一个更易理解的实例：

装饰模式为已有类动态附加额外的功能就像LOL、王者荣耀等类Dota游戏中，英雄升级一样。每次英雄升级都会附加一个额外技能点学习技能。具体的英雄就是ConcreteComponent，技能栏就是装饰器Decorator，每个技能就是ConcreteDecorator；

//Component 英雄接口 
public interface Hero {
    //学习技能
    void learnSkills();
}
//ConcreteComponent 具体英雄盲僧
public class BlindMonk implements Hero {
    
    private String name;
    
    public BlindMonk(String name) {
        this.name = name;
    }

    @Override
    public void learnSkills() {
        System.out.println(name + "学习了以上技能！");
    }
}
//Decorator 技能栏
public class Skills implements Hero{
    
    //持有一个英雄对象接口
    private Hero hero;
    
    public Skills(Hero hero) {
        this.hero = hero;
    }

    @Override
    public void learnSkills() {
        if(hero != null)
            hero.learnSkills();
    }    
}
//ConreteDecorator 技能：Q
public class Skill_Q extends Skills{
    
    private String skillName;

    public Skill_Q(Hero hero,String skillName) {
        super(hero);
        this.skillName = skillName;
    }

    @Override
    public void learnSkills() {
        System.out.println("学习了技能Q:" +skillName);
        super.learnSkills();
    }
}
//ConreteDecorator 技能：W
public class Skill_W extends Skills{

    private String skillName;

    public Skill_W(Hero hero,String skillName) {
        super(hero);
        this.skillName = skillName;
    }

    @Override
    public void learnSkills() {
        System.out.println("学习了技能W:" + skillName);
        super.learnSkills();
    }
}
//ConreteDecorator 技能：E
public class Skill_E extends Skills{
    
    private String skillName;
    
    public Skill_E(Hero hero,String skillName) {
        super(hero);
        this.skillName = skillName;
    }

    @Override
    public void learnSkills() {
        System.out.println("学习了技能E:"+skillName);
        super.learnSkills();
    }
}
//ConreteDecorator 技能：R
public class Skill_R extends Skills{    
    
    private String skillName;
    
    public Skill_R(Hero hero,String skillName) {
        super(hero);
        this.skillName = skillName;
    }
    
    @Override
    public void learnSkills() {
        System.out.println("学习了技能R:" +skillName );
        super.learnSkills();
    }
}
//客户端：召唤师
public class Player {
    public static void main(String[] args) {
        //选择英雄
        Hero hero = new BlindMonk("李青");
        
        Skills skills = new Skills(hero);
        Skills r = new Skill_R(skills,"猛龙摆尾");
        Skills e = new Skill_E(r,"天雷破/摧筋断骨");
        Skills w = new Skill_W(e,"金钟罩/铁布衫");
        Skills q = new Skill_Q(w,"天音波/回音击");
        //学习技能
        q.learnSkills();
    }
}

输出：

学习了技能Q:天音波/回音击
学习了技能W:金钟罩/铁布衫
学习了技能E:天雷破/摧筋断骨
学习了技能R:猛龙摆尾
李青学习了以上技能！

《HeadFirst 设计模式》里的装饰器模式的代码。

package HeadFirst设计模式.装饰者模式_星巴兹订单系统;
 
/**
 *区别于Shape，Beverage采用抽象类
 *通常装饰者模式是采用抽象类，但是在Java中可以使用接口
 */
public abstract class Beverage {
    public String description = "Unknown Beverage";
 
    public String getDescription() {
        return description;
    }
 
    public abstract double cost();
}

package HeadFirst设计模式.装饰者模式_星巴兹订单系统.被装饰者;

import HeadFirst设计模式.装饰者模式_星巴兹订单系统.Beverage;
/**
 *被装饰类继承Beverage抽象类，最终会通过装饰者动态添加上新的行为
 */
public class DarkRoast extends Beverage {

    public DarkRoast() {
        description = "DarkRoast";
    }

    @Override    
    public double cost() {
        return 0.99;
    }
}

package HeadFirst设计模式.装饰者模式_星巴兹订单系统.装饰者;
import HeadFirst设计模式.装饰者模式_星巴兹订单系统.Beverage;

/**
 *这是继承Beverage的抽象装饰者，接下来所有具体的装饰者都要继承CondimentDecorator
 */
public abstract class CondimentDecorator extends Beverage {

    /**     
     *所有的调料装饰者都必须重新实现该方法，因为调料的该方法应该得到扩充，方法实现不同于原来方法     
     */    
    public abstract String getDescription();

}

package HeadFirst设计模式.装饰者模式_星巴兹订单系统.装饰者;

import HeadFirst设计模式.装饰者模式_星巴兹订单系统.Beverage;

/**
 *摩卡，是具体的装饰者
 *用一个实例变量记录饮料（被装饰者）
 *装饰者和被装饰者通过组合来增强功能，实现功能的扩展，用组合来替代继承
 */
public class Mocha extends CondimentDecorator {
    Beverage beverage;

    public Mocha(Beverage beverage) {
        this.beverage = beverage;
    }

    @Override    
    public String getDescription() {
        return beverage.getDescription() + ", Mocha";
    }

    @Override    
    public double cost() {
        return 0.20 + beverage.cost();
    }
}

测试：

package HeadFirst设计模式.装饰者模式_星巴兹订单系统;

import HeadFirst设计模式.装饰者模式_星巴兹订单系统.被装饰者.DarkRoast;
import HeadFirst设计模式.装饰者模式_星巴兹订单系统.装饰者.Mocha;


public class TestStarbuzzCoffee {
    public static void main(String[] args) {
        Beverage beverage1 = new DarkRoast();
        beverage1 = new Mocha(beverage1);
        beverage1 = new Mocha(beverage1);
        System.out.println(beverage1.getDescription()+ " $" + beverage1.cost());
    }
}

输出：

/Library/.../Java/设计模式/bin HeadFirst设计模式.装饰者模式_星巴兹订单系统.TestStarbuzzCoffee
DarkRoast, Mocha, Mocha $1.39

Process finished with exit code 0

Swift 实现:

protocol Shape {
    func draw()
}

protocol ShapeDecorator: Shape {
    var decoratedShape: Shape { get }
}

class Decorator {
    struct Rectangle: Shape {
        func draw() {
            print("Shape: Rectangle")
        }
    }

    struct Circle: Shape {
        func draw() {
            print("Shape: Circle")
        }
    }

    struct RedShapeDecorator: ShapeDecorator {
        var decoratedShape: Shape

        func draw() {
            decoratedShape.draw()
            setRedBorder(decoratedShape)
        }

        func setRedBorder(_ shape: Shape) {
            print("Border Color: Red")
        }

    }
    func execute() {
        let circle = Circle()
        let redCircle = RedShapeDecorator(decoratedShape: Circle())
        let redRectangle = RedShapeDecorator(decoratedShape: Rectangle())
        print("Circle with normal border:")
        circle.draw()
        print("Circle of red border")
        redCircle.draw()
        print("Circle of red rectangle")
        redRectangle.draw()
    }
}