Definition

Attach additional responsibilities to an object dynamically. Decorators provide a flexible alternative to subclassing for extending functionality.

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Frequency of use:??Medium

裝飾模式的UML類圖如下：

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一個生動的例子：Pizza（手抓餅）

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Component：

namespace DecoratorDemo {public abstract class Pizza{public string Description { get; set; }public abstract string GetDescription();public abstract double CalculateCost();} }

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ConcreteComponent:

using System;namespace DecoratorDemo {public class LargePizza : Pizza{public LargePizza(){Description = "Large Pizza";}public override string GetDescription(){return Description;}public override double CalculateCost(){return 9.00;}} }using System;namespace DecoratorDemo {public class MediumPizza : Pizza{public MediumPizza(){Description = "Medium Pizza";}public override string GetDescription(){return Description;}public override double CalculateCost(){return 6.00;}} }using System;namespace DecoratorDemo {public class SmallPizza : Pizza{public SmallPizza(){Description = "Small Pizza";}public override string GetDescription(){return Description;}public override double CalculateCost(){return 3.00;}} }

Decorator

using System;namespace DecoratorDemo {public class PizzaDecorator : Pizza{protected Pizza _pizza;public PizzaDecorator(Pizza pizza){_pizza = pizza;}public override string GetDescription(){return _pizza.Description;}public override double CalculateCost(){return _pizza.CalculateCost();}} }

ConcreteDecorator：

using System;namespace DecoratorDemo {public class Cheese : PizzaDecorator{public Cheese(Pizza pizza): base(pizza){Description = "Cheese";}public override string GetDescription(){return String.Format("{0}, {1}", _pizza.GetDescription(), Description);}public override double CalculateCost(){return _pizza.CalculateCost() + 1.25;}} }using System;namespace DecoratorDemo {public class Ham : PizzaDecorator{public Ham(Pizza pizza): base(pizza){Description = "Ham";}public override string GetDescription(){return String.Format("{0}, {1}", _pizza.GetDescription(), Description);}public override double CalculateCost(){return _pizza.CalculateCost() + 1.00;}} }using System;namespace DecoratorDemo {public class Peppers : PizzaDecorator{public Peppers(Pizza pizza): base(pizza){Description = "Peppers";}public override string GetDescription(){return String.Format("{0}, {1}", _pizza.GetDescription(), Description);}public override double CalculateCost(){return _pizza.CalculateCost() + 2.00;}} }

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Program：

using System;namespace DecoratorDemo {class Program{static void Main(string[] args){Pizza largePizza = new LargePizza();largePizza = new Cheese(largePizza);largePizza = new Ham(largePizza);largePizza = new Peppers(largePizza);Console.WriteLine(largePizza.GetDescription());Console.WriteLine("{0:C2}", largePizza.CalculateCost());Console.ReadKey();}} }

DebugLZQ： ?手抓餅+奶酪+火腿+辣醬

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通過使用裝飾模式，可以在運行時動態地擴充一個類的功能。

原理是：增加一個修飾類包裹原來的類。裝飾類實現新的功能，但是，在不需要用到新功能的地方，它可以直接調用原來的類中的方法。修飾類必須和原來的類有相同的接口。

修飾模式是類繼承的另外一種選擇。類繼承在編譯時候增加行為，而裝飾模式是在運行時增加行為。

當有幾個相互獨立的功能需要擴充時，這個區別就變得很重要。在有些面向對象的編程語言中，類不能在運行時被創建，通常在設計的時候也不能預測到有哪幾種功能組合。這就意味著要為每一種組合創建一個新類。相反，修飾模式是面向運行時候的對象實例的,這樣就可以在運行時根據需要進行組合。

其示例性代碼：

using System;namespace DoFactory.GangOfFour.Decorator.Structural {/// <summary>/// MainApp startup class for Structural /// Decorator Design Pattern./// </summary>class MainApp{/// <summary>/// Entry point into console application./// </summary>static void Main(){// Create ConcreteComponent and two DecoratorsConcreteComponent c = new ConcreteComponent();ConcreteDecoratorA d1 = new ConcreteDecoratorA();ConcreteDecoratorB d2 = new ConcreteDecoratorB();// Link decorators d1.SetComponent(c);d2.SetComponent(d1);d2.Operation();// Wait for user Console.ReadKey();}}/// <summary>/// The 'Component' abstract class/// </summary>abstract class Component{public abstract void Operation();}/// <summary>/// The 'ConcreteComponent' class/// </summary>class ConcreteComponent : Component{public override void Operation(){Console.WriteLine("ConcreteComponent.Operation()");}}/// <summary>/// The 'Decorator' abstract class/// </summary>abstract class Decorator : Component{protected Component component;public void SetComponent(Component component){this.component = component;}public override void Operation(){if (component != null){component.Operation();}}}/// <summary>/// The 'ConcreteDecoratorA' class/// </summary>class ConcreteDecoratorA : Decorator{public override void Operation(){base.Operation();Console.WriteLine("ConcreteDecoratorA.Operation()");}}/// <summary>/// The 'ConcreteDecoratorB' class/// </summary>class ConcreteDecoratorB : Decorator{public override void Operation(){base.Operation();AddedBehavior();Console.WriteLine("ConcreteDecoratorB.Operation()");}void AddedBehavior(){}} } View Code

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下面再給出一個示例程序，來自程杰的大話設計模式：

View Code using System;
using System.Collections.Generic;
using System.Text;

namespace 裝飾模式
{
class Program
{
static void Main(string[] args)
{
Person xc = new Person("小菜");

Console.WriteLine("\n第一種裝扮：");

Sneakers pqx = new Sneakers();
BigTrouser kk = new BigTrouser();
TShirts dtx = new TShirts();

pqx.Decorate(xc);
kk.Decorate(pqx);
dtx.Decorate(kk);
dtx.Show();

Console.WriteLine("\n第二種裝扮：");

LeatherShoes px = new LeatherShoes();
Tie ld = new Tie();
Suit xz = new Suit();

px.Decorate(xc);
ld.Decorate(px);
xz.Decorate(ld);
xz.Show();

Console.WriteLine("\n第三種裝扮：");
Sneakers pqx2 = new Sneakers();
LeatherShoes px2 = new LeatherShoes();
BigTrouser kk2 = new BigTrouser();
Tie ld2 = new Tie();

pqx2.Decorate(xc);
px2.Decorate(pqx);
kk2.Decorate(px2);
ld2.Decorate(kk2);

ld2.Show();

Console.Read();
}
}

//Person類
class Person
{
public Person()
{ }

private string name;
public Person(string name)
{
this.name = name;
}

public virtual void Show()
{
Console.WriteLine("裝扮的{0}", name);
}
}

//裝飾類
class Finery : Person
{
protected Person component;

//打扮
public void Decorate(Person component)
{
this.component = component;
}

public override void Show()
{
if (component != null)
{
component.Show();
}
}
}

//具體裝飾類
class TShirts : Finery
{
public override void Show()
{
Console.Write("大T恤 ");
base.Show();
}
}

class BigTrouser : Finery
{
public override void Show()
{
Console.Write("垮褲 ");
base.Show();
}
}

class Sneakers : Finery
{
public override void Show()
{
Console.Write("破球鞋 ");
base.Show();
}
}

class Suit : Finery
{
public override void Show()
{
Console.Write("西裝 ");
base.Show();
}
}

class Tie : Finery
{
public override void Show()
{
Console.Write("領帶 ");
base.Show();
}
}

class LeatherShoes : Finery
{
public override void Show()
{
Console.Write("皮鞋 ");
base.Show();
}
}
}

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使用裝飾模式需要注意的地方：

◇在發生“類爆炸”的情況下，應及時反思工程的設計；

◇在類中，不要過多的將“是否具有某種裝飾”用boolean來表示；

◇Decorator（裝飾）模式的關鍵在于“動態地實現功能擴展”；

◇裝飾器的安裝順序很重要，應努力做到裝飾器的安裝順序不影響最終的裝飾效果。

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應用實例：

裝備大兵！無任何裝備時（核心功能）可以用拳腳搏擊；裝備了步槍，可以正常射擊；裝備了重機槍，可以掃射；裝備了火箭筒，可以防空。

類圖：

代碼實現：

using System;namespace DecoratorPattern {/// <summary>/// MainApp startup class for Structural /// Observer Design Pattern./// </summary>class MainApp{static void Main(string[] args){// 定義新兵Soldier soldier = new Soldier();// 三種裝備RifleEquipment rifle = new RifleEquipment();MachineGunEquipment machineGun = new MachineGunEquipment();RocketGunEquipment rocketGun = new RocketGunEquipment();// 將三種裝備全部交給新兵 rifle.SetComponent(soldier);machineGun.SetComponent(rifle);rocketGun.SetComponent(machineGun);// 攻擊，除了拳腳功夫外，新兵還可以使用步槍，機槍，火箭炮.最終執行的是rocketGun.Attack(). rocketGun.Attack();Console.Read();}}/// <summary>/// 裝備類，相當于Component/// </summary>public abstract class Equipment{public abstract void Attack();}/// <summary>/// 士兵類，繼承自Equipment/// </summary>public class Soldier : Equipment{public Soldier(){// 構造函數 }/// <summary>/// 沒有任何武器裝備下的核心功能/// </summary>public override void Attack(){Console.WriteLine("用拳腳攻擊！");}}public abstract class EquipDecorator : Equipment{protected Equipment equipment;/// <summary>/// 增加裝備，使用該方法來動態地給士兵增加裝備/// </summary>/// <param name="equipment"></param>public void SetComponent(Equipment equipment){this.equipment = equipment;}/// <summary>/// 攻擊/// </summary>public override void Attack(){//如果有裝備，就用裝備進行攻擊if (equipment != null){equipment.Attack();}}}/// <summary>/// 步槍/// </summary>public class RifleEquipment : EquipDecorator{public override void Attack(){base.Attack();Console.WriteLine("步槍射擊，啪！");}}/// <summary>/// 機槍/// </summary>public class MachineGunEquipment : EquipDecorator{public override void Attack(){base.Attack();Console.WriteLine("機槍掃射，突突突！");}}/// <summary>/// 火箭筒/// </summary>public class RocketGunEquipment : EquipDecorator{public override void Attack(){base.Attack();Console.WriteLine("火箭炮射擊，唰......！");}} }

輸出結果：

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優點

　1 每個裝飾對象只關心自己的功能，不需要關心如何被添加到對象鏈當中。它是由Decorator的SetComponent方法來實現的，因而它們的職責是單一的。

　2 類的核心職責與動態添加的職責是分離的。如果再向主類中添加新的功能，一是違反了開放封閉原則，二是增加了主類的復雜度。

　3?比靜態繼承更靈活?與對象的靜態繼承相比，Decorator模式提供了更加靈活的向對象添加職責的方式，可以使用添加和分離的方法，用裝飾在運行時刻增加和刪除職責.

缺點

　1?產生許多小對象，采用Decorator模式進行系統設計往往會產生許多看上去類似的小對象，這些對象僅僅在他們相互連接的方式上有所不同。

適用場景

　1 當需要為已有功能動態地添加更多功能時。

　2 類的核心功能無需改變，只是需要添加新的功能時。

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總結

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