Dependency Injection

We talked a little about the dependency injection pattern in the flutter_modular session, which can be accessed by clicking here. We strongly recommend reading it mainly for those who do not know or want to know better this pattern of projects.

Instance registration

The strategy for building an instance with its dependencies comprises of registering all objects in a module and manufacturing them on demand or in single-instance form(singleton). This 'registration' is called Bind.

There are a few ways to build a Bind to register object instances:

• Bind.scoped: Builds an instance that survives the entire request, being destroyed at the end of the request.
• Bind.singleton: Builds an instance only once when the module starts.
• Bind.lazySingleton: Build an instance only once when prompted.
• Bind.factory: Builds an instance on demand.
• Bind.instance: Adds an existing instance.

We register the binds in AppModule:

class AppModule extends Module {
  @override
  List<Bind> get binds => [
    Bind.factory((i) => XPTOEmail()),
    Bind.factory<EmailService>((i) => XPTOEmailService(i())),
    Bind.scoped((i) => Client(i()))
  ];
  
  ...
}

Note that we placed an i() instead of the dependencies instance. This will be responsible to allocate the dependencies automatically.

To get a resolved instance use Modular.get:

final client = Modular.get<Client>();

// or set a default value
final client = Modular.get<Client>(defaultValue: Client());

TIP

A default constructor Bind() is the same as Bind.lazySingleton();

AsyncBind

flutter_modular can also resolve asynchronous dependencies. To do this, use AsyncBind instead of Bind:

class AppModule extends Module {
  @override
  List<Bind> get binds => [
    AsyncBind<SharedPreferences>((i) => SharedPreferences.getInstance()),
    ...
  ];
  ...
}

By now we need to transform the AsyncBind into synchronous Bind in order to resolve the other instances. Therefore, we use Modular.isModuleReady() passing the module type in generics;

await Modular.isModuleReady<AppModule>();

This action will convert all AsyncBinds to synchronous Binds and singletons.

TIP

We can get the asynchronous instance directly too without having to convert to a synchronous bind using Modular.getAsync();

Auto Dispose

For registered objects that are not part of this list, we can implement a Disposable interface on the instance where we want to run an algorithm before dispose:

class MyController implements Disposable {
  final controller = StreamController();

  @override
  void dispose() {
    controller.close();
  }
}

shelf_modular also offers a singleton removal option from the dependency injection system by calling the Modular.dispose() method even with an active module:

Modular.dispose<MySingletonBind>();