Dart Lesson 84 of 102 3 min read

Imports in Dart: import, as, show and hide

Learn how to import libraries in Dart: core libraries, packages and your own files, with prefixes (as) and the show and hide keywords.

In Dart, every .dart file is a library. To use something defined in another library, you import it.

Three kinds of import #

// 1. A library built into the Dart SDK
import 'dart:math';

// 2. A library from a package (your own project or one from pub.dev)
import 'package:http/http.dart';

// 3. A file in your project, by relative path
import 'utils/formatters.dart';

Imports go at the top of the file, before any other code.

The built-in libraries #

import 'dart:math';

void main() {
  print(sqrt(144));
  print(max(3, 8));
  print(pi.toStringAsFixed(3));
}

12.0
8
3.142

Importing your own files #

Suppose the project looks like this:

my_app/
  pubspec.yaml
  bin/
    main.dart
  lib/
    models/
      user.dart
    utils/
      formatters.dart

Files under lib/ can be imported from anywhere with a package: path, which starts with the package name from pubspec.yaml.

// In bin/main.dart
import 'package:my_app/models/user.dart';
import 'package:my_app/utils/formatters.dart';

Files in the same folder tree can also use relative paths.

// In lib/models/user.dart
import '../utils/formatters.dart';

Use package: imports to reach into lib/ from outside it, and pick one style within lib/ and stick to it.

as: give an import a prefix #

A prefix puts everything from the library behind a name. It solves two problems: name clashes, and “where does this function come from?”

import 'dart:math' as math;

void main() {
  print(math.sqrt(49));
  print(math.max(10, 20));
  print(math.Random().nextInt(10) < 10);
}

7.0
20
true

Some packages are designed to be used with a prefix. The http package has top-level functions called get and post, which would be confusing bare.

import 'package:http/http.dart' as http;
import 'package:path/path.dart' as p;

// http.get(...), p.join(...)

When two libraries define the same name, a prefix on one of them resolves it:

import 'package:shapes/circle.dart';
import 'package:drawing/circle.dart' as drawing;

// Circle()          -> from shapes
// drawing.Circle()  -> from drawing

show: import only some names #

import 'dart:math' show pi, sqrt;

void main() {
  print(sqrt(pi));
  // print(max(1, 2)); // error: max was not imported
}

1.7724538509055159

show documents exactly what you depend on and keeps autocomplete tidy.

hide: import everything except some names #

import 'dart:math' hide Random;

// Our own Random class no longer clashes with dart:math's.
class Random {
  int next() => 4;
}

void main() {
  print(Random().next());
  print(sqrt(16));
}

4
4.0

show and hide can be combined with a prefix: import 'dart:math' as math show pi;.

Deferred loading #

On the web, a large library can be downloaded only when it is needed, which makes the first page load faster.

import 'package:my_app/heavy_charts.dart' deferred as charts;

Future<void> showReport() async {
  await charts.loadLibrary();
  charts.drawSalesChart();
}

export: pass a library on #

export makes another library’s names available to anyone who imports your file. It is how a package offers one tidy entry point. See creating libraries.

Good habits #

• Order imports: dart: first, then package:, then relative. dart format and the linter help.
• Remove unused imports. The analyzer flags them.
• Never import from another package’s lib/src/ folder. That is its private code and may change without warning.
• Use a prefix for libraries with very general names such as http, path or math.

Try it yourself #

Create two files, geometry.dart with a function area for rectangles and circles.dart with a function area for circles. Import both into main.dart and call each one, first using prefixes and then using show and hide.