Ricky and Morty

An Android app displaying Ricky and Morty characters.

Motivation

To implement state machine into an Android app.

Every user-facing is a state machine. A finite state machine is a device that can be in exactly one of the finite states at a given time. For example, a light switch.

A state machine transitions from one state to another depending on the current state and the input given. It has three components:

State: Status of the state machine e.g. whether the light switch is on or off.
Events: Inputs to the state machine like pressing a button
Transitions: They move the state machine from one state to another when it receives an event.

Implementation

Define states, events, and side effects using Kotlin's sealed interface:

sealed interface CharacterState {
    data object Initial : CharacterState
    data object Loading : CharacterState

    data class CharacterPagedData(val data: Flow<PagingData<Character>>) : CharacterState
}

sealed interface CharacterEvent {
    data object OnFetchCharacters : CharacterEvent
    data class OnFetchingCharacters(val characters: Flow<PagingData<Character>>) : CharacterEvent
}

sealed interface CharacterSideEffect {
    data object FetchCharacters : CharacterSideEffect
}

Create a state machine interface:

interface StateMachine<State, Event, SideEffect> {
    
    val state: StateFlow<StateOutput<State, SideEffect?>>
    
    suspend fun accept(event: Event)
}

The state variable holds the current state of the FSM.
The accept(event: Event) method receives the event sent to the FSM. The state machine will process the event and update the value of state.

Then inject the state machine the ViewModel class:

class CharacterViewModel(
    val stateMachine: StateMachine<CharacterState, CharacterEvent, CharacterSideEffect>,
    // Other dependencies
) : ViewModel() {
    // The rest of the code
}

BKinya/RickyAndMorty

Ricky and Morty

Motivation

Implementation

Resources