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
