Dart IOSink
last modified April 4, 2025
The IOSink class in Dart provides a stream-based interface for
performing I/O operations. It's commonly used for writing to files, sockets,
and other byte-oriented sinks.
IOSink implements the StreamSink interface and provides methods
for both synchronous and asynchronous writing. It's part of Dart's core
dart:io library.
Basic Definition
IOSink is an abstract class that represents a destination for
byte data. It provides methods to write data and manage the output stream.
Key features include buffered writing, encoding support, and stream control. It's used with files, stdout, and network sockets in Dart applications.
Basic IOSink Usage
This example shows basic file writing using IOSink.
import 'dart:io';
void main() async {
var file = File('output.txt');
var sink = file.openWrite();
sink.write('Hello, ');
sink.writeln('Dart!');
await sink.close();
print('File written successfully');
}
We create a file sink, write text to it, and properly close the resource.
The openWrite method returns an IOSink for file operations.
$ dart main.dart File written successfully
Writing Different Data Types
This example demonstrates writing various data types to a sink.
import 'dart:io';
void main() async {
var sink = stdout;
sink.write('String: ');
sink.writeln('Dart');
sink.write('Integer: ');
sink.writeln(42);
sink.write('Double: ');
sink.writeln(3.14);
sink.write('Boolean: ');
sink.writeln(true);
await sink.flush();
}
IOSink provides methods to write different data types with automatic conversion to strings. We use stdout as the sink in this example.
$ dart main.dart String: Dart Integer: 42 Double: 3.14 Boolean: true
Buffered Writing
This example shows how IOSink handles buffered writing.
import 'dart:io';
void main() async {
var file = File('buffered.txt');
var sink = file.openWrite();
for (var i = 1; i <= 10; i++) {
sink.write('Line $i\n');
if (i % 5 == 0) {
await sink.flush();
print('Flushed at line $i');
}
}
await sink.close();
}
The sink buffers writes for efficiency. We manually flush every 5 lines to demonstrate buffer control. Normally, closing the sink flushes automatically.
$ dart main.dart Flushed at line 5 Flushed at line 10
Error Handling
This example demonstrates proper error handling with IOSink.
import 'dart:io';
void main() async {
var sink = File('nonexistent/path/output.txt').openWrite();
try {
sink.write('Test data');
await sink.close();
} on IOException catch (e) {
print('Error writing file: $e');
await sink.close();
}
}
We handle potential I/O errors when writing to a file. The sink is properly closed in both success and error cases to prevent resource leaks.
$ dart main.dart Error writing file: FileSystemException: Cannot open file, path = 'nonexistent/path/output.txt' (OS Error: No such file or directory, errno = 2)
Using with Encoding
This example shows how to specify character encoding when writing.
import 'dart:io';
import 'dart:convert';
void main() async {
var file = File('encoded.txt');
var sink = file.openWrite(encoding: latin1);
sink.write('Special chars: ñçá');
await sink.close();
var content = await file.readAsString(encoding: latin1);
print(content);
}
We specify Latin-1 encoding when creating the sink to handle special characters correctly. The same encoding is used when reading back the file.
$ dart main.dart Special chars: ñçá
Best Practices
- Always close: Ensure sinks are properly closed
- Use await: Await close/flush operations
- Handle errors: Implement error handling for I/O operations
- Specify encoding: Set encoding when working with text
- Consider buffering: Use flush strategically for large writes
Source
This tutorial covered Dart's IOSink class with practical examples
showing file operations, stdout writing, buffering, and error handling.
Author
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