Libraries

Remarks#

The import and library directives can help you create a modular and shareable code base. Every Dart app is a library, even if it doesn’t use a library directive. Libraries can be distributed using packages. See Pub Package and Asset Manager for information about pub, a package manager included in the SDK.

Using libraries

Use import to specify how a namespace from one library is used in the scope of another library.

import 'dart:html';

The only required argument to import is a URI specifying the library. For built-in libraries, the URI has the special dart: scheme. For other libraries, you can use a file system path or the package: scheme. The package: scheme specifies libraries provided by a package manager such as the pub tool. For example:

import 'dart:io';
import 'package:mylib/mylib.dart';
import 'package:utils/utils.dart';

Libraries and visibility

Unlike Java, Dart doesn’t have the keywords public, protected, and private. If an identifier starts with an underscore _, it’s private to its library.

If you for example have class A in a separate library file (eg, other.dart), such as:

library other;

class A {
  int _private = 0;

  testA() {
    print('int value: $_private');  // 0
    _private = 5;
    print('int value: $_private'); // 5
  }
}

and then import it into your main app, such as:

import 'other.dart';

void main() {
  var b = new B();
  b.testB();    
}

class B extends A {
  String _private;

  testB() {
    _private = 'Hello';
    print('String value: $_private'); // Hello
    testA();
    print('String value: $_private'); // Hello
  }
}

You get the expected output:

String value: Hello
int value: 0
int value: 5
String value: Hello

Specifying a library prefix

If you import two libraries that have conflicting identifiers, then you can specify a prefix for one or both libraries. For example, if library1 and library2 both have an Element class, then you might have code like this:

import 'package:lib1/lib1.dart';
import 'package:lib2/lib2.dart' as lib2;
// ...
var element1 = new Element(); // Uses Element from lib1.
var element2 =
    new lib2.Element();       // Uses Element from lib2.

Importing only part of a library

If you want to use only part of a library, you can selectively import the library. For example:

// Import only foo and bar.
import 'package:lib1/lib1.dart' show foo, bar;

// Import all names EXCEPT foo.
import 'package:lib2/lib2.dart' hide foo;

Lazily loading a library

Deferred loading (also called lazy loading) allows an application to load a library on demand, if and when it’s needed. To lazily load a library, you must first import it using deferred as.

import 'package:deferred/hello.dart' deferred as hello;

When you need the library, invoke loadLibrary() using the library’s identifier.

greet() async {
  await hello.loadLibrary();
  hello.printGreeting();
}

In the preceding code, the await keyword pauses execution until the library is loaded. For more information about async and await, see more examples here asynchrony support or visit the asynchrony support part of the language tour.