# Libraries, Imports and pub.dev Packages — Dart

Source: https://www.skillbyai.com/en/dart/t-packages

> Organise code into libraries and use and publish packages.

## Libraries and packages

Every Dart file is a **library**. Import other libraries with `import 'package:shop/models/order.dart';` (from your own or another package's `lib/` folder), `import 'dart:convert';` for core libraries, or relative paths within the same package. Control names with **`as`** (prefixes, as in `import 'package:http/http.dart' as http;`), **`show`** and **`hide`**. **Privacy** is library-level: names starting with `_` are visible only within their library. Conventionally, public API lives in `lib/`, implementation details in `lib/src/`, and a top-level `lib/shop.dart` **exports** the public parts. Packages come from **pub.dev**, which shows popularity, pub points (quality checks), platform support and verified publishers. Version constraints use **caret syntax**: `^1.2.0` allows `>=1.2.0 <2.0.0`. `dart pub outdated` shows available updates, `dart pub upgrade` updates within constraints, and `dependency_overrides` handles temporary conflicts. Publishing your own package requires a good `README`, `CHANGELOG`, licence, example and `dart pub publish --dry-run` first.

## Libraries inside a package

Public API in lib/, implementation in lib/src/, and an export file exposing what users should see.

![A folder tree with a top-level file glowing as the public entrance and a nested folder of greyed files behind it.](assets/figures/dart/section-7-map.svg) — Figure 7.1 — Package layout with a public export file.

## Imports, prefixes and an export file

Public API is exported; implementation stays in lib/src.

```dart
// lib/shop.dart (public entry point of package:shop)
library;

export 'src/models/order.dart' show Order, OrderStatus;
export 'src/services/checkout.dart' show Checkout;

// lib/src/services/checkout.dart
import 'dart:convert';
import 'package:http/http.dart' as http;
import '../models/order.dart';

class Checkout {
  Checkout(this._client);
  final http.Client _client;               // private to this library

  Future<Order> place(Order draft) async {
    final response = await _client.post(
      Uri.parse('https://api.example.com/orders'),
      headers: {'content-type': 'application/json'},
      body: jsonEncode(draft.toJson()),
    );
    return Order.fromJson(jsonDecode(response.body) as Map<String, dynamic>);
  }
}

// in an app:
// import 'package:shop/shop.dart';     // one import gives the public API
```

## Check pub points and maintenance

Before adding a package, look at its pub.dev score, last release date, open issues and whether a verified publisher maintains it. Every dependency is code you will have to upgrade later.

**Quiz:** What does the version constraint ^1.2.0 allow in pubspec.yaml?

- [x] Any version from 1.2.0 up to but not including 2.0.0
- [ ] Exactly 1.2.0
- [ ] Any version at all
- [ ] Only 1.2.x patch releases

*Answer:* Any version from 1.2.0 up to but not including 2.0.0. Caret syntax allows compatible versions within the same major version.