return the path to the
    // temp directory on our test environment instead.
    if (methodCall.method == 'getApplicationDocumentsDirectory') {
      return directory.path;
    }
    return null;
  });
});

Complete Example

import 'dart:async';
import 'dart:io';

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

void main() {
  runApp(
    new MaterialApp(
      title: 'Reading and Writing Files',
      home: new FlutterDemo(storage: new CounterStorage()),
    ),
  );
}

class CounterStorage {
  Future<String> get _localPath async {
    final directory = await getApplicationDocumentsDirectory();

    return directory.path;
  }

  Future<File> get _localFile async {
    final path = await _localPath;
    return new File('$path/counter.txt');
  }

  Future<int> readCounter() async {
    try {
      final file = await _localFile;

      // Read the file
      String contents = await file.readAsString();

      return int.parse(contents);
    } catch (e) {
      // If we encounter an error, return 0
      return 0;
    }
  }

  Future<File> writeCounter(int counter) async {
    final file = await _localFile;

    // Write the file
    return file.writeAsString('$counter');
  }
}

class FlutterDemo extends StatefulWidget {
  final CounterStorage storage;

  FlutterDemo({Key key, @required this.storage}) : super(key: key);

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

class _FlutterDemoState extends State<FlutterDemo> {
  int _counter;

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

  Future<File> _incrementCounter() async {
    setState(() {
      _counter++;
    });

    // write the variable as a string to the file
    return widget.storage.writeCounter(_counter);
  }

  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      appBar: new AppBar(title: new Text('Reading and Writing Files')),
      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),
      ),
    );
  }
}