Small dependency injection library.

Pure, dependency free library for dependency injection without code generation and with small as possible boilerplate.

Features

• Multitype dependency registration.
• Scope based dependency registration.
• Dependency free and without code generation.
• Simple injection API

Getting started

Dependency registration

This library uses Module primitive for dependency registration code. When you want to register dependency for injection you should do next things.

1. Define module where dependency registration code will be declared.

class AppModule extends Module {
  AppModule() : super({});
}

You can define more than one module. The amount of modules depends on your app architecture.

2. Declare one or more dependency registration instances assigned to static final variables. The registration of dependency performed by Manage class.

class AppModule extends Module {
  AppModule() : super({});

  static final service = Manage(Service.new);
}

The full code of new Manage instance look like this:

class AppModule extends Module {
  AppModule() : super({});

  static final service = Manage<RegistrationType>(
    ServiceFactory,
    scope: Scope.unique,
    dependsOn: [],
  );
}

In example above you can see some optional parameters. Parameter scope used to provide specific scope in which instance of dependency will be managed. Parameter dependsOn used to initialize registration of other dependencies necessary for this dependency.

You should keep in mind that value of static variables initialized on first use. This means that until you access the variable its value will not be created. Thats why we should use other static variables in dependsOn parameter.

The factory you provide at construction of Manage instance can be class constructor

Manage(Service.new);

or you can provide closure

Manage(()=>Service());

In simple cases the generic type for Manage instance will be inferred automatically. But in some cases you should define generic parameter explicitly.

Manage<Service>(()=>Service());

3. Use some static variables in super constructor

class AppModule extends Module {
  AppModule() : super({service});

  static final service = Manage(Service.new, dependsOn: [client]);
  static final client = Manage(HttpClient.new);

}

This step is necessary because some static variables may not be initialized. This is necessary only for those variables that have not been used anywhere else. In real code, we usually encounter a situation where some dependencies depend on others. In our case, accessing variables in the constructor that no one has referred to yet will lead to initialization of the values of these variables and the entire graph of variables on which it depends.

In example above when we use service variable in constructor its value will be created and also will be created value for client variable.

Scopes

Scope is a mechanism which help control injection behaviour. This library provide three different implementation of Scope

• unique - create new instance every time
• singleton - create new instance only once and instance will never be dropped
• cached - create new instance and store it until reset. When reset method called than previously created instances will be dropped.

Every instance provided from scope created by factory provided at construction of Manage instance. If you use cached or singleton scope than instances provided from scope will be stored in pair with its factory. This allows us to make registration with different type and same factory and at the time of access same instance will be shared with different types

4. Mix Managed mixin where you wnt to use dependency

class Service with Managed {
}

The Managed mixin redefine noSuchMethod to implement injection behaviour (See mixin implementation for more details).

5. Define provider method to inject dependency

class Service with Managed {
  T _httpClient<T extends HttpClient>();
}

In example below we define method with generic parameter T which should extend HttpClient and without method body. Such declaration at runtime will delegate all invocation to noSuchMethod where we have access to invocation details via Invocation class. The invocation class contains information about concrete values of generic parameters at the moment of call. In our example value of generic parameter will be HttpClient because we bound generic parameter by concrete type it must implement. And because we not specify generic parameter at the moment of call the value of parameter become equal to bounded type. And because we have concrete dependency type from generic parameter value we can find Manage instance with equal type ant then get instance of type using concrete type factory and provided scope.

To provide dependency instance without Managed mixin and provider method you can use module static variable directly.

final service = AppModule.service();

For more information about usage and implementation see source code and test.

I will be back in future with more examples.

Thank you.