Dagger 2

Introduction

Dagger is made by Google. Dagger allows you to

• Scope dependencies
• Bind single instance to life cycles
• Only need to build them once
• Generates the code at compile time

Example Without Dagger

fun buildCar: Car =  
    Car(SturdyFrame(),
    Wheels(),
    RocketEngine())

With Dagger

fun buildCar: Car = 
    DaggerAppComponent
    .builder()
    .build()
    .buildCar()

Modules

Modules in Dagger are responsible for providing object we want to inject. They contain the methods which return the objects. Modules are decorated with @module and the objects are decorated with @provides

@Module
fun CarModule {
    @Provides
    fun provideEngine() : Engine = Engine()

    @Provides
    fun provideFrame() : Frame = Frame()

    @Provides
    fun provideWheels() : Wheels = Wheels()

    @Provides
    fun provideCar(engine: Engine, wheels: Wheels, frame: Frame) : Car {
      return Car(frame, wheels, engine)
    }
}

To share a module across modules add the include to the modules decorator

@Module(includes = [EngineModule::class])
fun CarModule {
    @Provides
    fun provideFrame() : Frame = Frame()
...
    @Provides
    fun provideCar(engine: Engine, wheels: Wheels, frame: Frame) : Car {
      return Car(frame, wheels, engine)
    }
}

Components

A Dagger Component is something which contains a set of modules. The component is an interface which allows access to our module instances. The underlying code is generated at build time so the interface is simple to write. We just need the @Component keyword and the modules required

@Component(modules = [
  NetworkModule::class, 
  ContextModule::class, 
  CarModule::class ])
interface AppCommponent {
   fun okHttpClient(): okHttpClient
   fun car(): Car
...
}

When creating a component we only need to list the dependencies at the top as Dagger knows to include child dependencies.
Using the component we just to the following

class SomeActivity: AppCompatActivity() {

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        var component = DaggerAppComponent
            .builder()
            .contextModule(ContextModule(context!!)),
            .networkModule(NetworkModule()),
            .build()

        var car = component.car()
        var client = component.OkHttpClient()
    }
}

Failing to pass external dependencies will fail at runtime and not build time