Finite State Machine for Unity

A simple yet powerful hierarchical finite state machine for the Unity game engine. It is scalable and customisable by being class-based, but also supports functions (or lambdas) for fast prototyping.

It has a special focus on the temporal aspects of state transitions, making it ideal for timing and synchronisation sensitive problems.

• Fast prototyping

• Hierarchical features

• Timing of state changes

• Multiple state change patterns

• Control flow of OnLogic

• Unity coroutines

• Scalable (class-based)

Why use UnityHFSM?

• State machines are an amazingly easy way to write and organise game logic. (As soon as you have states, e.g. Walk, Run, Sneak, Jump, you can use a state machine)

• It's easy and straightforward to use

• It helps you tame complex behaviour (e.g. weapon handling -> cooling, ammo, reload, fire) or write AI for NPCs

• Helps you write self documenting code, that is maintainable and readable

• Reduces the boilerplate code required to write a state machine

• UnityHFSM is lightweight and efficient, making it perfect for solving small and big problems

• It is especially designed for Unity, and supports Coroutines, which would otherwise be difficult to implement in a state machine

• No GC Allocations for state changes / updates / ... after setting up the state machine (-> No unwanted GC related lag spikes because of the state machine)

• The code is well documented

• It is not a component (MonoBehaviour) and therefore has a low overhead

• By being hierarchical, it can reduce the amount of duplicate code between states

Example

Simple State Machine

Here's a simple state machine for an enemy spy in your game.

As you can see the enemy will try to stay outside of the player's scanning range while extracting intel. When the player goes too far away, it will follow the player again.

The idea:

• Initialise the state machine

• Add states:

  fsm.AddState( new State(
      onEnter,
      onLogic,
      onExit
  ));

• Add transitions

  fsm.AddTransition( new Transition(
      from,
      to,
      condition
  ));

• Run the state machine

  void Update {
      fsm.OnLogic()
  }

Initialising the state machine

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using FSM;    // Import the required classes for the state machine

public class EnemyController : MonoBehaviour
{
    private StateMachine fsm;
    public float playerScanningRange = 4f;
    public float ownScanningRange = 6f;

    void Start()
    {
        fsm = new StateMachine(this);
    }
}

Adding states

    float DistanceToPlayer()
    {
        // This implementation is an example and may differ for your scene setup
        Vector3 player = PlayerController.Instance.transform.position;
        return Vector2.Distance(transform.position, player);
    }

    void MoveTowardsPlayer(float speed)
    {
        // This implementation is an example and may differ for your scene setup
        Vector3 player = PlayerController.Instance.transform.position;
        transform.position = Vector2.MoveTowards(transform.position, player, speed * Time.deltaTime);
    }

    void Start()
    {
        fsm = new StateMachine(this);

        // Empty state without any logic
        fsm.AddState("ExtractIntel", new State());

        fsm.AddState("FollowPlayer", new State(
            onLogic: (state) => MoveTowardsPlayer(1)
        ));

        fsm.AddState("FleeFromPlayer", new State(
            onLogic: (state) => MoveTowardsPlayer(-1)
        ));

        // This configures the entry point of the state machine
        fsm.SetStartState("FollowPlayer");
        // Initialises the state machine and must be called before OnLogic() is called
        fsm.Init()
    }

Although this example is using lambda expressions for the states' logic, you can of course just pass normal functions.

Adding transitions

    void Start()
    {
        // ...

        fsm.AddTransition(new Transition(
            "ExtractIntel", 
            "FollowPlayer",
            (transition) => DistanceToPlayer() > ownScanningRange
        ));

        fsm.AddTransition(new Transition(
            "FollowPlayer", 
            "ExtractIntel",
            (transition) => DistanceToPlayer() < ownScanningRange
        ));

        fsm.AddTransition(new Transition(
            "ExtractIntel", 
            "FleeFromPlayer",
            (transition) => DistanceToPlayer() < playerScanningRange
        ));

        fsm.AddTransition(new Transition(
            "FleeFromPlayer",
            "ExtractIntel",
            (transition) => DistanceToPlayer() > playerScanningRange
        ));
    }

Initialising and running the state machine

    void Start() 
    {
        // ...

        fsm.Init();
    }

    void Update()
    {
        fsm.OnLogic();
    }

Hierarchical State Machine

Because StateMachine inherits from StateBase, it can be treated as a normal state, therefore allowing for the nesting of state machines together with states.

Expanding on the previous example

So that you can see a visual difference, the enemy should be spinning when it enters the "SendData" state.

The idea:

• Create a separate state machine for the nested states (States in Extract Intel)

• Add the nested states to the new state machine

• Add the new state machine to the main state machine as a normal state

Separate FSM for the ExtractIntel state

    void Start()
    {
        // This is the main state machine
        fsm = new StateMachine(this);

        // This is the nested state machine
        StateMachine extractIntel = new StateMachine(this, needsExitTime: false);
        fsm.AddState("ExtractIntel", extractIntel);

        // ...
    }

Adding States and Transitions

    void RotateAtSpeed(float speed)
    {
        transform.eulerAngles += new Vector3(0, 0, speed * Time.deltaTime);
    }

    void Start()
    {
        fsm = new StateMachine(this);

        StateMachine extractIntel = new StateMachine(this, needsExitTime: false);
        fsm.AddState("ExtractIntel", extractIntel);

        extractIntel.AddState("SendData", new State(
            onLogic: (state) =>
            {
                // When the state has been active for more than 5 seconds,
                // notify the fsm that the state can cleanly exit
                if (state.timer > 5)
                    state.fsm.StateCanExit();

                // Make the enemy turn at 100 degrees per second
                RotateAtSpeed(100f);
            },
            // This means the state won't instantly exit when a transition should happen
            // but instead the state machine waits until it is given permission to change state
            needsExitTime: true
        ));

        extractIntel.AddState("CollectData", new State(
            onLogic: (state) => {if (state.timer > 5) state.fsm.StateCanExit();},
            needsExitTime: true
        ));

        // A transition without a condition
        extractIntel.AddTransition(new Transition("SendData", "CollectData"));
        extractIntel.AddTransition(new Transition("CollectData", "SendData"));
        extractIntel.SetStartState("CollectData");

        // ...
    }

What is fsm.StateCanExit() and needsExitTime? (See the next paragraph below)

Timing of state changes

When needsExitTime is set to false, the state can exit any time (because of a transition), regardless of its state (Get it? :) ). If it is set to true this cannot happen (unless a transition has the forceInstantly property set to true). This is very useful when you do not want an action to be interrupted before it has ended, like in this case.

But when is the right time for the state machine to finally change states? This is where the fsm.StateCanExit() method comes in and another argument for the State constructor: canExit. fsm.StateCanExit() notifies the state machine that the state can cleanly exit.

1. When a transition should happen, the state machine calls activeState.RequestExit(), this in turn calling the canExit function. If the state can exit, the canExit function has to call fsm.StateCanExit() and if not, it doesn't call fsm.StateCanExit().

2. If the state couldn't exit when canExit was called, the active state has to notify the state machine at a later point in time, that it can exit, by calling the fsm.StateCanExit() method.

State Change Patterns

The state machine supports three ways of changing states:

1. Using Transition objects as described earlier. You can even have multiple transitions that connect the same two states. They are checked on every OnLogic call and can be seen as a type of polling.

   fsm.AddTransition( new Transition(
       from,
       to,
       condition
   ));

2. Calling the RequestStateChange method: Instead of using Transition objects to manage state changes, each state can individually also manage its own transitions by directly calling the RequestStateChange method.

   fsm.RequestStateChange(state, forceInstantly: false);

   Example

   fsm.AddState("FollowPlayer", new State(
       onLogic: (state) =>
       {
           MoveTowardsPlayer(1);
   
           if (DistanceToPlayer() < ownScanningRange)
           {
               fsm.RequestStateChange("ExtractIntel");
           }
       }
   ));

3. Using "Trigger Transitions": These are normal transitions that are only checked when a certain trigger (an event) is activated.

   These are really handy when a polling-based solution does not fit or is not efficient enough. Trigger Transitions let you effortlessly leverage the efficiency of event-based transitions, in combination with the full power of the existing high-level transition types.

   fsm.AddTriggerTransition(triggerName, transition);

   Example

   // Flappy Bird Example
   fsm.AddTriggerTransition(
       "OnCollision",
       new Transition("Alive", "Dead")
   );
   
   // Later
   fsm.Trigger("OnCollision");

Therefore UnityHFSM supports both polling-based and event-based transitions, as well as the feature to bypass the concept of transitions all together. That's pretty cool.

There is also a slight variation of the Transition state change behaviour, that allows you to change to a specific state from any other state (a "global" transition as opposed to a "local" / "direct" transition). They have the same forceInstantly / needsExitTime handling as normal transitions.

fsm.AddTransitionFromAny( new Transition(
    from,
    to,
    condition 
));

// For Trigger Transitions
fsm.AddTriggerTransitionFromAny(
    triggerName,
    transition
);

Example

fsm.AddTransitionFromAny( new Transition(
    "",    // From can be left empty, as it has no meaning in this context
    "Dead",
    t => (health <= 0)
));

// For Trigger Transitions
fsm.AddTriggerTransitionFromAny(
    "OnDamage",
    new Transition("", "Dead", t => (health <= 0))
);

Control flow of OnLogic

Every StateMachine's OnLogic method manages the automatic transitions via Transition (TransitionBase) objects and the active state's logic function.

Here's what happens:

1. The state machine checks all global transitions (transitions from any state) and sees if a transition should occur. If this is the case, the state machine will advance to the new state, and call the new state's OnLogic function.

2. If this is not the case, the fsm checks all direct transitions (transitions that go directly from the active state to another state) and sees if a transition should occur. If this is the case, the state machine will move on to the new state, and call the new state's OnLogic function.

3. If this is not the case, the fsm will finally call the currently active state's OnLogic function.

Because global transitions are checked first, they have the highest priority. The reason for this can be illustrated with the following example: When the health of the player drops below 0, regardless of which state the fsm is in, it should transition to the Dead state.

As you can see on the steps mentioned above, only one transition can occur per OnLogic call. This has many reasons, one of which being that the state machine does not get stuck in an infinite loop.

Unity Coroutines

By using the CoState class you can run coroutines. This class handles the following things automatically:

• Starting the Coroutine

• Running the Coroutine again once it has completed

• Terminating the Coroutine on state exit

As a result of a limitation of the C# language, you can sadly not use lambda expressions to define IEnumerators (=> Coroutines).

In this example, we can replace the SendData state with a more advanced one, which makes the spy turn in one direction for two seconds, and the in the other direction for the same duration.

    IEnumerator SendData(CoState state)
    {
        while (state.timer.Elapsed < 2)
        {
            RotateAtSpeed(100f);
            // Wait until the next frame
            yield return null;
        }

        while (state.timer.Elapsed < 4)
        {
            RotateAtSpeed(-100f);
            yield return null;
        }

        state.timer.Reset();
        // Because needsExitTime is true, we have to tell the FSM when it can
        // safely exit the state
        state.fsm.StateCanExit();
        yield break;
    }

    void Start()
    {
        // ...
        extractIntel.AddState("SendData", new CoState(
            onLogic: SendData,
            needsExitTime: true
        ));
        // ...
    }

Class based architecture

Because the states, transitions and the state machine itself are implemented in a object oriented manner, custom state and transition classes can be created. By inheriting from the common base classes (StateBase, TransitionBase), custom states and transitions can be developed. This is also how CoState, TransitionAfter, ... have been implemented internally.

Creating your own states

Simply inherit from the base class StateBase and override the methods you need.

    class CustomSendData : StateBase {
        // Important: The constructor must call StateBase's constructor (here: base(...))
        // because it declares whether the state needsExitTime
        public CustomSendData() : base(needsExitTime: false)
        {
            // Optional initialisation code here
        }

        public override void OnEnter()
        {
            // Write your code for OnEnter here
            // If you don't have any, you can just leave this entire method override out
        }

        public override void OnLogic()
        {
            // The MonoBehaviour can be accessed from inside the state with this.mono or simply mono
            this.mono.transform.eulerAngles += new Vector3(0, 0, 100 * Time.deltaTime);
        }
    }

    void Start()
    {
        // ...
        extractIntel.AddState("SendData", new CustomSendData());
        // ...
    }

More documentation coming soon...