StateFlow don't emit when the new value same last value

Question

I have a login form. I use StateFlow to send LoginResult (after call API) from ViewModel to Activity. In the Activity, I will show an error dialog if login failed.
It works well for the first time but from the second time I login failed, the error dialog won't show again. I tested both .value and .emit on StateFlow

private val _loginResult = MutableStateFlow(LoginResult())
val loginResult: StateFlow<LoginResult> = _loginResult

fun login(email: String, password: String) {
    viewModelScope.launch {
        when (val result = loginRepository.login(email, password)) {
            is Result.Fail-> {
                _loginResult.value = LoginResult(error = "Login failed")
                // _loginResult.emit(LoginResult(error = "Login failed")) same issue
            }
            ...
       }
   }
}

StateFlow does not emit duplicate value. you can user SharedFlow to solve your problem. — Omid Faraji, Jan 06 '21 at 08:44
@OmidFaraji yes, I have solved it in my answer. It takes me some time to solve this problem so I post it here — Linh, Jan 06 '21 at 08:47

score 4 · Answer 1 · answered Jan 06 '21 at 08:26

4

For this case, I use SharedFlow so my Activity still able to collect the new value even it same the last value

private val _loginResult = MutableSharedFlow<LoginResult>()
val loginResult: SharedFlow<LoginResult> = _loginResult

...
_loginResult.emit(LoginResult(error = "Login failed"))

Another possible solution but it's not good is change my current data class LoginResult(...) to class LoginResult. Then every new instance of LoginResult will different

answered Jan 06 '21 at 08:26

Linh

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Why is SharedFlow better than StateFlow for this? – IgorGanapolsky Mar 08 '23 at 15:58
@IgorGanapolsky so normally, I use StateFlow to store UI state, and SharedFlow to emit event. we shouldn't use StateFlow for event, because if you rotate the screen, the event can trigger again. – Linh Mar 09 '23 at 02:00

score 0 · Answer 2 · answered Mar 10 '21 at 17:17

That's by default. What you could do is to emit one value when the login is in progress, before emitting the result. That way you would have a sequence like:

in_progress -> error -> in_progress -> error

Apps usually display a spinner or something when the in_progress or loading or whatever is emitted.

StateFlow don't emit when the new value same last value

2 Answers2

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