Finite State Machine (FSM)¶

This package provides a lightweight, composable finite state machine for orchestrating asynchronous workflows. It is designed to be simple enough for embedded or real‑time adjacent scenarios while remaining idiomatic for .NET applications.

Interfaces: IState, IFiniteStateMachine<TState>
Implementation: FiniteStateMachine<TState>
Cross‑cutting: integrates cleanly with Diagnostics and Logging and Async Helpers

Why use an FSM?¶

An FSM is a robust way to model systems that have a small number of modes of operation with well‑defined rules for moving between them. Typical examples include: - Device and protocol controllers (Idle → Connecting → Connected → Error → Reconnect) - UI or workflow wizards (Start → Collecting Input → Validating → Completed/Failed) - Stream/transport handlers (Listening → Handshaking → Streaming → Closing)

Benefits - Deterministic behaviour: states explicitly define entry, run, and exit behaviour. - Clear separation of concerns: each state has a single responsibility. - Testability: transitions and hooks are easy to verify (see the included specifications). - Safe shutdown/transition: the framework cancels the running state and waits for it to exit before activating the next state. - Observability: transitions are published via IObservable so other components can react.

Real‑time considerations - Bounded transitions: when TransitionTo() is called, the framework cancels the old state and only starts the new state after the old state’s RunAsync completes and OnExit has run. This prevents overlap and race conditions. - No UI thread dependence: states run on background tasks (Task.Run) and use ContinueOnAnyThread semantics, avoiding message‑pump deadlocks. - Cooperative cancellation: each state’s RunAsync(CancellationToken) cooperates to exit promptly on cancellation, allowing responsive hand‑overs.

Programming model¶

Implement the IState interface for each state and drive transitions through IFiniteStateMachine<TState>.

public interface IState
{
    string DisplayName { get; }
    void OnEnter();
    void OnExit();
    Task RunAsync(CancellationToken cancelOnExit);
}

public interface IFiniteStateMachine<TState> where TState : class, IState
{
    void TransitionTo(TState newState);
    void StartStateMachine(TState initialState);
    void StopStateMachine();

    IObservable<TState> ObservableStates { get; }
    TState? CurrentState { get; }
}

Key runtime guarantees - Cancellation first: TransitionTo(next) cancels the current state’s token before waiting. - Orderly exit/enter: after the current state’s run task completes, OnExit() is called, then OnEnter() for the next state, then RunAsync() for the next state is started. - Transition completion signal: the FSM exposes a StateChanged event (ManualResetEvent) that is set once a transition fully completes, useful in tests or host loops. - Observable transitions: each activation is pushed to ObservableStates.

Minimal example¶

Below is a simplified controller with three states: Idle, Connecting, Connected. The Connecting state simulates work and then transitions to Connected; Connected waits until cancelled.

using System;
using System.Threading;
using System.Threading.Tasks;
using TA.Utils.Core.Diagnostics;
using TA.Utils.Core.StateMachine;

public class IdleState : IState
{
    public string DisplayName => "Idle";
    private readonly Action<IState> go;

    public IdleState(Action<IState> transition)
        => go = transition;

    public void OnEnter() { /* noop */ }
    public void OnExit()  { /* noop */ }

    public Task RunAsync(CancellationToken cancel)
    {
        // Immediately move to Connecting
        go(new ConnectingState(go));
        return Task.CompletedTask;
    }
}

public class ConnectingState : IState
{
    public string DisplayName => "Connecting";
    private readonly Action<IState> go;

    public ConnectingState(Action<IState> transition) => go = transition;

    public void OnEnter() { /* start timers, allocate resources */ }
    public void OnExit()  { /* release temporary resources */ }

    public async Task RunAsync(CancellationToken cancel)
    {
        try
        {
            // Simulate connection establishment
            await Task.Delay(500, cancel).ConfigureAwait(false);
            go(new ConnectedState());
        }
        catch (OperationCanceledException)
        {
            // Transition or stop was requested
        }
    }
}

public class ConnectedState : IState
{
    public string DisplayName => "Connected";
    public void OnEnter() { /* mark online */ }
    public void OnExit()  { /* mark offline */ }

    public async Task RunAsync(CancellationToken cancel)
    {
        // Run until cancelled by a transition or StopStateMachine()
        try
        {
            while (!cancel.IsCancellationRequested)
                await Task.Delay(100, cancel).ConfigureAwait(false);
        }
        catch (OperationCanceledException) { }
    }
}

// Host
var log = new DegenerateLoggerService();
var fsm = new FiniteStateMachine<IState>(log);

// Helper to allow states to request transitions without holding fsm directly.
void Go(IState next) => fsm.TransitionTo(next);

var idle = new IdleState(Go);
fsm.StartStateMachine(idle);

Notes - States can trigger transitions by holding an Action<IState> (or a reference to the FSM) passed via constructor DI. - Keep OnEnter/OnExit fast; perform long work inside RunAsync. - Always observe the cancelOnExit token and exit promptly on cancellation.

Observing transitions¶

Other components can subscribe to state activations.

var subscription = fsm.ObservableStates.Subscribe(s =>
{
    Console.WriteLine($"Now in state: {s.DisplayName}");
});

In tests or synchronous orchestration, you can wait for a transition to complete:

fsm.TransitionTo(new ConnectedState());
fsm.StateChanged.WaitOne(); // transition completed (enter/run scheduled)

Error handling and logging¶

Exceptions in RunAsync should generally be caught inside the state and handled by transitioning to an error/recovery state.
Exceptions in OnExit are caught by the FSM and logged, to avoid derailing the transition.
Use Diagnostics and Logging to attach per‑state context (e.g., logger name, correlation ids).

Timeouts, timers, and external events¶

Use Task.WhenAny, Task.Delay(timeout, cancel) or external event sources to decide when to transition.
For periodic work, loop inside RunAsync with await Task.Delay(...) observing cancelOnExit.
For external events (I/O, message bus), subscribe in OnEnter and unsubscribe in OnExit.

Example: transition on timeout or signal, whichever comes first.

public class WaitingForSignal : IState
{
    public string DisplayName => "WaitingForSignal";
    private readonly Action<IState> go;
    private readonly TaskCompletionSource<bool> signal = new();

    public WaitingForSignal(Action<IState> transition) => go = transition;
    public void OnEnter() { /* wire external event => signal.TrySetResult(true) */ }
    public void OnExit()  { /* unwire */ }

    public async Task RunAsync(CancellationToken cancel)
    {
        var timeout = Task.Delay(TimeSpan.FromSeconds(2), cancel);
        var completed = await Task.WhenAny(signal.Task, timeout).ConfigureAwait(false);
        if (completed == signal.Task) go(new ConnectedState());
        else go(new IdleState(go));
    }
}

Best practices¶

Keep state objects small, immutable where possible, and focused on one responsibility (SRP).
Prefer composition: delegate specialized work (I/O, parsing) to collaborators injected into the state.
Avoid blocking waits inside RunAsync—always prefer async and honor cancellation.
Design explicit Error/Recovering states rather than mixing error handling into every state.
Publish enough metadata via logging to trace complex sequences (see Diagnostics and Logging and correlation guidance).

API reference (summary)¶

IState
DisplayName – human‑readable identifier for diagnostics/UI
OnEnter() – called after previous state exits, before RunAsync
OnExit() – called during transition out, after RunAsync completes
RunAsync(CancellationToken) – the main loop/logic; must observe cancellation
IFiniteStateMachine<TState>
StartStateMachine(TState initial) – activates the initial state
TransitionTo(TState next) – cancels current state, waits, then activates next
StopStateMachine() – cancels and exits, completes the observable, resets to inactive
ObservableStates – IObservable<TState> emitting each activated state
CurrentState – currently active state or null when stopped