Dart RandomAccessFile
last modified April 4, 2025
The RandomAccessFile class in Dart provides random access to files.
It allows reading and writing at specific positions within a file.
RandomAccessFile is part of Dart's dart:io library. It supports
asynchronous operations and is essential for advanced file manipulation.
Basic Definition
RandomAccessFile represents a file opened for random access.
It provides methods for reading, writing, and seeking within the file.
Key features include position manipulation, synchronous/asynchronous operations, and direct byte access. It's particularly useful for binary file operations.
Opening and Reading a File
This example shows how to open a file and read its contents.
import 'dart:io';
void main() async {
var file = File('data.txt');
var raf = await file.open(mode: FileMode.read);
var contents = await raf.read(await raf.length());
print(String.fromCharCodes(contents));
await raf.close();
}
We open a file in read mode, read its entire contents, then close it. The read operation returns bytes which we convert to a string.
$ dart main.dart File contents here...
Writing to a File
This example demonstrates writing data to a file at specific positions.
import 'dart:io';
void main() async {
var file = File('output.dat');
var raf = await file.open(mode: FileMode.write);
await raf.writeFrom([65, 66, 67]); // ABC
await raf.setPosition(0);
await raf.writeFrom([68, 69]); // DE
await raf.close();
}
We open a file in write mode, write some bytes, then overwrite the beginning. The setPosition method moves the file pointer to the specified offset.
$ dart main.dart $ hexdump output.dat 0000000 44 45 43
Seeking and Reading
This example shows how to seek to specific positions and read data.
import 'dart:io';
void main() async {
var file = File('data.bin');
var raf = await file.open(mode: FileMode.read);
await raf.setPosition(10);
var data = await raf.read(4);
print('Data at position 10-13: $data');
await raf.close();
}
We open a binary file, seek to position 10, then read 4 bytes from that point. This demonstrates random access capabilities of the class.
$ dart main.dart Data at position 10-13: [1, 2, 3, 4]
File Locking
This example demonstrates file locking during operations.
import 'dart:io';
void main() async {
var file = File('shared.dat');
var raf = await file.open(mode: FileMode.append);
await raf.lock();
try {
await raf.writeFrom([1, 2, 3]);
} finally {
await raf.unlock();
await raf.close();
}
}
We acquire an exclusive lock before writing to prevent concurrent access. The lock is released in a finally block to ensure it always happens.
$ dart main.dart
Reading and Writing Structures
This example shows reading and writing structured binary data.
import 'dart:io';
import 'dart:typed_data';
void main() async {
var file = File('records.dat');
var raf = await file.open(mode: FileMode.write);
// Write a record
var buffer = ByteData(8);
buffer.setInt32(0, 1234);
buffer.setFloat32(4, 56.78);
await raf.writeFrom(buffer.buffer.asUint8List());
// Read it back
await raf.setPosition(0);
var data = await raf.read(8);
var reader = ByteData.view(data.buffer);
print('Int: ${reader.getInt32(0)}');
print('Float: ${reader.getFloat32(4)}');
await raf.close();
}
We write a structured record containing an integer and float, then read it back. ByteData is used to handle different data types in the binary file.
$ dart main.dart Int: 1234 Float: 56.78
Best Practices
- Always close: Ensure files are closed after use
- Error handling: Use try-catch for file operations
- Buffering: Consider buffering for small writes
- Positioning: Be mindful of position changes
Source
Dart RandomAccessFile Documentation
This tutorial covered Dart's RandomAccessFile class with practical examples showing file operations, positioning, locking, and structured data handling.
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