Head First设计模式之状态模式

一、定义

定义：允许对象在内部状态改变时改变它的行为, 对象看起来好像修改了它的类。

主要解决：对象的行为依赖于它的状态（属性），并且可以根据它的状态改变而改变它的相关行为。

何时使用：代码中包含大量与对象状态有关的条件语句。

如何解决：将各种具体的状态类抽象出来。

关键代码：通常命令模式的接口中只有一个方法。而状态模式的接口中有一个或者多个方法。而且，状态模式的实现类的方法，一般返回值，或者是改变实例变量的值。也就是说，状态模式一般和对象的状态有关。实现类的方法有不同的功能，覆盖接口中的方法。状态模式和命令模式一样，也可以用于消除 if...else 等条件选择语句。

二、结构图

Context类：维护一个ConcreteState子类的一个实例，这个实例定义当前的状态。

State类：抽象状态类，定义一个接口以封装与Context的一个特定状态相关的行为。

ConcreteStateA,ConcreteStateB,ConcreteStateC类：具体状态类，每一个子类实现一个与Context的一个状态相关的行为。

三、适用场景

1、行为随状态改变而改变的场景。

2、条件、分支语句的代替者。

四、优缺点

优点： 1、封装了转换规则。 2、枚举可能的状态，在枚举状态之前需要确定状态种类。 3、将所有与某个状态有关的行为放到一个类中，并且可以方便地增加新的状态，只需要改变对象状态即可改变对象的行为。 4、允许状态转换逻辑与状态对象合成一体，而不是某一个巨大的条件语句块。 5、可以让多个环境对象共享一个状态对象，从而减少系统中对象的个数。

缺点： 1、状态模式的使用必然会增加系统类和对象的个数。 2、状态模式的结构与实现都较为复杂，如果使用不当将导致程序结构和代码的混乱。 3、状态模式对"开闭原则"的支持并不太好，对于可以切换状态的状态模式，增加新的状态类需要修改那些负责状态转换的源代码，否则无法切换到新增状态，而且修改某个状态类的行为也需修改对应类的源代码。

五、实现

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

namespace DesignPatterns.StatePattern
{
    class Program
    {
        static void Main(string[] args)
        {
            // Open a new account
            Account account = new Account("Jim Johnson");
            // Apply financial transactions
            account.Deposit(500.0);
            account.Deposit(300.0);
            account.Deposit(550.0);
            account.PayInterest();
            account.Withdraw(2000.00);
            account.Withdraw(1100.00);
            // Wait for user
            Console.ReadKey();
        }
    }

    class Account
    {
        private State _state;
        private string _owner;
        // Constructor
        public Account(string owner)
        {
            // New accounts are 'Silver' by default
            this._owner = owner;

            this._state = new SilverState(0.0, this);
        }

        // Properties
        public double Balance
        {
            get { return _state.Balance; }
        }
        public State State
        {
            get { return _state; }
            set { _state = value; }
        }
        public void Deposit(double amount)
        {
            _state.Deposit(amount);
            Console.WriteLine("Deposited {0:C} --- ", amount);
            Console.WriteLine("Balance = {0:C}", this.Balance);
            Console.WriteLine("Status = {0}",this.State.GetType().Name);
            Console.WriteLine("");
        }
        public void Withdraw(double amount)
        {
            _state.Withdraw(amount);
            Console.WriteLine("Withdrew {0:C} --- ", amount);
            Console.WriteLine(" Balance = {0:C}", this.Balance);
            Console.WriteLine(" Status = {0}\n",
                this.State.GetType().Name);
        }

        public void PayInterest()
        {
            _state.PayInterest();
            Console.WriteLine("Interest Paid --- ");
            Console.WriteLine(" Balance = {0:C}", this.Balance);
            Console.WriteLine(" Status = {0}\n",
                this.State.GetType().Name);
        }
    }

    /// <summary>
    /// The 'State' abstract class
    /// </summary>
    abstract class State
    {
        protected Account account;
        protected double balance;
        protected double interest;
        protected double lowerLimit;
        protected double upperLimit;

        // Properties
        public Account Account
        {
            get { return account; }
            set { account = value; }
        }
        public double Balance
        {
            get { return balance; }
            set { balance = value; }
        }
        public abstract void Deposit(double amount);
        public abstract void Withdraw(double amount);
        public abstract void PayInterest();

    }

    /// <summary>
    /// A 'ConcreteState' class
    /// <remarks>
    /// Red indicates that account is overdrawn
    /// </remarks>
    /// </summary>
    class RedState : State
    {
        private double _serviceFee;
        // Constructor
        public RedState(State state)
        {
            this.balance = state.Balance;
            this.account = state.Account;
            Initialize();
        }
        private void Initialize()
        {
            // Should come from a datasource
            interest = 0.0;
            lowerLimit = -100.0;
            upperLimit = 0.0;
            _serviceFee = 15.00;
        }

        public override void Deposit(double amount)
        {
            balance += amount;
            StateChangeCheck();
        }

        public override void Withdraw(double amount)
        {
            amount = amount - _serviceFee;
            Console.WriteLine("No funds available for withdrawal!");
        }

        public override void PayInterest()
        {
            // No interest is paid
        }

        private void StateChangeCheck()
        {
            if (balance > upperLimit)
            {
                account.State = new SilverState(this);
            }
        }
    }
    /// <summary>

    /// A 'ConcreteState' class
    /// <remarks>
    /// Silver indicates a non-interest bearing state
    /// </remarks>
    /// </summary>
    class SilverState : State
    {
        // Overloaded constructors
        public SilverState(State state)
            :
            this(state.Balance, state.Account)
        {

        }
        public SilverState(double balance, Account account)
        {
            this.balance = balance;
            this.account = account;
            Initialize();
        }

        private void Initialize()
        {
            // Should come from a datasource
            interest = 0.0;
            lowerLimit = 0.0;
            upperLimit = 1000.0;
        }

        public override void Deposit(double amount)
        {
            balance += amount;
            StateChangeCheck();
        }
        public override void Withdraw(double amount)
        {
            balance -= amount;
            StateChangeCheck();
        }
        public override void PayInterest()
        {
            balance += interest * balance;
            StateChangeCheck();

        }
        private void StateChangeCheck()
        {
            if (balance < lowerLimit)
            {
                account.State = new RedState(this);
            }
            else if (balance > upperLimit)
            {
                account.State = new GoldState(this);
            }
        }
    }
    /// <summary>
    /// A 'ConcreteState' class
    /// <remarks>
    /// Gold indicates an interest bearing state
    /// </remarks>
    /// </summary>
    class GoldState : State
    {
        // Overloaded constructors
        public GoldState(State state)
            : this(state.Balance, state.Account)
        {
        }
        public GoldState(double balance, Account account)
        {
            this.balance = balance;
            this.account = account;
            Initialize();
        }
        private void Initialize()
        {
            // Should come from a database
            interest = 0.05;
            lowerLimit = 1000.0;
            upperLimit = 10000000.0;
        }
        public override void Deposit(double amount)
        {
            balance += amount;
            StateChangeCheck();

        }
        public override void Withdraw(double amount)
        {
            balance -= amount;
            StateChangeCheck();
        }

        public override void PayInterest()
        {
            balance += interest * balance;
            StateChangeCheck();
        }

        private void StateChangeCheck()
        {
            if (balance < 0.0)
            {
                account.State = new RedState(this);
            }
            else if (balance < lowerLimit)
            {
                account.State = new SilverState(this);
            }
        }
    }
}

现实中还有其他很多例子，比如自动贩卖机、电梯等。

状态模式的关键在于 状态变化时引起行为的变化，它是被动的触发

策略模式的差别在于，它是由外部（client）主动引起行为的变化，可以随意控制它想要执行的行为。

参考文章：

http://www.cnblogs.com/ywqu/archive/2010/01/26/1656418.html

http://www.runoob.com/design-pattern/state-pattern.html

http://www.cnblogs.com/JsonShare/p/7246915.html

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