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When the project scale grows, splitting code into multiple files and modules becomes essential. This chapter introduces Dart's package management system pub, the dependency configuration file pubspec.yaml, import/export syntax, and how to create custom libraries. * * * ## pubspec.yaml Configuration pubspec.yaml is the core configuration file for each Dart project, declaring project metadata and dependencies. A typical pubspec.yaml file structure is as follows: ## Example # File path: pubspec.yaml name: my_dart_app # Project name (required, lowercase + underscore) description: A Dart sample project # Project description version: 1.0.0 # Version number # publish_to: none # If you don't want to publish to pub.dev, uncomment this line environment: sdk: '>=3.0.0 <4.0.0' # Dart SDK version range dependencies: # Third-party packages required at runtime http: ^1.1.0 # HTTP client path: ^1.8.0 # Path utility dev_dependencies: # Dependencies needed only during development (testing, linting, etc.) test: ^1.24.0 # Testing framework lints: ^2.0.0 # Official lint rules # Optional: Executable entry point # executables: # my_app: main Version number notation guide: | Notation | Meaning | Example | | --- | --- | --- | | ^1.1.0 | Compatible with 1.1.0 to 2.0.0 (exclusive) | Most common, recommended | | 1.1.0 | Exact version | Less flexible | | >=1.1.0 <1.5.0 | Version range | Used when precise control is needed | | any | Any version | Not recommended | > The ^ symbol (caret) is Dart's default version constraint method. ^1.1.0 is equivalent to >=1.1.0 <2.0.0. This