Packages and pub

Why packages?

You should not write an HTTP client, a date formatter or a PDF generator from scratch. A package is reusable code published for others: Flutter apps commonly use http for networking, provider or flutter_bloc for state management, shared_preferences for settings, and many more. pub.dev hosts tens of thousands of them.

pubspec.yaml

name: chai_shop
description: Orders and sales for a tea stall.
version: 1.0.0+1

environment:
  sdk: ^3.5.0                 # Dart SDK versions this project supports

dependencies:
  flutter:
    sdk: flutter
  http: ^1.2.0                # used in the app
  intl: ^0.19.0

dev_dependencies:
  flutter_test:
    sdk: flutter
  flutter_lints: ^5.0.0       # only needed while developing

flutter:
  uses-material-design: true
  assets:
    - assets/images/

dependencies are packages your app uses at run time. dev_dependencies are only for development, like test tools, linters and code generators. They are not shipped in your app.

The version line 1.0.0+1 is your app's own version: 1.0.0 is the name users see, and the number after + is the build number that must increase for every store upload (module 11).

Adding a package

flutter pub add http          # in a Flutter project
dart pub add intl             # in a pure Dart project
flutter pub add --dev build_runner
flutter pub get               # download everything listed in pubspec.yaml
flutter pub outdated          # see which packages have newer versions

import 'package:http/http.dart' as http;
import 'package:intl/intl.dart';

void main() {
  print(NumberFormat.currency(locale: 'en_IN', symbol: '₹').format(1234567.5));   // ₹12,34,567.50
}

Versions: caret and lock file

Packages follow semantic versioning: MAJOR.MINOR.PATCH. A major change may break your code. ^1.2.0 (caret syntax) means "1.2.0 or any later version below 2.0.0". When you run pub get, pub picks versions that satisfy every constraint and writes the exact choices into pubspec.lock. Commit pubspec.lock for apps, so every developer and the build server use the same versions.

To update packages, run flutter pub upgrade (within your constraints) or change the constraint in pubspec.yaml for a new major version, then read the package's changelog for breaking changes and run your tests.

Choosing a good package

Common mistakes

• Tabs or wrong indentation in pubspec.yaml: "Error on line 12". YAML needs consistent spaces.
• Adding a package for every small task. Each dependency is code you must trust and update.
• Choosing an abandoned package because a tutorial used it. Check the last publish date.
• Editing pubspec.lock by hand. Change pubspec.yaml and let pub update the lock file.