CSharp设计模式读书笔记（21）：状态模式(学习难度：★★★☆☆，使用频率：★★★☆☆)

模式角色与结构：

示例代码：(本示例在具体状态类中实现状态切换，也可以在环境类中实现状态切换。状态模式一定程度上违背开闭原则)

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace CSharp.DesignPattern.StatePattern
{
    class Program
    {
        static void Main(string[] args)
        {
            Account account = new Account(0.0);
            account.Deposit(1000); // 1000
            account.Withdraw(2000); // -1000
            account.Deposit(3000); // 2000
            account.Withdraw(4000); // -2000
            account.Withdraw(1000); // -3000
            account.ComputeInterest();
            Console.ReadLine();
        }
    }

    // 环境类
    class Account 
    {
        public Account(double balance)
        {
            this._balance = balance;
            this._state = new NormalState(this);
        }

        public void Deposit(double amount)
        {
            _state.Deposit(amount);
        }

        public void Withdraw(double amount)
        {
            _state.Withdraw(amount);
        }

        public void ComputeInterest()
        {
            _state.ComputeInterest();
        }

        public double Balance
        {
            get { return _balance; }
            set { _balance = value; }
        }
        internal AccountState State
        {
            get { return _state; }
            set { _state = value; }
        }

        private double _balance = 0;
        private AccountState _state;
    }

    // 抽象状态类
    abstract class AccountState
    {
        protected Account _account;

        public virtual void Deposit(double amount)
        {
            _account.Balance += amount;
            StateCheck();
        }
        public virtual void Withdraw(double amount)
        {
            _account.Balance -= amount;
            StateCheck();
        }
        public abstract void ComputeInterest();
        public abstract void StateCheck();
    }

    // 具体状态类
    class NormalState : AccountState
    {
        public NormalState(Account account)
        {
            this._account = account;
        }

        public override void ComputeInterest() 
        {
            Console.WriteLine("No interest...");
        }
        // 状态转换
        public override void StateCheck()
        {
            if (_account.Balance > -2000 && _account.Balance <= 0)
            {
                _account.State = new OverdraftState(this._account);
                Console.WriteLine("OverdraftState...");
            }
            else if (_account.Balance == -2000)
            {
                _account.State = new RestrictedState(this._account);
                Console.WriteLine("RestrictedState...");
            }
            else if (_account.Balance < -2000)
            {
                Console.WriteLine("Forbidden...");
            }
        }
    }

    // 具体状态类
    class OverdraftState : AccountState
    {
        public OverdraftState(Account account)
        {
            this._account = account;
        }

        public override void ComputeInterest()
        {
            Console.WriteLine("Compute interest...");
        }
        // 状态转换
        public override void StateCheck()
        {
            if (_account.Balance > 0)
            {
                _account.State = new NormalState(this._account);
                Console.WriteLine("NormalState...");
            }
            else if (_account.Balance == -2000)
            {
                _account.State = new RestrictedState(this._account);
                Console.WriteLine("RestrictedState...");
            }
            else if (_account.Balance < -2000)
            {
                Console.WriteLine("Forbidden...");
            }
        }
    }

    // 具体状态类
    class RestrictedState : AccountState
    {
        public RestrictedState(Account account)
        {
            this._account = account;
        }

        public override void Withdraw(double amount)
        {
            Console.WriteLine("Account is restricted, withdraw failed...");
        }

        public override void ComputeInterest()
        {
            Console.WriteLine("Compute interest...");
        }
        // 状态转换
        public override void StateCheck()
        {
            if (_account.Balance > 0)
            {
                _account.State = new NormalState(this._account);
                Console.WriteLine("NormalState...");
            }
            else if (_account.Balance > -2000)
            {
                _account.State = new OverdraftState(this._account);
                Console.WriteLine("OverdraftState...");
            }
        }
    }
}