In today’s quick tip, we will learn how easy it is to read and write files in Dart. We will use the File object of the ‘dart:io’ library to achieve this.

Prefer a video?

To follow along, generate the console-full project with stagehand:

$ mkdir my_app && cd my_app
$ stagehand console-full

And see the solution from the video below:

import 'dart:io';

main() async {
  var file = File('data.txt');
  var contents;

  if (await file.exists()) {
    // Read file
    contents = await file.readAsString();
    print(contents);

    // Write file
    var fileCopy = await File('data-copy.txt').writeAsString(contents);
    print(await fileCopy.exists());
    print(await fileCopy.length());
  }
}

Extending the solution

1. How to better specify the file path

Install the path package by adding the dependency to your pubspec.yaml file:

dependencies:
  path: ^1.6.2

And running pub get in your terminal.

Import this at the top of bin/main.dart and amend the path as follows:

import 'dart:io';
import 'package:path/path.dart';

void main() async {
  var pathToFile = join(dirname(Platform.script.toFilePath()), '..', 'data.txt');
  var file = File(pathToFile);
  ..
  ..
}

2. How to read the file line by line

We can create a stream to read the file:

import 'dart:convert'; // Contains the `Utf8Decoder` and `LineSplitter` stream transformers
..
..
  if (await file.exists()) {
    // Read file
    contents = StringBuffer();
    var contentStream = file.openRead();

    contentStream
      .transform(Utf8Decoder())
      .transform(LineSplitter())
      .listen((String line) => contents.write(line), // Add line to our StringBuffer object
        onDone: () => print(contents.toString()), // Call toString() method to receive the complete data
        onError: (e) => print('[Problems]: $e'));
    ..
    ..
  }
..
..

A good use case for this would be processing a file containing environment variables, like the file contents below:

PORT=8080
API_KEY=lkjsk453lkslfkl5
API_USER=jimmyo

Please leave your feedback in the comments, and let me know what you would like to see demonstrated next!

Further reading:

1. The File class

2. Free Dart screencasts on Egghead.io