Core

This is the core module for the Orbit framework. It provides all the basic parts of Orbit.

You will need this module (or modules that include it) to get started!

implementation("org.orbit-mvi:orbit-core:<latest-version>")

See architecture if you're interested in learning more about MVI and how its concepts map onto Orbit's components.

Orbit container

A Container is the heart of the Orbit MVI system. It retains the state, allows you to listen to side effects and state updates and allows you to modify the state through the orbit function which executes Orbit operators of your desired business logic.

Subscribing to the container

Container exposes flows that emit updates to the container state and side effects.

State emissions are conflated
Side effects are cached by default if no observers are listening. This can be changed via Container Settings

data class ExampleState(val seen: List<String> = emptyList())

sealed class ExampleSideEffect {
   data class Toast(val text: String)
}

class ExampleContainerHost(scope: CoroutineScope): ContainerHost<ExampleState, ExampleSideEffect> {
    
    // create a container
    override val container = scope.container<ExampleState, ExampleSideEffect>(ExampleState())

    fun doSomethingUseful() = intent {
        ...
    }
}

private val scope = CoroutineScope(Dispatchers.Main)
private val viewModel = ExampleContainerHost(scope)

fun main() {

    // subscribe to updates
    // On Android, use ContainerHost.observe() from the orbit-viewmodel module
    scope.launch {
        viewModel.container.stateFlow.collect {
            // do something with the state
        }
    }
    scope.launch {
        viewModel.container.sideEffectFlow.collect {
            // do something with the side effect
        }
    }

    viewModel.doSomethingUseful()
    
    // Ensure the main function does not complete so we can do something useful with the container.
}

ContainerHost

A ContainerHost is not strictly required to work with an Orbit Container. However, Orbit's syntax is defined as an extension on this class. Additionally it simplifies and organises your business logic and so is highly recommended. A ContainerHost typically defines MVI flows (your business logic and Orbit operators to be invoked on the Container) as functions that can be called by e.g. the UI.

In a typical implementation you would subclass Android's ViewModel and implement ContainerHost in order to create an Orbit-enabled Android ViewModel.

class ExampleViewModel(
    savedStateHandle: SavedStateHandle
) : ViewModel(), ContainerHost<ExampleState, ExampleSideEffect> {
    // create a container
    val container = container<ExampleState, ExampleSideEffect>(ExampleState(), savedStateHandle)

    …
}

Core operators

The Core module contains built-in Orbit operators. Here's how they map to MVI concepts:

MVI Operation	Orbit DSL	Purpose
block	`intent { ... }`	Contains business logic journeys, allows you to invoke other operators within
transformation	operations within `intent`	Run business operations to transform data
posted side effect	`postSideEffect(...)`	Sends one-off events to the side effect channel
reduction	`reduce { ... }`	Atomically updates the Container's state
-	`repeatOnSubscription { ... }`	Helps collect infinite flows only when there are active subscribers

Operators are invoked through the intent block in a ContainerHost. For more information about which threads these operators run on please see Threading.

Transformation

class Example : ContainerHost<ExampleState, ExampleSideEffect> {
    ...

    fun simpleExample() = intent {
        anotherApiCall(apiCall()) // just call suspending functions
    }
}

Transformations change upstream data into a different type. Transformations can do a simple mapping or something more complex like call a backend API or subscribing to a Flow.

tip

Infinite Flows are best collected within a repeatOnSubscription block.

In Orbit, the transformations are simply suspend function calls in the block function. It is your responsibility to ensure you are using the correct context for your calls. Blocking code in your intent block will generally cause Orbit's "event loop" to be blocked, effectively preventing processing of new intents until that code completes.

Reduction

class Example : ContainerHost<ExampleState, ExampleSideEffect> {
    ...

    fun simpleExample(number: Int) = intent {
        val result = apiCall()
        reduce { state.copy(results = result) }
    }
}

Reducers take incoming events and the current state to produce a new state.

Side effect

class Example : ContainerHost<ExampleState, ExampleSideEffect> {
    ...

    fun simpleExample(number: Int) = intent {
        val result = apiCall()
        postSideEffect(ExampleSideEffect.Toast("result $result"))
        reduce { state.copy(results = result) }
    }
}

Working with any system will eventually generate side effects. We've made them a first class citizen in Orbit.

This functionality is commonly used for things like truly one-off events, navigation, logging, analytics etc.

You may post the side effect in order to send it to a Container's side effect flow. Use this for view-related side effects like Toasts, Navigation, etc.

Side effects are cached if there are no observers, guaranteeing critical events such as navigation are delivered after re-subscription.

caution

Container.sideEffectFlow is designed to be collected by only one observer. This ensures that side effect caching works in a predictable way. If your particular use case requires multi-casting use broadcast on the side effect flow, but be aware that caching will not work for the resulting BroadcastChannel.

Repeat on subscription

class Example : ContainerHost<ExampleState, ExampleSideEffect> {
    ...

    fun simpleExample() = intent(idlingResource = false) {
        repeatOnSubscription {
            expensiveFlow().collect {
                //
            }
        }
    }
}

Collecting flows directly in an intent block continues until the flow completes or cancels. Cancellation happens automatically when the Orbit coroutine scope cancels.

The lifecycle of the Orbit coroutine scope, especially when set to viewModelScope may outlive the lifecycle of the UI resulting in subscriptions continuing in the background.

For expensive subscriptions, such as location or Bluetooth, this may be undesirable, you only want to collect from the flow when the UI actively observes the state or sideEffect streams.

repeatOnSubscription provides functionality to start (and restart) its inner block when the state or sideEffect streams are being observed and stop when that is no longer the case.

Operator context

Each simple syntax operator lambda has a receiver that exposes the current state of the Container as state

perform("Toast the current state")
class Example : ContainerHost<ExampleState, ExampleSideEffect> {
    ...

    fun anotherExample(number: Int) = intent {
        val result = apiCall()
        postSideEffect(ExampleSideEffect.Toast("state $state"))
        reduce { state.copy(results = event.results) }
    }
}

note

reduce is a special operator, where state is captured when its lambda is invoked. This means that within a reduce block, your state is guaranteed not to change.

Container factories

perform("Toast the current state")
class Example : ContainerHost<ExampleState, ExampleSideEffect> {
    override val container = container<ExampleState, ExampleSideEffect>(ExampleState()) {
        // This block is an intent invoked when the container is first created
        reduce { ... }
    }
}

Containers are typically not created directly but through convenient factory functions. This allows you to pass through extra settings or an intent lambda to invoke when the Container is first created (important for containers that can be recreated from a saved state or live longer than the UI).

A typical use case for this is to collect Flows that we need to start observing right after the container is created.

perform("Toast the current state")
class Example(
    private val flow1: Flow<Int>,
    private val flow2: Flow<Int>,
): ContainerHost<ExampleState, ExampleSideEffect> {
    override val container = container<ExampleState, ExampleSideEffect>(ExampleState()) {
        coroutineScope {
            repeatOnSubscription {
            launch {
                flow1.collect {
                    reduce { ... }
                }
            launch {
                flow2.collect {
                    reduce { ... }
                }
            }
        }
    }
}

Extra Container factory functionality is provided via extension functions. One example is ViewModel saved state support via a SavedStateHandle.

Threading

Orbit is designed to provide a sane default threading model to cater for most of the typical use cases. That being said you are not constrained and are free to switch threads if you need to (e.g. for database access). Typically that is done by switching your coroutine context.

Threading guarantees

Calls to Container.intent` do not block the caller. The operations within are offloaded to an event-loop style background coroutine.
Generally it is good practice to make sure long-running operations are done in a switched coroutine context in order not to block the Orbit "event loop".

Error handling

It is good practice to handle all of your errors within your intents. By default Orbit doesn't handle or process any exceptions because it cannot make assumptions about how you respond to errors. However you could install default exception handler via Container Settings property orbitExceptionHandler -> if defined exceptions are caught here so parent scope is not affected and Orbit container would continue to operate normally.

Core

Orbit container​

Subscribing to the container​

ContainerHost​

Core operators​

Transformation​

Reduction​

Side effect​

Repeat on subscription​

Operator context​

Container factories​

Threading​

Threading guarantees​

Error handling​