What is Hilt and what is it used for in Android app development?

Hilt is an opinionated dependency injection library for Android.

Used for:

• Simplifying dependency injection: Automates the creation and provision of dependencies.
• Enhancing testability: Facilitates creating test versions of dependencies.
• Integration with Android components: Initially supports standard Android classes (Activity, Fragment, ViewModel).

Code examples:

// Example of using @HiltAndroidApp for initializing Hilt
import dagger.hilt.android.HiltAndroidApp

@HiltAndroidApp
class MyApplication : Application() {
    // Hilt setup is automatically handled
}

// Example of injecting a dependency into an Activity
import android.os.Bundle
import androidx.appcompat.app.AppCompatActivity
import dagger.hilt.android.AndroidEntryPoint
import javax.inject.Inject

@AndroidEntryPoint
class MainActivity : AppCompatActivity() {

    @Inject // Hilt injects the MyService dependency
    lateinit var myService: MyService

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        myService.doSomething()
    }
}

// Dependency interface
interface MyService {
    fun doSomething()
}

// Dependency implementation
class MyServiceImpl @Inject constructor() : MyService { // @Inject indicates how Hilt creates an instance
    override fun doSomething() {
        println("Doing something...")
    }
}

// Module for providing dependencies
import dagger.Binds
import dagger.Module
import dagger.hilt.InstallIn
import dagger.hilt.components.SingletonComponent
import javax.inject.Singleton

@Module
@InstallIn(SingletonComponent::class) // Sets the module in SingletonComponent (application lifetime)
abstract class ServiceModule {

    @Binds // Binds interface to implementation
    @Singleton // Defines the lifetime of the dependency (singleton)
    abstract fun bindMyService(myServiceImpl: MyServiceImpl): MyService
}