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Core Concepts

Let's visit each of the core concepts by examining a simple example. We'll create a simple "counter" program that can increment and decrement an Int state. In MVU architecture, state is changed by dispatching and handling messages. We'll look at this more in a moment. First let's take a look at the model.


The model is a representation of the state of your application. In this case, the model contains just one property which is an Int named count with a default value of 0. By convention, we define the model as a data class so it can be mutated easily. While your application is running, the model will be changing from one state to the next every time a message is dispatched.

data class Model(
    val count: Int = 0



Before looking at the update function, let's talk about messages. Messages describe the way we want the application state to change. In the case of our counter app, we have two messages that can be dispatched and handled: Increment and Decrement. Increment adds 1 to the current value of count and Decrement subtracts 1 from count. By convention, messages are defined as part of a sealed class. This allows us to use an exhaustive list of types when interpreting messages.

sealed class Msg {
    object Increment : Msg()
    object Decrement : Msg()

Update function

The update function uses these two concepts to take a previous state and transform it to the next state. When a message is dispatched, the update function is called with the dispatched message and the current state as arguments. The function must determine what state to return given these arguments.

So far we have mentioned two conventions: models are data classes, and messages are sealed classes. You can see in the function below how those modifiers are leveraged. The message type is able to be determined in an exhaustive manner using the when block. The new state is created by mutating the old state with the copy function.

val update: Update<Model, Msg> = { msg, model ->
    when (msg) {
        Msg.Increment -> model.copy(count = model.count + 1) to none()
        Msg.Decrement -> model.copy(count = model.count - 1) to none()


The job of the view function is to transform the current state into something that can be rendered in a UI. This could be an HTML string, a tree of widgets, or in multiplatform projects it will be a simple data structure. We call this output type view properties or Props.

View properties

There are two types of data you may want to store in your view properties. First is information that you want to be rendered to the UI, and in this case we want to show the current count. The second category of data is message generators. Your rendering code will know how to display properties and dispatch messages created from generators. If you are only targetting one platform, then you may decide to return components specific to that platform, but since Oolong is a multiplatform library it is convention to use a hierarchical data structure.

class Props(
    val count: Int,
    val increment: (Dispatch<Msg>) -> Unit,
    val decrement: (Dispatch<Msg>) -> Unit

View function

The view function, as mentioned above, takes the current state its argument and returns view properties. In our counter example we populate the view properties with:

  • count - the current count.
  • increment - a function which dispatches the Increment message.
  • decrement - a function which dispatches the Decrement message.
val view: View<Model, Props> = { model ->
        { dispatch -> dispatch(Msg.Increment) },
        { dispatch -> dispatch(Msg.Decrement) }

Putting it all together

Now that we've built the core components of our application we need a few more things to complete it, namely a way to create an initial application state and a way to render the view properties.

Initialization function

To get the runtime loop started, we first need to know what the initial state is. We do this by definiting an initialization function. This function is similar to the update function, however it takes no arguments. By convention, it is often desireable to define defaults in the model class and simply return a new instance from the init function.

val init: Init<Model, Msg> = {
    Model() to none()

Render function

We also need to know how to render the view properties returned by the view function. Each target platform does this by implementing a render function which takes the view properties and a dispatch function as arguments. The dispatch function can be invoked to send messages to the update function.

val render: Render<Msg, Props> = { props, dispatch ->
    // Platform specific rendering
    countLabel.text = "${props.count}"
    incrementButton.setOnClickListener { dispatch(props.increment()) }
    decrementButton.setOnClickListener { dispatch(props.decrement()) }


The Oolong runtime composes these core functions into a user interaction loop, continually moving from one state to the next. It also handles things like side-effects (which we'll see in the next chapter) and resource disposal. You can start this loop by calling Oolong.runtime.

val dispose = Oolong.runtime(