Is this a documentation bug? It says to not delay updating the textfield when the value changes. But...

# compose

[Mark]

Is this a documentation bug? It says to not delay updating the textfield when the value changes. But then the example does exactly that. Maybe there should be a comment explaining that this is what not to do? https://developer.android.com/jetpack/compose/text#state-mgmt

[Stylianos Gakis]

Which example are you referring to?

[Mark]

Do not delay making updates to the state: When you callonValueChange, update your TextField synchronously and immediately:

[Stylianos Gakis]

Yes, but which example is it that “does exactly that” in the compose text documentation that you linked?

[Mark]

The example directly following that colon

class SignUpViewModel(private val userRepository: UserRepository) : ViewModel() {
   fun updateUsername(input: String) {
        viewModelScope.launch {
            // async operation
            val isUsernameAvailable = userRepository.isUsernameAvailable(input)
            // ...
            if (!isUsernameAvailable) {
                // modify error state
            }
           username.value = input
       }
   }
}

[Mark]

Shouldn’t it be:

class SignUpViewModel(private val userRepository: UserRepository) : ViewModel() {
   fun updateUsername(input: String) {
        username.value = input
        viewModelScope.launch {
            // async operation
            val isUsernameAvailable = userRepository.isUsernameAvailable(input)
            // ...
            if (!isUsernameAvailable) {
                // modify error state
            }
       }
   }
}

[Stylianos Gakis]

Aha I see now, yes this is odd. On one had, right above this it does say “Do not delay” so it may be that they are giving this as an example of how not to do it? Regardless I do agree that this is not clear.
I’d create a bug report for the documentation and see what they respond with.

[Albert Chang]

viewModelScope uses Dispatchers.Main.immediate, which will run the job immediately and synchronously if it's already on main thread.

[Mark]

isUsernameAvailable takes time though, and who knows on which thread

[Albert Chang]

Read your second reply again.

[Albert Chang]

And all callbacks in compose are run on main thread.

[Mark]

Sorry, I don’t understand. If isUsernameAvailable takes ages, how can it possibly set the value immediately?

[Stylianos Gakis]

userRepository.isUsernameAvailable may switch dispatcher to say IO For example though, and this function will be waiting for that response to come back before it sets username.value. I feel like I am also missing smth here.

[Mark]

Even if it doesn’t switch dispatcher, it still takes potentially a duration longer than “immediate”.

[Stylianos Gakis]

If it doesn’t switch dispatcher, then it would work synchronously as it should, since it’s using Dispatchers.main.immediate, it’s just that then it’d make the UI hang as far as I understand at least.

[Mark]

It says synchronously and immediately so that still wouldn’t comply

[Landry Norris]

Wouldn't it make more sense to have the username update immediately, with some sort of method that asynchronously checks for error state? (bonus points for observing the username value and mapping it to potential error state). If the check takes some amount of time, such as when my phone has an sd card, or there's a lot of usernames taken, you don't want to slow down the text field's responsiveness.

[eygraber]

Another issue with not updating the state immediately is that the TextField internal state can get messed up and start showing incorrect values / treatment because of how it interacts with the IME

[Albert Chang]

Ok, I didn't notice the async operation comment. I guess that's an example of bad pattern.

[Colton Idle]

Filed a bug? I'll star 🌠

[Ale Stamato]

This doc is fixed now. TY for flagging and sorry for the confusion🙏

[Colton Idle]

we did it!

[eygraber]

@Ale Stamato the link in the note that should go to Effective state management for TextField in Compose instead goes to kotlinx StateFlow documentation