Compile-time Dependency Injection for Dart and Flutter

Compile-time dependency injection is a technique for managing the dependencies of an application at compile-time rather than at runtime. This provides several benefits, including improved performance, reduced code size, and better compile-time error checking. In Dart and Flutter, compile-time dependency injection is implemented using the @inject and @provides annotations, along with the Component and Module classes.

Getting Started

Adding the Dependency

To use this library in your Dart or Flutter project, you need to add it as a dependency in your pubspec.yaml file:

// dart
$ dart pub add inject_annotation
$ dart pub add inject_generator build_runner --dev

// flutter
$ flutter pub add inject_annotation
$ flutter pub add inject_generator build_runner --dev

Generating the Code

To generate the code, you need to run the build runner:

// dart
$ dart run build_runner build

// flutter
$ flutter pub run build_runner build

The Component

To use compile-time dependency injection in your Dart or Flutter application, you need to create a Component class. This is an abstract class annotated with @component or @Component([]) if you also have modules.

Inside the Component, you can define methods that return instances of the classes you need ( e.g., Repository get repository in the example below).

@component
abstract class MainComponent {
  static const create = g.MainComponent$Component.create;

  @inject
  Repository get repository;
}

@injecting Types

To add a type to the dependency graph, you annotate its class with @inject. For example:

@inject
class Repository {
  const Repository(this.apiClient);

  final FakeApiClient apiClient;

  Future<String> getGreeting({required String name}) => apiClient.getGreeting(name: name);
}

Note that you cannot add the @inject annotation to classes from 3rd party libraries.

Modules

Modules are classes annotated with @module. There, you can define dependencies with the @provides annotation.

Methods annotated with @provides tell how to provide an instance of a class. Function parameters are the dependencies of this type.

@module
class ApiModule {
  @provides
  @singleton
  FakeApiClient apiClient() => FakeApiClient();
}

You can then include the module in your Component:

@component([ApiModule])
abstract class MainComponent {
  static const create = g.MainComponent$Component.create;

  @inject
  Repository get repository;
}

@singleton

The @singleton annotation is used to indicate that only one instance of the provided type should be created and shared across the application. This can help improve performance and reduce memory usage by avoiding unnecessary object creation.

To use @singleton, simply add it as an annotation to the method that provides the instance:

@module
class ApiModule {
  @provides
  @singleton
  FakeApiClient apiClient() => FakeApiClient();
}

FAQ

What do you mean by compile-time?

All dependency injection is analyzed, configured, and generated at compile-time as part of a build process, and does not rely on any runtime setup or configuration (such as reflection with dart:mirrors). This provides the best experience in terms of code-size and performance (it's nearly identical to hand written code) and allows us to provide compile-time errors and warnings instead of relying on runtime.