C# Finite State Machine for Unity

FSM.cs

using System;
using System.Collections.Generic;
using UnityEngine;

/* Example usage:
public FSM GameFSM = new();

private void Awake()
{
	GameFSM.AddState("Init",
		onEnter: fsm =>
		{
			print("Init");
			fsm.ChangeState("Play");
		}
	);

	GameFSM.AddState("Play",
		onEnter: _ => print("Play"),
		onUpdate: fsm =>
		{
			if (Input.GetKeyDown(KeyCode.Space))
				fsm.Trigger("OnLevelCompleted");
		},
		onExit: _ => print("Exiting Play state"),
		triggers: new()
		{
			("OnLevelCompleted", fsm => fsm.ChangeState("LevelCompleted")),
			("OnPlayerDeath", fsm => fsm.ChangeState("GameOver"))
		}
	);

	GameFSM.AddState("LevelCompleted",
		onEnter: _ => print("Level was completed")
	);

	GameFSM.AddState("GameOver",
		onEnter: _ => print("Game Over!")
	);

	GameFSM.ChangeState("Init"); // Set starting state
}

void FixedUpdate()
{
	GameFSM.FixedUpdate();
}

void Update()
{
	GameFSM.Update();
}

void LateUpdate()
{
	GameFSM.LateUpdate();
}
 */
public class FSM
{
	public string CurrentState = string.Empty;

	public Dictionary<string, Dictionary<string, Action<FSM>>> StatesTable = new();

	public FSM AddState(string stateName, Action<FSM> onEnter = null, Action<FSM> onFixedUpdate = null, Action<FSM> onUpdate = null, Action<FSM> onLateUpdate = null, Action<FSM> onExit = null, List<(string triggerName, Action<FSM> action)> triggers = null)
	{
		if (StatesTable.ContainsKey(stateName))
		{
			Debug.Log($"FSM AddState - State name ({stateName}) already exists.");
			return this;
		}
		else
		{
			string.Intern(stateName);
		}

		// Assign actions
		onEnter ??= DoNothing;
		onFixedUpdate ??= DoNothing;
		onUpdate ??= DoNothing;
		onLateUpdate ??= DoNothing;
		onExit ??= DoNothing;
		void DoNothing(FSM fsm) { }

		StatesTable.Add(stateName, new Dictionary<string, Action<FSM>>
		{
			["OnEnter"] = onEnter,
			["OnFixedUpdate"] = onFixedUpdate,
			["OnUpdate"] = onUpdate,
			["OnLateUpdate"] = onLateUpdate,
			["OnExit"] = onExit
		});

		// Add triggers
		triggers ??= new();
		foreach (var (triggerName, action) in triggers)
		{
			StatesTable[stateName].Add(triggerName, action);
			string.Intern(triggerName);
		}

		return this;
	}

	public void ChangeState(string targetState)
	{
		if (!StatesTable.ContainsKey(targetState))
		{
			Debug.Log($"FSM ChangeState - No matching state ({targetState}) in machine.");
			return;
		}

		if (!string.IsNullOrWhiteSpace(CurrentState))
			StatesTable[CurrentState]["OnExit"](this);

		CurrentState = string.Intern(targetState);

		StatesTable[targetState]["OnEnter"](this);
	}

	public void Trigger(string triggerName)
	{
		if (!StatesTable[CurrentState].ContainsKey(triggerName))
		{
			Debug.Log($"FSM Trigger - No matching trigger ({triggerName}) handler in the current state ({CurrentState}).");
			return;
		}

		StatesTable[CurrentState][triggerName](this);
	}

	public void FixedUpdate()
	{
		StatesTable[CurrentState]["OnFixedUpdate"](this);
	}

	public void Update()
	{
		StatesTable[CurrentState]["OnUpdate"](this);
	}

	public void LateUpdate()
	{
		StatesTable[CurrentState]["OnLateUpdate"](this);
	}
}