android : @AndroidEntryPoint and @HiltViewModel usage

 @AndroidEntryPoint is an annotation used in Hilt, which is Google’s recommended library for Dependency Injection (DI) built on top of Dagger.

What does it actually do?

When you mark a class with @AndroidEntryPoint, Hilt generates an individual Hilt component for that specific Android class (in this case, your MainActivity).

Member Injection: It allows the class to receive dependencies from Hilt. Without this, you couldn't use the @Inject annotation to get your ViewModels, Repositories, or API services into the Activity.

Lifecycle Management: It ensures that the dependencies are tied to the Activity's lifecycle. Hilt will automatically handle creating the component in onCreate() and destroying it when the Activity is destroyed.

There are a few things to remember how to use this. We need to use viewModel delegate


@dagger.hilt.android.AndroidEntryPoint
class MainActivity : ComponentActivity() {
    private val viewModel: HomeViewModel by viewModels()

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        enableEdgeToEdge()
        setContent {
            GetMyHomeTheme {
                GetMyHomeApp(viewModel = viewModel)
            }
        }
    }
}


How the "Chain" works

When you call by viewModels(), Hilt does the following behind the scenes:

  1. Lookups: It looks at the HomeViewModel constructor.

  2. Fulfillment: It sees it needs PropertySearchBackendApi and OkHttpClient.

  3. Search: It searches your Hilt Modules (classes marked with @Module) to find instructions on how to create those specific objects.

  4. Delivery: It creates the objects, creates the ViewModel, and hands it to your Activity.


@HiltViewModel // <--- Use this for ViewModels
class HomeViewModel @Inject constructor(
    private val propertySearchBackendApi: PropertySearchBackendApi,
    private val graphQLClient: OkHttpClient
) : ViewModel() { ... }


Can you have more than one AndroindEntry point - Yes you can 

Here is a full example, we will create DetailActivity, then define the DetailScreen and finally DetailViewModel. 


@AndroidEntryPoint
class DetailActivity : ComponentActivity() {
    private val viewModel: DetailViewModel by viewModels()

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        enableEdgeToEdge()
        setContent {
            GetMyHomeTheme {
                DetailScreen(viewModel = viewModel)
            }
        }
    }
}

And then we will have our DetailScreen here. This is our main layout and it should contains all our UI, at this point it is just a Button that calls viewModel.sayHi(). 

@Composable
fun DetailScreen(viewModel: DetailViewModel) {
    Box(
        modifier = Modifier
            .fillMaxSize()
            .background(BackgroundDark),
        contentAlignment = Alignment.Center
    ) {
        Text(
            text = "Detail Screen",
            color = TextPrimary,
            fontSize = 24.sp
        )
        Button(
            onClick = { viewModel.sayHi() },   <---- calling view model function sayHi()
            modifier = Modifier
                .fillMaxWidth()
                .height(56.dp),
            colors = ButtonDefaults.buttonColors(
                containerColor = PrimaryPurple
            ),
            shape = RoundedCornerShape(12.dp)
        ) {
            Row(
                verticalAlignment = Alignment.CenterVertically,
                horizontalArrangement = Arrangement.Center
            )
            {
                Text("Detail View", fontSize = 16.sp, fontWeight = FontWeight.Medium)
            }
        }
    }
}


And finally our view model 

@HiltViewModel

class DetailViewModel @Inject constructor(
    private val propertySearchBackendApi: PropertySearchBackendApi,
    private val graphQLClient: OkHttpClient
) : ViewModel() {

    fun sayHi()
    {
        val test = propertySearchBackendApi.toString()
    }
}


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