Bootstrapping

This is an article about how to initialize (bootstrap) an Android app written in Kotlin.

It will also give hints on how to configure a Kodein module for dependency retrieval.

The code used in this article is presently used in my project Victor Events.

Application class

This is the entire code of the Application class used in the project:

class EventsApplication : Application() {

    init {
        Kodein.global.addImport(module)
    }

    override fun onCreate() {
        super.onCreate()
        bootstrap.boot()
    }

    fun login(user: FirebaseUser) = bootstrap.login(user)

    fun logout() = bootstrap.logout()
}

Kodein module

The first few lines of the Application class create and import a Kodein module implemented in this way:

val Context.module get() = KodeinModuleBuilder(this).build()

inline fun <reified R : Any> instance() = Kodein.global.instance<R>()

...

class KodeinModuleBuilder(private val ctx: Context) {

    fun build() = Kodein.Module("main") {
        bind<Bootstrap>() with singleton { Bootstrap() }
	...
        bind<Geocoder>() with singleton { Geocoder(ctx, Locale.getDefault()) }
        ...
    }
}

As you see, there is an extention property defined for Context, so that when I use the property module inside Application class, an instance of the module is created, and the application context is passed to it.

Thus passed application Context us then used to create, for example, binding for Geocoder, because Geocoder needs to have Context passed in its constructor.

Bootstrap

This is the code of the Bootstrap class:

val bootstrap by instance<Bootstrap>()

class Bootstrap {

    fun boot() {
        login.setCurrentUser(FirebaseAuth.getInstance().currentUser)
        db.commDao.getAllAsync { comms.addAll(it) }
        db.messageDao.getAllAsync { messages.addAll(it) }
    }

    fun login(user: FirebaseUser) {
        login.setCurrentUser(user)
    }

    fun logout() {
    }
}

The file first defines a global constant bootstrap that is retrieved lazily when the function onStart() inside Application class calls:

bootstrap.boot()

It also defines two other functions (login() and logout()) that can be called at any moment from the Application class. The functions may be removed in future versions, because they are not clearly related to bootstrapping. I put them there, because I did not want them to be global, and this particular class seemed to be appropriate for containing all the code that should be called from Application class, but I may choose a different solution later.

Conclusion

This article showed where to put code that should be run when the application starts, but you do not want to put it directly in the code of Application’s onCreate() function.

It also showed how to capture application Context using Kodein and lazy dependency retrieval.

Lastly, it also showed that although the Bootstrap class may be also used to hold other functions that you do not want to be global, and for example you may want them to have access to some values captured by Bootstrap, but it may be more appropriate to put them elsewhere. For example, you may want to create a class called Architecture, and retrieve its instance depending on your product’s flavor or hardware.