概述：
状态模式（state）：当一个对象的内在状态改变时允许改变其行为，这个对象看起来像是改变了其类。
适用场合：
1.当一个对象的转换条件表达式过于复杂时，通常这个状态由一个或者多个枚举表示，通常有多个操作包含这一相同的的条件结构，state模式将一个条件分支放到一个类中，这使得你可以根据对象自身的情况将对象的状态作为对象，这一对象不依赖于其他对象而独立变化，把状态的判断逻辑放到表示不同状态的一系列类中。
2.一个对象的行为取决于它的状态，并且它必须在运行时根据状态立即改变行为。
类图：
代码示例：
1.维护一个ConcreteState子类的实例，这个实例定义当前状态
/// <summary>
/// 维护一个ConcreteState子类的实例，这个实例定义当前的状态
/// </summary>
class Context
{
private State state;
/// <summary>
/// 定义context的初始状态
/// </summary>
/// <param name="state"></param>
public Context(State state)
{
this.state = state;
}
/// <summary>
/// 可读写的状态属性，用于读取当前状态和设置新状态
/// </summary>
public State State
{
get { return state; }
set
{
state = value;
Console.WriteLine("当前的状态："+state.GetType().Name);
}
}
/// <summary>
/// 对请求作出处理，并设置下一状态
/// </summary>
public void Request()
{
state.Handle(this);
}
}
2.抽象状态类
/// <summary>
/// 抽象状态类，每个子类实现与Context的一个状态相关的行为
/// </summary>
abstract class State
{
public abstract void Handle(Context context);
}
3.具体状态类
class ConcreteStateA:State
{
public override void Handle(Context context)
{
//设置状态A的下一状态是状态B
context.State = new ConcreteStateB();
}
}
class ConcreteStateB : State
{
public override void Handle(Context context)
{
//状态B的下一状态是状态A
context.State = new ConcreteStateA();
}
}
应用示例：摘自 灵动生活
View Code
class Account
{
private AccountState _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 AccountState 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 AccountState
{
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 : AccountState
{
private double _serviceFee;
// Constructor
public RedState(AccountState 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 : AccountState
{
// Overloaded constructors
public SilverState(AccountState 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 : AccountState
{
// Overloaded constructors
public GoldState(AccountState 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);
}
}
}
/// <summary>
/// 测试状态模式案例
/// </summary>
static void TestStateSample()
{
// 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();
}
小结：
状态类的主要作用就是消去大量条件判断