1. 什么是有限状态机？

有限状态机是一种用来进行对象行为建模的工具，主要是描述对象在它的生命周期内所经历的状态序列，以及如何响应来自外界的各种事件。
状态机可归纳为4个要素，即现态、条件、动作、次态。“现态”和“条件”是因，“动作”和“次态”是果。

2. 如何使用Stateless实现状态流转？

public class Order
    {
        public Order()
        {
            StateMachineInit();
        }

        [Key]
        public Guid Id { get; set; }
        public string UserId { get; set; }
        public ApplicationUser User { get; set; }
        public ICollection<LineItem> OrderItems { get; set; }
        public OrderStateEnum State { get; set; }
        public DateTime CreateDateUTC { get; set; }
        public string TransactionMetadata { get; set; }
        StateMachine<OrderStateEnum, OrderStateTriggerEnum> _machine;

        public void PaymentProcessing()
        {
            _machine.Fire(OrderStateTriggerEnum.PlaceOrder);
        }

        public void PaymentApprove()
        {
            _machine.Fire(OrderStateTriggerEnum.Approve);
        }

        public void PaymentReject()
        {
            _machine.Fire(OrderStateTriggerEnum.Reject);
        }

        private void StateMachineInit()
        {
            _machine = new StateMachine<OrderStateEnum, OrderStateTriggerEnum>(() => State, s => State = s);

            _machine.Configure(OrderStateEnum.Pending)
                .Permit(OrderStateTriggerEnum.PlaceOrder, OrderStateEnum.Processing)
                .Permit(OrderStateTriggerEnum.Cancel, OrderStateEnum.Cancelled);

            _machine.Configure(OrderStateEnum.Processing)
                .Permit(OrderStateTriggerEnum.Approve, OrderStateEnum.Completed)
                .Permit(OrderStateTriggerEnum.Reject, OrderStateEnum.Declined);

            _machine.Configure(OrderStateEnum.Declined)
                .Permit(OrderStateTriggerEnum.PlaceOrder, OrderStateEnum.Processing);

            _machine.Configure(OrderStateEnum.Completed)
                .Permit(OrderStateTriggerEnum.Return, OrderStateEnum.Refund);
        }
    }

3. 第三方支付的基本流程是什么？

引导客户携带着回调url到第三方支付平台支付，支付成功后，第三方支付平台将用户引导回商家。