A simple state pattern implements for Unity Engine

CharacterFSM.cs

public class CharacterFSM : FsmBehaviour<CharacterFSM>
{
	public float variable = 1f;

	protected override State<CharacterFSM> InitState()
	{
		return new Natural();
	}

	class Natural : State<CharacterFSM>, ITickable
	{
		public override void OnEnter()
		{
			Debug.Log("enter natural");
		}

		public void Tick(float deltaTime)
		{
			Debug.Log("nature OnUpdate");
			if (Input.anyKey)
			{
				TransitionTo(new NewState());
			}
		}
	}

	class NewState : State<CharacterFSM>, ITickable
	{
		public void Tick(float deltaTime)
		{
			Debug.Log($"new OnUpdate: {context.variable}");
		}
	}
}

FsmBehaviour.cs

public abstract class FsmBehaviour<T> : MonoBehaviour, IStateMachine<T> where T : FsmBehaviour<T>
{
	private State<T> _state;

	protected virtual void Awake()
	{
		_state = InitState();
	}

	protected virtual void FixedUpdate()
	{
		if (_state is ITickable tickable)
		{
			tickable.Tick(Time.deltaTime);
		}
	}

	public void TransitionTo(State<T> nextState)
	{
		nextState.context = this as T;
		_state?.OnExit();
		_state = nextState;
		_state.OnEnter();
	}

	protected abstract State<T> InitState();
}

public interface ITickable
{
	void Tick(float deltaTime);
}

State.cs

public abstract class State<T> where T : IStateMachine<T>
{
	public T context;

	protected void TransitionTo(State<T> nextState)
	{
		context.TransitionTo(nextState);
	}

	public virtual void OnEnter()
	{
	}

	public virtual void OnExit()
	{
	}
}

public interface IStateMachine<T> where T : IStateMachine<T>
{
	void TransitionTo(State<T> nextState);
}