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Hilt ‐ Constructor Injection

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Devrath edited this page Oct 8, 2023 · 4 revisions

Observation

  • Here wheels and engine are constructor injected on the car. This is where the constructor injection happens.
  • Also notice the @Inject annotation on the constructor of the implementations that helps hilt to create objects.
  • Implementations are instantiated as soon as the Activity is loaded.
  • The Injected class address is changed, you can notice that's because when the activity is destroyed and recreated, All the instances of the implementations are re-initiated again.
  • If you repeatedly access the same reference without the rotation of activity, the address of instance remains same.

Output

// ----------> Class is loaded for the first time 
Wheels constructor is invoked !
Engine constructor is invoked !
Car constructor is invoked !
// ----------> Clicking the button to access the instance to perform an action the first time
Car instance -> 137539380
Car is Driving with com.istudio.di.modules.dagger.demos.concepts.implementations.vehicle.Wheels@52ea15d and with com.istudio.di.modules.dagger.demos.concepts.implementations.vehicle.Engine@4f33ad2
// ----------> Device is rotated
Wheels constructor is invoked !
Engine constructor is invoked !
Car constructor is invoked !
// ----------> Clicking the button to access the instance to perform an action the second time
Car instance -> 145556911
Car is Driving with com.istudio.di.modules.dagger.demos.concepts.implementations.vehicle.Wheels@dc0edbc and with com.istudio.di.modules.dagger.demos.concepts.implementations.vehicle.Engine@efcb145
// ----------> Clicking the button to access the instance to perform an action a third time but now without rotation
Car instance -> 145556911
Car is Driving with com.istudio.di.modules.dagger.demos.concepts.implementations.vehicle.Wheels@dc0edbc and with com.istudio.di.modules.dagger.demos.concepts.implementations.vehicle.Engine@efcb145

Code

Implementation

Engine.kt

class Engine @Inject constructor() {
    init {
        printLog("Engine constructor is invoked !")
    }
}

Wheels.kt

class Wheels @Inject constructor() {
    init {
        PrintUtils.printLog("Wheels constructor is invoked !")
    }
}

Car.kt

class Car @Inject constructor( var wheels: Wheels, var engine: Engine ) {
    init { printLog("Car constructor is invoked !") }
    fun start() { printLog("Car is Driving with $wheels and with $engine") }
}

Activity

MyActivity.kt

@AndroidEntryPoint
class HiltConstructorInjectionActivity : AppCompatActivity() {

    private lateinit var binding: ActivityHiltConstructorInjectionBinding

    @Inject lateinit var car: Car

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        binding = ActivityHiltConstructorInjectionBinding.inflate(layoutInflater)
        setContentView(binding.root)
        setOnClickListeners();
    }

    private fun setOnClickListeners() {
        binding.apply {
            initiateId.setOnClickListener {
                PrintUtils.printLog("Car instance -> "+car.hashCode().toString())
                car.start()
            }
        }
    }

}
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