Using state machine in Kotlin and sending updates

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02-09-2024

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Keeping Your UI in Sync: State Machines and Updates in Kotlin

State machines are powerful tools for managing complex workflows and system behaviors. In Kotlin, libraries like KStateMachine offer a convenient way to define state transitions and handle events. But how do you keep your user interface (UI) updated when the state machine changes?

This article explores a common scenario: you have a Kotlin state machine managing application logic, and you want to reflect state changes in your UI. We'll build upon a Stack Overflow question https://stackoverflow.com/questions/71574809/how-to-notify-ui-about-a-state-change-in-a-kotlin-state-machine and explore effective ways to connect your state machine to the UI.

The Challenge: Bridging the Gap

The provided code snippet demonstrates a basic state machine implementation in Kotlin using KStateMachine. However, it lacks a mechanism to communicate state changes to the UI. This is where we need to introduce a communication channel between the state machine and the UI components.

Solution 1: Using a Shared Data Object

One approach is to use a shared data object, like a LiveData object (in the case of Android) or a simple Observable class, to store the current state and signal updates to the UI.

Example:

import androidx.lifecycle.LiveData
import androidx.lifecycle.MutableLiveData

// ... (Rest of your state machine code)

@Singleton
class StateManager() : IStateManager {
    private var stateMachine: StateMachine? = null
    private val _currentState = MutableLiveData<State>()
    val currentState: LiveData<State> = _currentState

    // ... (Rest of your state machine code)

    private suspend fun startupMachine() {
        // ... (Rest of your state machine code)

        stateMachine!!.onStateChanged { state, _ ->
            _currentState.postValue(state)
        }
    }

    // ... (Rest of your code)
}

In this modified code:

1. We introduce a _currentState MutableLiveData to hold the current state of the state machine.
2. We create a currentState LiveData as a read-only wrapper for _currentState.
3. In the onStateChanged callback of the state machine, we post the new state to _currentState, triggering UI updates.

How it Works:

• UI components can observe the currentState LiveData.
• When the state machine transitions to a new state, the onStateChanged callback is invoked, updating _currentState.
• The LiveData notifies observers, triggering UI re-rendering based on the new state.

Solution 2: Using Event Bus

An alternative solution involves using an event bus. An event bus is a centralized mechanism for distributing events to interested subscribers. This approach is particularly useful when you have multiple UI components that need to be updated based on state changes.

Example:

import org.greenrobot.eventbus.EventBus
import org.greenrobot.eventbus.Subscribe
import org.greenrobot.eventbus.ThreadMode

// ... (Rest of your state machine code)

@Singleton
class StateManager() : IStateManager {
    private var stateMachine: StateMachine? = null

    // ... (Rest of your state machine code)

    private suspend fun startupMachine() {
        // ... (Rest of your state machine code)

        stateMachine!!.onStateChanged { state, _ ->
            EventBus.getDefault().post(StateChangeEvent(state))
        }
    }

    // ... (Rest of your code)
}

// Define a custom event class for state changes
class StateChangeEvent(val state: State)

// In your UI component
class MyUIComponent : ViewModel() {
    init {
        EventBus.getDefault().register(this)
    }

    @Subscribe(threadMode = ThreadMode.MAIN)
    fun onStateChange(event: StateChangeEvent) {
        // Update UI based on event.state
    }

    override fun onCleared() {
        super.onCleared()
        EventBus.getDefault().unregister(this)
    }
}

In this example:

1. We define a custom event class StateChangeEvent to encapsulate state change information.
2. In the onStateChanged callback, we post a StateChangeEvent to the EventBus.
3. UI components subscribe to the EventBus and handle StateChangeEvent to update themselves.

Advantages of Event Bus:

• Decouples state machine logic from UI components.
• Allows multiple UI components to react to state changes.
• Promotes modularity and testability.

Remember:

• Always choose the approach that best suits your project's specific needs and complexity.
• Consider using a reactive programming library like RxKotlin for more sophisticated event handling and UI updates.

By implementing a communication channel between your state machine and UI, you can ensure that your UI remains synchronized with the changing state of your application. This leads to a more responsive and user-friendly experience.