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.
To follow along, generate the console-full project with stagehand:
$ mkdir my_app && cd my_app
$ stagehand console-full
See the solution in action below:
Here's the full solution:
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 example
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=jermaineo
Please leave your feedback in the comments, and let me know what you would like to see demonstrated next!
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Further reading:
Top comments (3)
Thanks for your article!
If I were lazy to look up the documentation myself, I'd like to extend this example by two things:
Thanks @sandordargo for your feedback. I've updated the article on those points.
You're awesome, man!