Syntriax.Engine/Engine.Systems/StateMachine/StateMachine.cs

using Engine.Core;
using Engine.Core.Serialization;

namespace Engine.Systems.StateMachine;

public class StateMachine : Behaviour, IUpdate
{
    public Event<StateMachine, StateChangedArguments> OnStateChanged { get; } = new();

    private readonly Event<IState, IState.StateTransitionReadyArguments>.EventHandler delegateOnStateTransitionReady = null!;

    private IState _state = new State();

    public StateMachine()
    {
        delegateOnStateTransitionReady = OnStateTransitionReady;
    }

    [Serialize]
    public IState State
    {
        get => _state;
        set
        {
            if (_state == value)
                return;

            IState previousState = _state;
            previousState.OnStateTransitionReady.RemoveListener(delegateOnStateTransitionReady);

            _state = value;
            previousState.TransitionFrom(value);
            value.TransitionTo(_state);
            OnStateChanged?.Invoke(this, new(value, previousState));

            value.OnStateTransitionReady.AddListener(delegateOnStateTransitionReady);
        }
    }

    private void OnStateTransitionReady(IState sender, IState.StateTransitionReadyArguments args)
    {
        State = args.ToState;
        while (State.GetNextState() is IState nextState)
            State = nextState;
    }

    public void Update()
    {
        if (State is null)
            return;

        while (State.GetNextState() is IState nextState)
            State = nextState;

        State.Update();
    }

    public readonly record struct StateChangedArguments(IState CurrentState, IState PreviousState);
}