Dart ZLibDecoder

last modified April 4, 2025

The ZLibDecoder class in Dart provides functionality to decompress data compressed with the zlib format. It's part of Dart's dart:io and dart:convert libraries.

ZLibDecoder handles the decompression of zlib-encoded data streams, commonly used in network protocols and file compression. It supports both raw and wrapped zlib streams.

Basic Definition

ZLibDecoder is a converter that transforms zlib-compressed byte data into its original uncompressed form. It implements the Codec interface for stream processing.

Key features include support for streaming decompression, memory efficiency, and handling of both zlib-wrapped and raw compressed data formats.

Basic ZLibDecoder Usage

This example shows basic decompression of a zlib-compressed byte list.

import 'dart:convert';

void main() {
  // Compressed data (zlib format)
  final compressed = [120, 156, 75, 76, 74, 6, 0, 2, 130, 1, 69];
  
  final decoder = ZLibDecoder();
  final decompressed = decoder.convert(compressed);
  
  print('Decompressed: ${String.fromCharCodes(decompressed)}');
}

We create a ZLibDecoder instance and use its convert method to decompress the byte list. The compressed data represents the string "Hello" in zlib format.

$ dart main.dart
Decompressed: Hello

Streaming Decompression

This example demonstrates streaming decompression of zlib data.

import 'dart:convert';
import 'dart:async';

Future<void> main() async {
  // Simulate streaming compressed data
  final stream = Stream.fromIterable([
    [120, 156],
    [75, 76, 74, 6],
    [0, 2, 130, 1, 69]
  ]);
  
  final decoder = ZLibDecoder();
  final decompressed = await decoder.bind(stream).toList();
  
  final result = decompressed.expand((x) => x).toList();
  print('Decompressed: ${String.fromCharCodes(result)}');
}

We use the bind method to connect a stream of compressed data chunks to the decoder. This is memory-efficient for large compressed data streams.

$ dart main.dart
Decompressed: Hello

Handling Raw ZLib Data

This example shows how to decompress raw zlib data without headers.

import 'dart:convert';

void main() {
  // Raw compressed data (no zlib headers)
  final rawCompressed = [75, 76, 74, 6, 0, 2, 130, 1, 69];
  
  final decoder = ZLibDecoder(raw: true);
  final decompressed = decoder.convert(rawCompressed);
  
  print('Decompressed: ${String.fromCharCodes(decompressed)}');
}

By setting the raw parameter to true, we tell the decoder to expect raw compressed data without zlib headers. This is useful for certain protocols.

$ dart main.dart
Decompressed: Hello

Error Handling

This example demonstrates proper error handling during decompression.

import 'dart:convert';

void main() {
  // Invalid compressed data
  final invalidData = [1, 2, 3, 4, 5];
  
  final decoder = ZLibDecoder();
  
  try {
    final decompressed = decoder.convert(invalidData);
    print('Decompressed: $decompressed');
  } on FormatException catch (e) {
    print('Decompression failed: ${e.message}');
  }
}

We wrap the decompression in a try-catch block to handle potential FormatExceptions that occur with invalid compressed data.

$ dart main.dart
Decompression failed: Invalid zlib data

Decompressing Large Files

This example shows how to decompress large zlib-compressed files efficiently.

import 'dart:io';
import 'dart:convert';

Future<void> main() async {
  final inputFile = File('compressed.zlib');
  final outputFile = File('decompressed.txt');
  
  final decoder = ZLibDecoder();
  
  await outputFile.openWrite().addStream(
    inputFile.openRead().transform(decoder)
  );
  
  print('File decompressed successfully');
}

We use file streams and the transform method to decompress large files without loading the entire content into memory. This is memory-efficient for big files.

$ dart main.dart
File decompressed successfully

Best Practices

• Stream processing: Use streams for large compressed data
• Error handling: Always handle FormatException
• Resource cleanup: Close streams properly
• Memory limits: Be mindful of decompressed size
• Format awareness: Know if data is raw or wrapped

Source

Dart ZLibDecoder Documentation

This tutorial covered Dart's ZLibDecoder class with practical examples showing basic decompression, streaming, error handling, and file operations.

Author

My name is Jan Bodnar, and I am a passionate programmer with extensive programming experience. I have been writing programming articles since 2007. To date, I have authored over 1,400 articles and 8 e-books. I possess more than ten years of experience in teaching programming.

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