📦 Lesson 4: Packages & Libraries

🧠 Core Concept

A package is a reusable collection of Dart code with a well-defined public API. Instead of copying utility functions between projects, you create a package once and import it everywhere. Dart's package manager (pub) handles downloading, versioning, and dependency resolution for you.

🧠 Every Dart package follows a standard layout:

Dart enforces conventions that make packages predictable. When you run dart create -t package super_utils, you get this structure:

🔑 Critical Convention: lib/src/ is Private

Files inside lib/src/ are conventionally private. Package consumers should never import them directly. Instead, you re-export the public parts from your main library file.

This convention lets package authors refactor internal code without breaking consumers.

🧠 Every package has one pubspec.yaml file at its root

This file tells Dart your package's name, version, dependencies, and environment requirements. It's the single source of truth for package configuration.

🧠 Your main library file acts as the "front door" to your package

Users should only ever import package:your_package/your_package.dart. Inside that file, you use export statements to expose exactly what you want them to use.

📋 Example: What's Inside a Utility File?

Here's what lib/src/string_utils.dart might contain:

// lib/src/string_utils.dart
// These functions are "private" to the package structure
// but become public when exported from super_utils.dart

String reverse(String str) {
  return str.split('').reversed.join('');
}

int countVowels(String str) {
  return str
    .toLowerCase()
    .replaceAll(RegExp(r'[^aeiou]'), '')
    .length;
}

bool isPalindrome(String str) {
  final cleaned = str.toLowerCase().replaceAll(RegExp(r'[^a-z0-9]'), '');
  return cleaned == reverse(cleaned);
}

🧠 Three types of dependencies in pubspec.yaml

🧠 When you have multiple related packages, a workspace ties them together

A workspace lets you manage multiple packages in one repository. One dart pub get resolves dependencies for all packages, ensuring consistency.

🧠 pub.dev is the official Dart/Flutter package repository

With over 40,000 packages, you'll find solutions for HTTP, JSON, databases, CLI tools, and more. Always check package scores (likes, pub points, popularity) before adopting a dependency.