zg-statemachine/StateMachine.cs

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using ZelleGames.Log;

namespace ZelleGames.StateMachine
{
    /// <summary>
    /// Generic abstract state machine with OnEnter and OnExit features
    /// </summary>
    public abstract class StateMachine : MonoBehaviour
    {
        /// <summary>
        /// Readonly State currently executing
        /// </summary>
        public State CurrentState { get; protected set; }

        /// <summary>
        /// Drive the current state if present
        /// </summary>
        private void Update()
        {
            if (CurrentState != null)
            {
                CurrentState.Update(Time.deltaTime);
            }
        }

        /// <summary>
        /// Drive the current state's FixedUpdate method if state is present
        /// </summary>
        private void FixedUpdate()
        {
            if (CurrentState != null)
            {
                CurrentState.FixedUpdate(Time.fixedDeltaTime);
            }
        }

        /// <summary>
        /// Public facing function to change state based on a transition.
        /// Relies on an implementation of <see cref="GetNewState(Transition)"/>
        /// </summary>
        /// <param name="t">The transition to pass in.  Validitiy depends on the implementation of GetNewState</param>
        public void TransitionState(Transition t)
        {
            var newState = GetNewState(t);
            if (newState != null)
            {
                CurrentState?.OnExit();
                CurrentState = newState;
                CurrentState.StateMachine = this;
                CurrentState.OnEnter();
            } else
            {
                Logging.Log(LogLevel.WARN, $"Could not transition from {CurrentState} with {t}");
            }
        }

        /// <summary>
        /// Load a new state without consideration of the GetNewState function.
        /// This is sessentially the "goto" of state machine management, use only if absolutely needed.
        /// </summary>
        /// <param name="s"></param>
        public void ForceState(State s)
        {
            CurrentState.OnExit();
            CurrentState = s;
            CurrentState.OnEnter();
        }

        /// <summary>
        /// Derived classes should provide an implementation that returns a new <see cref="State"/> depending
        /// on the Transition passed in.
        /// </summary>
        /// <param name="t"></param>
        /// <returns>The new state for a valid transition for the current state, 
        /// Null otherwise to maintain current state</returns>
        protected abstract State GetNewState(Transition t);
    }

    /// <summary>
    /// Transitions are passed into the state machine to move from one state to the next.
    /// </summary>
    public abstract class Transition 
    {
        /// <summary>
        /// Generic object that can contain any information this transition needs to pass into the next state
        /// </summary>
        public object Context { get; }

        public Transition() : this(null){}

        public Transition(object ctx)
        {
            Context = ctx;
        }
    }

    /// <summary>
    /// Describes a finite state for a <see cref="StateMachine"/>
    /// in Unity.
    /// </summary>
    public abstract class State
    {
        /// <summary>
        /// Reference to the parent statemachine.  Derived classes can cast this reference into their specialized
        /// class for specific statemachine functionality, or use this reference as is to operate on the base class
        /// </summary>
        public StateMachine StateMachine;

        /// <summary>
        /// Called every frame by the <see cref="StateMachine"/>
        /// </summary>
        /// <param name="deltaTime"></param>
        public abstract void Update(float deltaTime);

        /// <summary>
        /// Called according to the Physics timing configuration by the <see cref="StateMachine"/>
        /// </summary>
        /// <param name="fixedDeltaTime"></param>
        public virtual void FixedUpdate(float fixedDeltaTime) { }

        /// <summary>
        /// Called on the frame the machine switches to this state
        /// </summary>
        public virtual void OnEnter() { }

        /// <summary>
        /// Called on the last frame before this state is replaced.
        /// </summary>
        public virtual void OnExit() { }
    }
}