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The import directive makes public symbols (classes, functions, and variables) declared in an external library available within the current library. It utilizes a Uniform Resource Identifier (URI) to locate the target resource and resolve its declarations into the current library’s scope.

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URI Schemes

Dart resolves imports based on the URI scheme provided in the string literal.
SchemeDescriptionExample
dart:References built-in libraries provided by the Dart SDK.import 'dart:io';
package:References files within the lib/ directory of a package (including the current package) as resolved by the package manager.import 'package:utils/calc.dart';
RelativeReferences files relative to the current file’s location in the file system.import '../models/user.dart';

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Basic Syntax

The directive must appear at the top of the Dart file, following any library declaration but preceding code definitions. 
// Imports the math library from the Dart SDK
import 'dart:math';

// Imports a file named 'api.dart' from the 'network' package
import 'package:network/api.dart';

// Imports a local file located in the parent directory
import '../constants.dart';

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Namespace Management

By default, import introduces all public symbols from the referenced library into the current library’s namespace. Dart libraries are encapsulated; importing a file does not add symbols to a global application-wide scope, but rather makes them visible only within the specific library declaring the import.

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Prefixing (as)

The as keyword binds the imported library to a named identifier. This prevents naming collisions by requiring symbols to be accessed using dot notation on the prefix. 
import 'package:lib1/utils.dart';
import 'package:lib2/utils.dart' as lib2;

void main() {
  // Uses Element from lib1 (imported into current scope)
  Element e1 = Element(); 
  
  // Uses Element from lib2 via the prefix
  lib2.Element e2 = lib2.Element(); 
}

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Filtering (show and hide)

To control which symbols are introduced into the current library’s scope, use show (inclusion list) or hide (exclusion list). 
// Only imports 'Client' and 'Response'; other symbols remain inaccessible
import 'package:http/http.dart' show Client, Response;

// Imports everything EXCEPT 'Logger'
import 'package:logging/logging.dart' hide Logger;

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Deferred Loading

The deferred as keywords allow a library to be loaded lazily at runtime. The library must be loaded asynchronously using loadLibrary() before its symbols are accessed. 
import 'package:heavy_lib/features.dart' deferred as features;

Future<void> loadFeatures() async {
  // Pauses execution until the library is loaded
  await features.loadLibrary();
  features.initialize();
}

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Exporting

The export directive exposes symbols from another library through the current library’s public API. It does not import those symbols into the current library’s scope; it only makes them available to consumers of the current library. 
// In file: lib/main_api.dart

// Users importing main_api.dart gain access to symbols in auth.dart and user.dart
export 'src/auth.dart';
export 'src/user.dart' show User, UserProfile; // Selective export
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