史帝芬心得筆記 -- Decorator Pattern (裝飾模式)

試著想一下，如果我們要寫一個牛排館的計費程式，這家牛排館有兩樣主菜及一些附餐，主菜為牛排、豬排，附餐有麵、生菜沙拉、飲料、甜點，客人點菜時，需點一樣主菜，附餐可多選也可不點，計費的方式就像自助餐一樣，每樣菜有它的價格，相加後即為客人要付的費用。我們希望未來這家牛排館增加主菜或附餐時，都不需要修改既有程式! 可能嗎? 下面我們來看看Decorator Pattern如何滿足這樣的需求!

程式 (C#)

using System;

namespace Decorator
{
    abstract class Meal
    {
        protected string description = "排餐";

        public virtual string Description
        {
            get { return description; }
            set { description = value; }
        }

        abstract public int cost();
    }

    class PorkChop : Meal
    {
        public PorkChop()
        {
            Description = "豬排";
        }

        public override int cost()
        {
            return 130;
        }
    }

    class Steak : Meal
    {
        public Steak()
        {
            Description = "牛排";
        }

        public override int cost()
        {
            return 150;
        }
    }

    abstract class Condiment : Meal
    {
        protected Meal meal;

        public override string Description
        {
            get { return meal.Description + " + " + description; }
            set { description = value; }
        }
    }

    class Noodle : Condiment
    {
        public Noodle(Meal meal)
        {
            Description = "麵";
            this.meal = meal;
        }

        public override int cost()
        {
            return 50 + meal.cost();
        }
    }

    class Salad : Condiment
    {
        public Salad(Meal meal)
        {
            Description = "生菜沙拉";
            this.meal = meal;
        }

        public override int cost()
        {
            return 60 + meal.cost();
        }
    }

    class Drink : Condiment
    {
        private bool isHot;

        public bool IsHot
        {
            get { return isHot; }
        }

        public Drink(Meal meal, bool isHot)
        {
            Description = (isHot) ? "熱飲" : "冷飲";
            this.meal = meal;
            this.isHot = isHot;
        }

        public override int cost()
        {
            return (isHot) ? (30 + meal.cost()) : (50 + meal.cost());
        }
    }

    class Dessert : Condiment
    {
        public Dessert(Meal meal)
        {
            Description = "甜點";
            this.meal = meal;
        }

        public override int cost()
        {
            return 40 + meal.cost();
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            Meal meal = new PorkChop();
            Console.WriteLine(meal.Description + " = NT$ " + meal.cost());

            Meal meal2 = new Noodle(meal);
            Console.WriteLine(meal2.Description + " = NT$ " + meal2.cost());

            Meal meal3 = new Drink(meal2, true);
            Console.WriteLine(meal3.Description + " = NT$ " + meal3.cost());

            Meal meal4 = new Dessert(meal3);
            Console.WriteLine(meal4.Description + " = NT$ " + meal4.cost());

            Console.ReadLine();
        }
    }
}

執行結果

豬排 = NT$ 130
豬排 + 麵 = NT$ 180
豬排 + 麵 + 熱飲 = NT$ 210
豬排 + 麵 + 熱飲 + 甜點 = NT$ 250

說明

Java I/O

Microsoft Forum