Hey :wave: I just published my first article on m...

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[satyan]

Hey 👋

I just published my first article on migrating from LiveData to StateFlow

https://fabernovel.github.io/2020-12-07/android-from-livedata-to-flows

Feel free if you have feedbacks to hit me on my Twitter 😉

[Daniel]

Thank you for the article!

How does StateFlow behave when the screen is recrated (rotation)?

Also if the answer is: The value is observed again, how would it be possible to make a SingleLiveDataEvent with StateFlow? With other words: A value wich is really only observed one time and then consumed

[allan.conda]

Oh, just in time, I’m literally doing this right now.

Seems there’s no info about savedStateHandle? That’s kinda the blocker for me right now.

[satyan]

@Daniel

How does StateFlow behave when the screen is recrated (rotation)?

Also if the answer is: The value is observed again, how would it be possible to make a SingleLiveDataEvent with StateFlow? With other words: A value wich is really only observed one time and then consumed

I had a part about SingleLiveEvent initially, but it was going to be too long.

You can use a SharedFlow instead of a StateFlow
On collect, a SharedFlow will emits its replay cache then emit the next values, so you can use a SharedFlow with a replay cache of 0

class MyViewModel {
  private val _toast = MutableSharedFlow<String>() // by default replay cache is 0
val toast: SharedFlow<String> = _toast

  fun foo() {
    viewModelScope.launch {
      // suspend until collected
      _toast.emit("Hello")
}
  }
}

[satyan]

On configuration changes. the Stateflow value will be collected again yes

[satyan]

@allan.conda

Oh, just in time, I’m literally doing this right now.

Seems there’s no info about savedStateHandle? That’s kinda the blocker for me right now.

What do you mean ?

I used savedStateHandle to save data but it’s only going to be use initially to restore the state.

class MyViewModel (savedStateHandle: SavedStateHandle) {
    val _state = MutableStateFlow(restoreState())
    val state: StateFlow<State> = _state

    private fun restoreState(): State {
       return savedStateHandle.get(KEY) ?: INITIAL_STATE
    }
}

⚠️ If your state is big, you should only save part of it and restore the full state. For example, if your state is a paginated list, instead of saving the whole list, you could save the current page/token and then restore the items using it

[allan.conda]

How do you save it? restoring is easy

[allan.conda]

I’m aware about the gotcha about large lists

[satyan]

When you’re updating the state, you can save the relevant part to the savedStateHandle:

fun foo() {
  viewmodelScope.launch {
    val user = userRepository.getUser(id)
     _state.value = mapper.mapState(user)   savedStateHandle[KEY] = user
  }
}

[satyan]

⚠️ You can only save types that can be save to a bundle inside the savedStateHandle.

So you could create a UserSavedState parcelable type

[allan.conda]

I see, so you simply update the savedStateHandle as well whenever you assigned a new value

[satyan]

Yep’ I’m only relying on savedStateHandle to save/restore a state not to hold the entire screen state.

[allan.conda]

SavedStateHandle.getLiveData returns a MutableLiveData which you could update directly.
I’m currently trying to come up with a similar api with MutableStateFlow.

[Daniel]

Thank you! @satyan