Machinery

namespace Machinery
{
    abstract class StateBase<TInput>
    {
        public event EventHandler<TInput> InputRequested;
        public virtual void Enter(StateBase<TInput> oldState, TInput input) { }
        public virtual void Exit(StateBase<TInput> newState, TInput input) { }
        public abstract ReactionBase<TInput> React(TInput input);
    }

    abstract class ReactionBase<TInput>
    {
        internal abstract StateBase<TInput> GetNewState();
    }

    sealed class DiscardReaction<TInput> : ReactionBase<TInput>
    {
        internal override StateBase<TInput> GetNewState() { return null; }

        public static readonly DiscardReaction<TInput> Default = new DiscardReaction<TInput>();
    }

    sealed class TransitReaction<TState, TInput> : ReactionBase<TInput> where TState : StateBase<TInput>
    {
        public TransitReaction(StateBase<TInput> newState)
        {
            if (newState == null)
                throw new ArgumentNullException(nameof(newState));

            _newState = newState;
        }

        internal override StateBase<TInput> GetNewState() { return _newState; }

        private readonly StateBase<TInput> _newState;
    }

    static class TransitReactionCompanion<TState, TInput> where TState : StateBase<TInput>, new()
    {
        public static readonly TransitReaction<TState, TInput> Default = new TransitReaction<TState, TInput>(new TState());
    }
}