Injecting in Android

Once you have declared some modules, and you have started Koin, how can you retrieve your instances in your Android Activity Fragments or Services?

Ready for Android Classes

Activity, Fragment & Service are extended with the KoinComponents extension. Any ComponentCallbacks class is accessible for the Koin extensions.

You gain access for the Kotlin extensions:

• by inject() - lazy evaluated instance from Koin container
• get() - eager fetch instance from Koin container

We can declare a property as lazy injected:

module {
    // definition of Presenter
    factory { Presenter() }
}

class DetailActivity : AppCompatActivity() {

    // Lazy inject Presenter
    override val presenter : Presenter by inject()

    override fun onCreate(savedInstanceState: Bundle?) {
        //...
    }
}

Or we can just directly get an instance:

override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)

    // Retrieve a Presenter instance
    val presenter : Presenter = get()
}  

info

if your class doesn't have extensions, just implement the KoinComponent interface in it to inject() or get() an instance from another class.

Using the Android Context in a Definition

Once your Application class configures Koin you can use the androidContext function to inject Android Context so that it can be resolved later when you need it in modules:

class MainApplication : Application() {

    override fun onCreate() {
        super.onCreate()

        startKoin {
            // inject Android context
            androidContext(this@MainApplication)
            // ...
        }
        
    }
}

In your definitions, The androidContext() & androidApplication() functions allows you to get the Context instance in a Koin module, to help you simply write expression that requires the Application instance.

val appModule = module {

    // create a Presenter instance with injection of R.string.mystring resources from Android
    factory {
        MyPresenter(androidContext().resources.getString(R.string.mystring))
    }
}

Android Scope & Android Context resolution

While you have a scope that is binding the Context type, you may need to resolve the Context but from different level.

Let's take a config:

class MyPresenter(val context : Context)

startKoin {
  androidContext(context)
  modules(
    module {
      scope<MyActivity> {
        scoped { MyPresenter( <get() ???> ) }
      }
    }
  )
}

To resolve the right type in MyPresenter, use the following:

• get() will resolve closest Context definition, here it will be the source scope MyActivity
• androidContext() will also resolve closest Context definition, here it will be the source scope MyActivity
• androidApplication() will also resolve Application definition, here it will be the source scope context object defined in Koin setup