Lesson 52 of 59

5 mins read

Libraries, Namespaces & Barrel Exports

In Plain English

Dart organizes code into **Libraries**. Files are connected using `import 'path/to/file.dart';`. To avoid namespace naming collisions, you can prefix imports with `as prefix`. You can filter imports with `show` (import only specific symbols) and `hide` (exclude symbols). **Barrel Files** use `export` statements to bundle and publish multiple internal library files under a single clean public import path.

The Mental Model

"A barrel file is like a retail store catalog: instead of visiting 20 separate factory warehouses across town, you flip open one catalog that showcases all available products in one place."

Deep Dive: How It Works

`import ... as prefix`: Solves naming clashes (e.g. `import "dart:math" as math;`).

`show` / `hide`: Selectively restrict imported symbol visibility.

Barrel Files: A single `feature.dart` containing `export "src/models.dart"; export "src/views.dart";` so consumers only import one file.

Syntax Blueprint

// Import with namespace
import "package:http/http.dart" as http;

// Selective import
import "dart:math" show pi, max;

// Barrel export file
export "src/user_model.dart";
export "src/user_repository.dart";

Import aliasing, symbol filtering, and library barrel exports.

Core Rules to Remember

Clean Public APIs: Barrel export files hide deep internal folder structures from consumers.

Collision Prevention with as: Disambiguate identical class names across third-party libraries.

Selective show/hide: Avoid symbol pollution in tight domain files.

Common Beginner Traps & How to Fix Them

Mistake:Importing relative `../lib/file.dart` instead of `package:my_app/file.dart`.

Why it happens: Inconsistent import paths create duplicate library instances in memory.

How to fix: Standardize on `package:` imports for all files in the `lib/` directory.

Pro Tip: In package design, keep internal files inside `lib/src/` and expose only public APIs through root `lib/my_package.dart`.

Live Interactive Example

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Import Aliasing and Math Library Integration

Python 3.12

Output Console

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How it works: Imported selective math functions with the math namespace prefix.

Your Turn: Micro Challenge

No pressure! Edit the starter code below and test your solution with instant feedback.

Micro Exercise

Use Math Library with Alias

Import `dart:math as math;`.
In `main()`, compute `math.pow(2, 5)` (which equals 32).
Print `2^5 = ${math.pow(2, 5)}`.

Sandbox Output

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