状态模式(State)
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1. 定义
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。2. UML 类图
3. 结构代码
using System;
namespace DoFactory.GangOfFour.State.Structural
{
///<summary>
/// MainApp startup class for Structural
/// State Design Pattern.
///</summary>
classMainApp
{
///<summary>
/// Entry point into console application.
///</summary>
staticvoid Main()
{
// Setup context in a state
Context c =newContext(newConcreteStateA());
// Issue requests, which toggles state
c.Request();
c.Request();
c.Request();
c.Request();
// Wait for user
Console.ReadKey();
}
}
///<summary>
/// The 'State' abstract class
///</summary>
abstractclassState
{
publicabstractvoid Handle(Context context);
}
///<summary>
/// A 'ConcreteState' class
///</summary>
classConcreteStateA :State
{
publicoverridevoid Handle(Context context)
{
context.State = new ConcreteStateB();
}
}
///<summary>
/// A 'ConcreteState' class
///</summary>
classConcreteStateB :State
{
publicoverridevoid Handle(Context context)
{
context.State = new ConcreteStateA();
}
}
///<summary>
/// The 'Context' class
///</summary>
classContext
{
privateState _state;
// Constructor
public Context(State state)
{
this.State = state;
}
// Gets or sets the state
publicState State
{
get {return _state; }
set
{
_state = value;
Console.WriteLine("State: " +
_state.GetType().Name);
}
}
publicvoid Request()
{
_state.Handle(this);
}
}
}
Output
State: ConcreteStateB
State: ConcreteStateA
State: ConcreteStateB
State: ConcreteStateA
4. 实例代码
using System;
namespace DoFactory.GangOfFour.State.RealWorld
{
///<summary>
/// MainApp startup class for Real-World
/// State Design Pattern.
///</summary>
classMainApp
{
///<summary>
/// Entry point into console application.
///</summary>
staticvoid Main()
{
// Open a new account
Account account =newAccount("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();
}
}
///<summary>
/// The 'State' abstract class
///</summary>
abstractclassState
{
protectedAccount account;
protecteddouble balance;
protecteddouble interest;
protecteddouble lowerLimit;
protecteddouble upperLimit;
// Properties
publicAccount Account
{
get {return account; }
set { account =value; }
}
publicdouble Balance
{
get {return balance; }
set { balance =value; }
}
publicabstractvoid Deposit(double amount);
publicabstractvoid Withdraw(double amount);
publicabstractvoid PayInterest();
}
///<summary>
/// A 'ConcreteState' class
///<remarks>
/// Red indicates that account is overdrawn
///</remarks>
///</summary>
classRedState :State
{
privatedouble _serviceFee;
// Constructor
public RedState(State state)
{
this.balance = state.Balance;
this.account = state.Account;
Initialize();
}
privatevoid Initialize()
{
// Should come from a datasource
interest = 0.0;
lowerLimit = -100.0;
upperLimit = 0.0;
_serviceFee = 15.00;
}
publicoverridevoid Deposit(double amount)
{
balance += amount;
StateChangeCheck();
}
publicoverridevoid Withdraw(double amount)
{
amount = amount - _serviceFee;
Console.WriteLine("No funds available for withdrawal!");
}
publicoverridevoid PayInterest()
{
// No interest is paid
}
privatevoid StateChangeCheck()
{
if (balance > upperLimit)
{
account.State = new SilverState(this);
}
}
}
///<summary>
/// A 'ConcreteState' class
///<remarks>
/// Silver indicates a non-interest bearing state
///</remarks>
///</summary>
classSilverState :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();
}
privatevoid Initialize()
{
// Should come from a datasource
interest = 0.0;
lowerLimit = 0.0;
upperLimit = 1000.0;
}
publicoverridevoid Deposit(double amount)
{
balance += amount;
StateChangeCheck();
}
publicoverridevoid Withdraw(double amount)
{
balance -= amount;
StateChangeCheck();
}
publicoverridevoid PayInterest()
{
balance += interest * balance;
StateChangeCheck();
}
privatevoid StateChangeCheck()
{
if (balance < lowerLimit)
{
account.State = new RedState(this);
}
elseif (balance > upperLimit)
{
account.State = new GoldState(this);
}
}
}
///<summary>
/// A 'ConcreteState' class
///<remarks>
/// Gold indicates an interest bearing state
///</remarks>
///</summary>
classGoldState :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();
}
privatevoid Initialize()
{
// Should come from a database
interest = 0.05;
lowerLimit = 1000.0;
upperLimit = 10000000.0;
}
publicoverridevoid Deposit(double amount)
{
balance += amount;
StateChangeCheck();
}
publicoverridevoid Withdraw(double amount)
{
balance -= amount;
StateChangeCheck();
}
publicoverridevoid PayInterest()
{
balance += interest * balance;
StateChangeCheck();
}
privatevoid StateChangeCheck()
{
if (balance < 0.0)
{
account.State = new RedState(this);
}
elseif (balance < lowerLimit)
{
account.State = new SilverState(this);
}
}
}
///<summary>
/// The 'Context' class
///</summary>
classAccount
{
privateState _state;
privatestring _owner;
// Constructor
public Account(string owner)
{
// New accounts are 'Silver' by default
this._owner = owner;
this._state =newSilverState(0.0,this);
}
// Properties
publicdouble Balance
{
get {return _state.Balance; }
}
publicState State
{
get {return _state; }
set { _state =value; }
}
publicvoid 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("");
}
publicvoid 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);
}
publicvoid PayInterest()
{
_state.PayInterest();
Console.WriteLine("Interest Paid --- ");
Console.WriteLine(" Balance = {0:C}",this.Balance);
Console.WriteLine(" Status = {0}\n",
this.State.GetType().Name);
}
}
}
Output
Balance = $500.00
Status = SilverState
Deposited $300.00 ---
Balance = $800.00
Status = SilverState
Deposited $550.00 ---
Balance = $1,350.00
Status = GoldState
Interest Paid ---
Balance = $1,417.50
Status = GoldState
Withdrew $2,000.00 ---
Balance = ($582.50)
Status = RedState
No funds available for withdrawal!
Withdrew $1,100.00 ---
Balance = ($582.50)
Status = RedState
该文章来自:http://www.dofactory.com/Patterns/PatternState.aspx
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