FullCode.kt

@AndroidEntryPoint
class MainActivity : ComponentActivity() {

    private val viewModel: FormValidationViewModel by viewModels()

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

        setContent {
            val emailState by viewModel.emailAddress.observeAsState()
            val isFormValid by viewModel.valid.observeAsState()
            MaterialTheme {
                MyCoolForm(
                    emailText = emailState ?: "",
                    onEmailChanged = { viewModel.emailAddress.postValue(it) },
                    isFormValid = isFormValid ?: false
                )
            }
        }
    }
}

@Composable
fun MyCoolForm(
    emailText: String = "",
    onEmailChanged: (String) -> Unit = {},
    isFormValid: Boolean = false
) {
    Column(
        horizontalAlignment = Alignment.CenterHorizontally,
        modifier = Modifier
            .fillMaxSize()
            .padding(16.dp)
    ) {
        OutlinedTextField(
            value = emailText,
            label = { Text("Email") },
            onValueChange = onEmailChanged,
            modifier = Modifier.fillMaxWidth()
        )
        Spacer(modifier = Modifier.size(4.dp))
        Button(enabled = isFormValid, onClick = {}) {
            Text(text = "Save")
        }
    }
}

@HiltViewModel
class FormValidationViewModel @Inject constructor() : ViewModel() {

    val emailAddress = MutableLiveData<String>("")

    val valid = MediatorLiveData<Boolean>().apply {
        addSource(emailAddress) {
            val valid = isFormValid(it)
            Log.d(it, valid.toString())
            value = valid
        }
    }

    fun isFormValid(emailAddress: String): Boolean {
        return Patterns.EMAIL_ADDRESS.matcher(emailAddress).matches()
    }
}