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
欢迎阅读本系列文章：Head First设计模式之目录