Dart read file
last modified January 28, 2024
In this article we show how to read files in Dart language. We read text and binary files.
The dart:io package is used to do IO operations, including reading
files. The File class contains methods for manipulating files and
their contents.
The try/catch keywords can be used to handle possible IO exceptions.
Note that readAsString and readAsStringSync should
not be used for reading large files.
sky blue cup cloud water war pen dog
This is a simple text file.
Dart read file with readAsString
The readAsString method reads the entire file contents as a string
using the given encoding. The encoding is utf-8 by default. It returns a
Future<String> that completes with the string once the file
contents has been read.
import 'dart:io';
void main() async {
var path = 'words.txt';
try {
var file = File(path);
var contents = await file.readAsString();
print(contents);
} catch (e) {
stderr.writeln('failed to read file: \n${e}');
}
}
In the example, we read a text file with readAsString. We use
try/catch keywords to explicitly handle exceptions.
import 'dart:io';
We import the dart:io package.
var file = File(path);
A File object is created.
var contents = await file.readAsString();
We read the contents of the file with readAsString method. With
await, we wait until the async operation finishes.
$ dart main.dart sky blue cup cloud water war pen dog
Dart read file with readAsStringSync
The readAsStringSync reads the contents of the given file into a
string synchronously.
import 'dart:io';
void main() {
var fileName = 'words.txt';
var contents = File(fileName).readAsStringSync();
print(contents);
}
The example reads a text file with readAsStringSync.
Dart self read file
The next program reads its own contents.
import 'dart:convert';
import 'dart:io';
void main() async {
var file = File(Platform.script.toFilePath());
print(await (file.readAsString(encoding: ascii)));
}
We determine the program file name with Platform.script.toFilePath
and then read the contents with readAsString.
$ dart main.dart
import 'dart:convert';
import 'dart:io';
void main() async {
var file = File(Platform.script.toFilePath());
print(await (file.readAsString(encoding: ascii)));
}
Dart read file with readAsLines
With readAsString, we read the file asynchronously line by line.
The method returns a future resolving in a list of strings.
import 'dart:io';
void main() {
var fileName = 'words.txt';
var myFile = new File(fileName);
myFile.readAsLines().then((lines) => lines.forEach((line) => print(line)));
}
The example reads a text file asynchronously line by line.
myFile.readAsLines().then((lines) => lines.forEach((line) => print(line)));
Here we use a callback to process the lines.
In the next example, we use the async/await syntax.
import 'dart:io';
void main() async {
var fileName = 'words.txt';
var myFile = new File(fileName);
var lines = await myFile.readAsLines();
lines.forEach((line) {
print(line);
});
}
The program reads the file with readAsLines; it uses the
async/await syntax.
Dart read file with readAsLinesSync
The readAsLinesSync reads the file synchronously line by line.
It returns a list of strings.
import 'dart:io';
void main() {
var fileName = 'words.txt';
var myFile = new File(fileName);
List<String> lines = myFile.readAsLinesSync();
lines.forEach((line) => print(line));
}
The program reads a text file line by line in a synchronous way.
Dart read file with Stream
The next program reads a file using a Stream.
import 'dart:convert';
import 'dart:io';
void main() {
var fileName = 'words.txt';
Stream<List<int>> stream = new File(fileName).openRead();
StringBuffer buffer = new StringBuffer();
stream.transform(utf8.decoder).transform(LineSplitter()).listen((line) {
buffer.writeln("${line}");
}, onDone: () => print(buffer.toString()), onError: (e) => print(e));
}
We read a text file with a Stream and a StringBuffer.
Stream<List<int>> stream = new File(fileName).openRead();
We open a stream with openRead.
StringBuffer buffer = new StringBuffer();
We will write data to the StringBuffer.
stream.transform(utf8.decoder).transform(LineSplitter()).listen((line) {
buffer.writeln("${line}");
}, onDone: () => print(buffer.toString()), onError: (e) => print(e));
We transform the stream with a decoder and a line splitter. The final line is written to the buffer. In the end, the buffer is printed to the console.
Dart read binary file
The readAsBytes method reads the entire file contents as a list of
bytes. It returns a Future<Uint8List>.
import 'dart:convert';
import 'dart:io';
import 'package:collection/collection.dart';
void main() async {
var fileName = 'favicon.ico';
var bytes = await File(fileName).readAsBytes();
String base64Image = base64.encode(bytes);
print(base64Image);
var output = bytes
.mapIndexed((idx, e) => e.toRadixString(16).padLeft(2, '0'))
.join(' ');
print(output);
}
In the program, we read a small favicon file.
var bytes = await File(fileName).readAsBytes();
We read the file into a list of bytes.
String base64Image = base64.encode(bytes);
We encode the bytes into base64 encoding.
var output = bytes
.mapIndexed((idx, e) => e.toRadixString(16).padLeft(2, '0'))
.join(' ');
Alternatively, we convert the bytes into hexadecimal numbers.
Source
In this article we have showed how to read files in Dart.
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