v1.0.0 (Feb 3, 2019)
Sep 3, 2018
Feb 3, 2019 (Retired)
Sebastian Sellmair (sellmair)
Collin Flynn (cdflynn)
Sebastian Sellmair (sellmair)
Balazs Toth (balazstothofficial)
Source code

State management library for Android

What is it

Quantum is a general purpose state management library designed for building easy, stable and thread safe Android applications. It was inspired by AirBnb's MvRx and tailored for building reliable ViewModels.


dependencies { 
    implementation "io.sellmair:quantum:1.0.0"
    // optional rx extensions
    implementation "io.sellmair:quantum-rx:1.0.0"
    // optional LiveData extensions
    implementation "io.sellmair:quantum-livedata:1.0.0"
Define a State

States should always be immutable. I highly recommend using kotlin data classes to make immutability easy ????


data class MyState(
    val isLoading: Boolean = false, 
    val error: Error? = null,
    val content: Content? = null, 
    val userLocation: Location? = null)
Create a Quantum

A Quantum is the owner of your state. It applies all reducers, invokes actions and publishes new states.


// Create a new Quantum with initial state. 
val quantum = Quantum.create(MyState())
Enqueue a Reducer

Reducers are functions that take the current state and create a new state. Reducers will always be called by a internal thread of the Quantum. Only one reducer will run at a time! Reducers are allowed to return the same (untouched) instance to signal a no-operation.

Example (simple reducer):

A simple reducer that that says hello to a certain user.

data class SimpleState(val name: String, val message: String = "" )

val quantum = Quantum.create(SimpleState("Julian"))

fun sayHello() = quantum.setState {
    copy(message = "Hello $name")

Unlike other "State Owner" concepts, Quantum allows reducers to dispatch async operations. This decision was made to give developers the option to handle side-effects inside a safer environment.

Example (load content):

Much more complicated reducer problem:
We want to

  • Load content from repository asyncronously
  • Ensure that only one loading operation is running at a time
  • Publish the content when fetched successfully
  • Publish the error when an error occurred
// Reducer that fetches the content (if not currently loading)
fun loadContent() = quantum.setState {
    // Do not try to load the content while currently loading
    // Returning this (the current / input state) signals the quantum that 
    // this reducer was a NOOP
    if(isLoading) return@setState this
    // Dispatch a async loading operation with myRepository (exemplary)
    // Copy the current state but set loading flag  
    copy(isLoading = true)

fun onContentLoaded(content: Content) = setState {
    // Content loaded: 
    // Copy current state and clear any error
    copy(content = content, error = null, isLoading = false)

fun onError(error: Error) = setState {
    // Copy current state but publish the error
    copy(error = error, isLoading = false)
Enqueue an Action

Actions are parts of your code that require the most recent state, but do not intend to change it. Actions will always be called by a internal thread of the Quantum and run after all reducers are applied.

val quantum = Quantum.create(SimpleState(name = "Balazs"))

quantum.setState {
    copy(name = "Paul")

quantum.withState {
    // will print 'Hello Paul'
    Log.i("Readme", "Hello $name")
Listen for changes

Listeners are invoked by Android's main thread by default. It is possible to configure the thread which invokes listeners by specifying an Executor.

Example: Without Extensions, Rare
quantum.addStateListener { state -> print(state.message) }
Example: Without Extensions, Function
fun onState(state: SimpleState){
  // be awesome

fun onStart() {

fun onStop() {
Example: Rx (recommended)
fun onStart() {
    quantum.rx.subscribe { state -> /* be awesome */ }
Nested Quantum / Map

It is possible to map a Quantum to create a 'Child-Quantum' which can enqueue reducers and actions as usual. The state of this child will be in sync with the parent Quantum.

Example: Child
data class ChildState(val name: String, val age: Int)

data class ParentState(val name: String, val age: Int, val children: List<ChildState>)

// Get the quantum instance of the parent state
val parentQuantum: Quantum<ParentState> =  /* ... */

// Create the child state
val childQuantum = parentQuantum
    .map { parentState ->  parentState.children }
    .connect { parentState, children -> parentState.copy(children = children) }

// Increase the age of all children
childQuantum.setState { children ->
     children.map { child -> child.copy(age=child.age++) }

It is possible to record all states created in a Quantum.

val quantum = Quantum.create(MyState()).apply { 
    history.enabled = true

fun debug(){
   for(state in quantum.history){

A Quantum has to be stopped if it's no longer needed, in order to stop the internal background thread and release all resources.

quantum.quit() // will quit as fast as possible
quantum.quitSafely() // will quit after all currently enqueued reducers / actions
ViewModel (Suggestion)

I suggest having one 'ViewState' for each ViewModel. The ViewModel itself might want to implement Quantum itself.

data class LoginState(
    val email: String = "",
    val password: String = "", 
    val user: User? = null)

class LoginViewModel(private val loginService: LoginService): 
    Quantum<LoginState> by Quantum.create(LoginState()) {
   fun setEmail(email: String) = setState {
        copy(email = email)
   fun setPassword(password: String) = setState {
        copy(password = password)
   fun login() = setState {
       val user = loginService.login(email, password)
       copy(user = user)
   override fun onCleared() {
        // Quit the quantum


It is possible to configure the defaults of Quantum for your whole application. For example: It is possible to specify the default threading mode, history settings, or even the thread pool that is shared for multiple Quantum instances.

Global configuration
// configure defaults
Quantum.configure {
    // Quantum instances will use the given thread pool by default
    this.threading.default.mode = Threading.Pool
    // Listeners are now invoked by a new background thread
    this.threading.default.callbackExecutor = Executors.newSingleThreadExecutor()
    // Override the default shared thread pool
    this.threading.pool = Executors.newCachedThreadPool()
    // Set history default to enabled with limit of 100 states
    this.history.default.enabled = true
    this.history.default.limit = 100
    // Get info's from quantum
    this.logging.level = LogLevel.INFO
Instance configuration
        // initial state
        initial = LoginState(), 
        // invoke listeners by background thread
        callbackExecutor = Executors.newSingleThreadExecutor(),
        // use thread pool 
        threading = Threading.Pool)