Dart write file

last modified January 28, 2024

In this article we show how to write to files in Dart language.

The dart:io package is used to do IO operations, including writing files. The File class contains methods for manipulating files and their contents.

Dart provides both synchronous and asynchronous methods for writing to files, such as writeAsStringSync and writeAsString.

The openWrite function creates a new independent IOSink for the file. It must be explicitly closed when no longer used, to free system resources.

Dart writeAsStringSync

The writeAsStringSync method synchronously writes a string to a file. It opens the file, writes the string in the given encoding, and closes the file.

import 'dart:io';

void main() {
  final fname = 'output.txt';
  final f = new File(fname);

  final data = 'an old falcon';
  f.writeAsStringSync(data);
}

In the program, we write a string into a file synchronously. The file is created if it does not exist. If it exists, it is overwritten.

import 'dart:io';

We import the dart:io library.

final fname = 'output.txt';
final f = new File(fname);

We create a new instance of File.

final data = 'an old falcon';
f.writeAsStringSync(data);

We write the string into the file.

Dart writeAsString

The writeAsString function opens the file, writes the string in the given encoding, and closes the file. It returns a Future<File> that completes with the file object once the entire operation has completed.

import 'dart:io';

void main() async {
  final fname = 'output2.txt';
  final f = new File(fname);

  final data = 'a stormy night';
  await f.writeAsString(data);
}

We write a string to a file with writeAsString function.

void main() async {

Since we use the asynchronous writeAsString function, we must add the async keyword to the main declaration.

await f.writeAsString(data);

With the await keyword we get the completed result of the async operation.

Dart writeAsBytesSync

The writeAsBytesSync member function synchronously writes a list of bytes to a file.

import 'dart:io';

void main() {
  final fname = 'bytes.txt';

  final text = "one 🐘 and three 🐋";
  final data = utf8.encode(text);
  final f = new File(fname);

  f.writeAsBytesSync(data);
}

In the example, we write a text message with emojis to a file using writeAsBytesSync.

final text = "one 🐘 and three 🐋";

We have the text with two emojis.

final data = utf8.encode(text);

We encode the text into a list of integers.

f.writeAsBytesSync(data);

The encoded text is written to the file synchronously.

$ dart main.dart 
$ cat bytes.txt
one 🐘 and three 🐋

Dart fetch image

In the next example, we fetch an image from an URL resource.

import 'dart:io';
import 'package:http/http.dart' as http;

void main() async {
  final res = await http.get(Uri.http('webcode.me', '/favicon.ico'));
  final bdata = res.bodyBytes;

  final fname = 'favicon.ico';
  final f = await File(fname);
  await f.writeAsBytes(bdata);
}

The example downloads a small favicon.

import 'package:http/http.dart' as http;

To create a HTTP GET request, we use the http library.

final bdata = res.bodyBytes;

With bodyBytes we retrieve the bytes comprising the body of the response.

final fname = 'favicon.ico';
final f = await File(fname);

We create the File object.

await f.writeAsBytes(bdata);

The bytes are written to the file.

Dart append to file

In the next example, we append data to a file.

import 'dart:io';

void main() async {
  final fname = 'words.txt';

  final words = ['cloud', 'blue', 'book', 'solid'];
  final f = new File(fname);

  for (final word in words) {
    await f.writeAsString('$word\n', mode: FileMode.append);
  }
}

To append data to a file, we use the FileMode.append mode.

Dart IOSink write

The IOSink's write method writes an object to the sink.

import 'dart:io';

void main() {
  final fname = 'test.txt';

  final f = File(fname);
  final sink = f.openWrite();
  sink.write('current datetime: ${DateTime.now()}\n');

  sink.close();
}

The example writes the current datetime to the sink.

final sink = f.openWrite();

From the File object, we open the sink with openWrite method.

sink.write('current datetime: ${DateTime.now()}\n');

The data is written to the sink with write.

sink.close();

We close the IOSink to free system resources.

Dart IOSink writeAll

The IOSink writeAll function iterates over the given objects and writes them in sequence.

import 'dart:io';

void main() {
  final fname = 'words2.txt';

  final words = ['sky', 'ocean', 'pen', 'rock', 'storm', 'falcon'];

  final f = File(fname);
  final sink = f.openWrite();
  sink.writeAll(words, "\n");

  sink.close();
}

In the example, we use writeAll to write the contents of a list to the sink.

Source

Dart I/O documentation

In this article we have showed how to write to files in Dart.

Author

My name is Jan Bodnar and I am a passionate programmer with many years of programming experience. I have been writing programming articles since 2007. So far, I have written over 1400 articles and 8 e-books. I have over eight years of experience in teaching programming.

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