设计模式24：State Pattern (状态模式)

英文原文：http://www.dofactory.com/Patterns/PatternState.aspx

一、State Pattern (状态模式)

Define: Allow an object to alter its behavior when its internal state changes. The object will appear to change its class.

定义：当一个对象的内部状态进行改变的时候允许对象改变其行为。这个对象将显然可以改变其类。

二、UML类图

Context  (Account)

• defines the interface of interest to clients
• maintains an instance of a ConcreteState subclass that defines the current state.

State  (State)

• defines an interface for encapsulating the behavior associated with a particular state of the Context.

Concrete State  (RedState, SilverState, GoldState)

• each subclass implements a behavior associated with a state of Context

三、State Pattern (状态模式)实例性代码

using System; using System.Collections.Generic; using System.Linq; using System.Text; namespace State_Pattern { /// <summary> /// MainApp startup class for Structural /// State Design Pattern. /// </summary> class MainApp { /// <summary> /// Entry point into console application. /// </summary> static void Main() { // Setup context in a state Context c = new Context(new ConcreteStateA()); // Issue requests, which toggles state c.Request(); c.Request(); c.Request(); c.Request(); // Wait for user Console.ReadKey(); } } /// <summary> /// The 'State' abstract class /// </summary> abstract class State { public abstract void Handle(Context context); } /// <summary> /// A 'ConcreteState' class /// </summary> class ConcreteStateA : State { public override void Handle(Context context) { context.State = new ConcreteStateB(); } } /// <summary> /// A 'ConcreteState' class /// </summary> class ConcreteStateB : State { public override void Handle(Context context) { context.State = new ConcreteStateA(); } } /// <summary> /// The 'Context' class /// </summary> class Context { private State _state; // Constructor public Context(State state) { this.State = state; } // Gets or sets the state public State State { get { return _state; } set { _state = value; Console.WriteLine("State: " + _state.GetType().Name); } } public void Request() { _state.Handle(this); } } }

四、使用State Pattern (状态模式)的实例代码

This real-world code demonstrates the State pattern which allows an Account to behave differently depending on its balance. The difference in behavior is delegated to State objects called RedState, SilverState and GoldState. These states represent overdrawn accounts, starter accounts, and accounts in good standing.

using System; using System.Collections.Generic; using System.Linq; using System.Text; namespace State_Pattern { /// <summary> /// MainApp startup class for Real-World /// State Design Pattern. /// </summary> class MainApp { /// <summary> /// Entry point into console application. /// </summary> static void Main() { // 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(); } } /// <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); } } } /// <summary> /// The 'Context' class /// </summary> 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); } } }