Dagger 2 Singleton between Android Activities

Question

I am trying to understand how @Singleton works on Dagger 2

build.gradle

implementation 'com.google.dagger:dagger:2.32'
kapt 'com.google.dagger:dagger-compiler:2.32'

Vehicle.kt

@Singleton
class Vehicle @Inject constructor() {

    var speed = 0
}

AppComponent.kt

@Singleton
@Component
interface AppComponent {

    fun inject(activity: MainActivity)

    fun inject(activity: SecondActivity)
}

MainActivity.kt

class MainActivity : AppCompatActivity() {

    @Inject
    lateinit var mVehicle: Vehicle

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

        DaggerAppComponent.create().inject(this)

        mVehicle.speed = 75
    }
}

SecondActivity.kt

class SecondActivity : AppCompatActivity() {

    @Inject
    lateinit var mVehicle: Vehicle

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

        DaggerAppComponent.create().inject(this)

        Log.d(TAG, "Vehicle speed: ${mVehicle.speed}")
    }
}

I expect to see the Vehicle speed 75 on SecondActivity, but it is 0. I am sure there must be something I am missing or misunderstand, just cannot figure it out.

Answer 1

The problem is based on the fact you're creating multiple instances of DaggerAppComponent. You have to create a component once, store it somewhere (object, Application), and then use it for injection. Every instance of the component represents a complete, independent dependency graph.

Something like this should work as expected:

@Singleton
@Component
interface AppComponent {
    fun inject(activity: FirstActivity)
    fun inject(activity: SecondActivity)
}

object ComponentHolder {
    val component: AppComponent by lazy { DaggerAppComponent.create() }
}

class FirstActivity : AppCompatActivity() {

    @Inject lateinit var mVehicle: Vehicle

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

        ComponentHolder.component.inject(this)

        mVehicle.speed = 75
    }
}

class SecondActivity : AppCompatActivity() {

    @Inject lateinit var mVehicle: Vehicle

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

        ComponentHolder.component.inject(this)
        
        Log.d("SecondActivity", "Vehicle speed: ${mVehicle.speed}")
    }
}