Overview

What is SavedStateFlow?

SavedStateFlow is a Kotlin StateFlow that can survive the Android process death.

Why SavedStateFlow?

SavedStateHandle is a great tool to persist state across the Android process death, however this only officially supports reactive updates through LiveData. There is a nice extension function that exists in the support library that can convert LiveData to a Flow, however using this directly in a ViewModel test requires setting up the test to work with LiveData and LiveData can technically have a null value.

This is where SavedStateFlow comes in being a simple wrapper around SavedStateHandle.getLiveData() while enabling non null initial values exposing the state as a StateFlow.

Basic Usage

Inject a SavedStateFlowHandle into a ViewModel by using the extension function on SavedStateHandle.

val savedStateHandle: SavedStateHandle = TODO()

val savedStateFlowHandle: SavedStateFlowHandle = 
    savedStateHandle.toSavedStateFlowHandle()

Info

Please refer to the samples to see how to get a reference to SavedStateHandle.

• manual injection using the AbstractSavedStateViewModelFactory
• The saved-state-flow-hilt artifact automatically scopes SavedStateFlowHandle to ViewModel's so there is no need to get a reference to a SavedStateHandle.

Once a SavedStateFlowHandle is created, inject it in a ViewModel and retrieve a SavedStateFlow.

class MainViewModel(
    savedStateFlowHandle: SavedStateFlowHandle,
    private val newsDataSource: NewsDataSource
) : ViewModel() {

    private val query: SavedStateFlow<String> =
        savedStateFlowHandle.getSavedStateFlow(
            viewModelScope = viewModelScope, // scope for updates to be collected
            key = "main-viewmodel-query-key", // unique key for the property
            defaultValue = "" // used when there is no previously saved value upon restoration
        )

    init {
        observeQuery()
    }

    fun updateQuery(query: String) {
        this.query.value = query
    }

    private fun observeQuery() {
        viewModelScope.launch {
            query.asStateFlow()
                .flatMapLatest { query ->
                    // fetch the results for the latest query
                    newsDataSource.fetchQuery(query)
                }
                .collect { results ->
                    // Update with the latest results
                }
        }
    }
}

Warning

Since SavedStateFlow is a wrapper around SavedStateHandle, the following note from the documentation should be observed.

State must be simple and lightweight. For complex or large data, you should use local persistence.