Mixing reactive and non-reactive could create ambiguity

It could create data ambiguity if the data has been produced by mixing reactive and non-reactive code. The reactive data may notified earlier without updating non-reactive data.

Let me explain an example scenario. Let's assume LoginRepository provides LoginState by observing LoginService. It is mixing reactive LoginService.loginStatusFlow with non-reactive LoginService.accountData. There could be a situation where loginStatusFlow has been notified isLogin=true but accountData not initializing yet. In this case LoginRepository could provide Logout with wrong userId or crashing the application.

Problem

sealed interface LoginState {
    data class Login(val userId: String) : LoginState
    object Logout : LoginState
}

class LoginRepository(
    private val loginService: LoginService
) {

    val loginState = loginService.loginStatusFlow.map { loginStatus ->
        if (loginStatus.isLogin) {
            val userId = loginService.accountData.userId
            LoginState.Login(userId = userId)
        } else {
            LoginState.Logout
        }
    }
}

The solution could be mixing only reactive data to produce new data. In the example, to combine two reactive loginStatusFlow and accountDataFLow solve the issue and ensure proper data emit by LoginRepository.

Solution

class LoginRepository(
    private val loginService: LoginService
) {

    val loginState = loginService
        .loginStatusFlow
        .combine(loginService.accountDataFLow) { loginStatus, accountData ->
        if (loginStatus.isLogin) {
            val userId = accountData.userId
            LoginState.Login(userId = userId)
        } else {
            LoginState.Logout
        }
    }
}