StateMachine.cs

namespace rpg {
    public class StateMachine<T> {

        private T entity;
        private State<T> currentState;
        private State<T> previousState;
        private State<T> globalState;

        public StateMachine(T entity, State<T> initialState, State<T> globalState = null) {
            this.entity = entity;
            this.currentState = initialState;
            this.globalState = globalState;
            initialState?.Enter(entity);
            globalState?.Enter(entity);
        }

        public void Update(float dt) {
            if(globalState  != null) globalState.Update(entity, dt);
            if(currentState != null) currentState.Update(entity, dt);
        }

        public void Draw() {
            if(globalState  != null) globalState.Draw(entity);
            if(currentState != null) currentState.Draw(entity);
        }

        public void ChangeState(State<T> newState) {
            previousState = currentState;
            previousState.Exit(entity);
            currentState = newState;
            currentState.Enter(entity);
        }

    }
}