UnityLearn – Beginner Programming – Finite State Machine – Pt. 02 – Finite State Machines – Steven Lilley

Novemeber 18, 2019

Beginner Programming: Unity Game Dev Courses

Unity Learn Course – Beginner Programming

Finite State Machines

Building the Machine

This part of the tutorial has a lot more hands on parts, so I skip some of the sections for note purposes when there is not much substance to them other than following inputs.

A key in finite state machines is that an object can only ever be in exactly one state at a time. This helps ensure each state be completely self contained.

Elements of a Finite State Machine:
Context: maintains an instance of a concrete state as the current state
Abstract State: defines an interface which encapsulates behaviors common to all concrete states
Concrete State: implements behaviors specific to a particular state of context

To get started in the tutorial, they created a public abstract class PlayerBaseState which will be the abstract state for this example. PlayerController_FSM is the context. They note that while in this case all the abstract state methods take the PlayerController_FSM (the context) in as a parameter in this case, that does not necessarily have to be the case for the general FSM pattern.

Concrete States

It is noted that the context in a FSM needs to hold a reference to a concrete state as the current state. This is done in the example by creating a variable which holds that of the type that is our abstract state, which is PlayerBaseState in this case. They then create a method called TransitionToState which takes a PlayerBaseState in as a parameter. It then sets the currentState to that parameter state, and then calls the new state’s EnterState method (all states have this method as it is dictated by the abstract class they all implement). This determines what actions should be done immediately upon entering this new state.

Example:

public void TransitionToState(PlayerBaseState state)
{
currentState = state;
currentState.EnterState(this);
}

The tutorial also shows a way to take control of the context’s general Unity methods and pass the work on to the concrete states instead. This example did this with Update and OnCollisionEnter. The abstract state, and in turn, all of the concrete states, have their own Update and OnCollisionEnter method. The context, PlayerController_FSM, then simply calls currentState.Update(this) in its Update method, and currentState.OnCollisionEnter(this) in its OnCollisionEnter method, so that the current concrete state’s logic for these methods are used without flooding the context itself with any more code.

Since it is necessary that your context has some initial state, they do this by simply calling the TransitionToState method within the Start method and entering the IdleState. IdleState is the initial state for this case.

Beginning the Implementation

Important benefits seen using this system:
While working on the concrete classes themselves, we never needed to go back to the PlayerController_FSM class (the context) to modify any code there. The entire behvior is handled within the concrete states and is abstracted from the character controller (the context). Setting expressions was much easier as no checks are needed and this can just be set in the EnterState method of each concrete state.

It is already clear that this method removes a lot of boolean checks from the overall code, and helps organize the code by ensuring any logic about a state is contained within the class for that state itself (with less bleeding into the code of other states).

Continuing the Implementation

It is worth noting that PlayerController_FSM holds a reference to every concrete state except the spinning state. This was done because they actually have the jumping state create a new spinning state on transition each time it is invoked. They apparently do this so that the local field for rotation within the spinning state is reset to 0 each time it is called, but it seems like there would be other ways to do this that seem less wasteful (such as resetting it to 0 when exiting the state). I am also not sure if this is intended behavior, but the spin also immediately cancels upon contacting the ground (resetting the player rotation to 0) with this setup, where as in the previous behavioral setup the spin completed even if the player contacted the ground.

Module Conclusion

Benefits of FSM:

More modular
Easier to read and maintain
Less difficult to debug
More extensible

Cons of FSM:

Take time to setup initially
More moving parts
Potentially less performant

Just something very notable with this approach, it seems much harder for me to break than the naive implementation. If I spammed key presses (like pressing jump and duck a lot) with the naive approach, sometimes I could break the system and have the player stuck in the duck position or be ducking while jumping. I have not been able to break it at all with the full FSM setup, which makes sense since transition behaviors solely exist within the states themselves so these inputs cannot be jumbled in any way.

SUMMARY

Using state machine systems appear way easier to use and build on than the “naive” approach of basic boolean behaviors (with lots of if statements and boolean checks). Not only was I very excited about how much easier this appears to work as a scalable option, it also just worked better and more cleanly when it was all put together.

The other version had small bugs that would pop up if you spammed all the different action keys (such as getting stuck ducking or being ducked in a jump), which were possible just because the key presses would get recorded before reaching the bools or if statements that should be telling them that they are not proper options. These very separated states make that type of error impossible as it is only concerned with a single state at a time.

This type of system just seems much cleaner, more organized, and less error prone than what I have done before and I am very excited to try and build a system like this for my own project (for both players and enemy AI).