深入浅出设计模式-010：迭代器模式（Iterator Pattern）

深入浅出设计模式-010：迭代器模式（Iterator Pattern）

一：餐厅举例    //菜单项    class MenuItem{        private String name;        private String description;        private Boolean vegetarian;        private Double price;        public MenuItem(String name, String description, Boolean vegetarian, Double price){            this.name = name;            this.description = description;            this.vegetarian = vegetarian;            this.price = price;        }        public String getName{            get { return name; }        }        public String getDescription{            get { return description; }        }        public Boolean getVegetatian{            get { return vegetarian; }        }        public Double getPrice{            get { return price; }        }    }    //用ArrayList    class PancakeHouseMenu{        private ArrayList menuItems;        public PancakeHouseMenu(){            menuItems = new ArrayList();            addItem("11", "11", true, 2);        }        public void addItem(String name, String description, Boolean vegetarian, Double price){            menuItems.Add(new MenuItem(name, description, vegetarian, price));        }        public ArrayList getMenuItems{            get { return menuItems; }        }    }    //用数组    class DinerMenu{        public const int MAX_ITEMS = 6;        int numberOfItems = 0;        MenuItem[] menuItems;        public DinerMenu(){            menuItems = new MenuItem[MAX_ITEMS];            addItem("21", "21", false, 2);        }        public void addItem(String name, String description, Boolean vegetarian, Double price){            if (numberOfItems < MAX_ITEMS - 1){                menuItems[numberOfItems++] = new MenuItem(name, description, vegetarian, price);            }        }        public MenuItem[] getMenuItems{            get { return menuItems; }        }    }    //遍历时    static void Main(string[] args)    {        PancakeHouseMenu pancakeHouseMenu = new PancakeHouseMenu();        ArrayList breakfastItems = pancakeHouseMenu.getMenuItems;

        DinerMenu dinerMenu = new DinerMenu();        MenuItem[] lunchItems = dinerMenu.getMenuItems;

        for (int index = 0; index < breakfastItems.Count; index++)        {            MenuItem menuItem = (MenuItem)breakfastItems[index];        }        for (int index = 0; index < lunchItems.Length; index++)        {            MenuItem menuItem = lunchItems[index];        }    }

二：我们希望的结果是：利用迭代器操作，屏蔽内部细节    static void Main(string[] args)    {        PancakeHouseMenu pancakeHouseMenu = new PancakeHouseMenu();               DinerMenu dinerMenu = new DinerMenu();

        Iterator iterator1 = pancakeHouseMenu.createIterator();        while (iterator1.hasNext())        {            MenuItem menuItem = (MenuItem)iterator1.next();        }        Iterator iterator2 = dinerMenu.createIterator();        while (iterator2.hasNext())        {            MenuItem menuItem = (MenuItem)iterator2.next();        }    }

    public interface Iterator{        Boolean hasNext();        Object next();    }    class DinerMenuIterator : Iterator{        MenuItem[] items;        int position = 0;        public DinerMenuIterator(MenuItem[] items){            this.items = items;        }        public Boolean hasNext(){            if (position >= items.Length || items[position] == null){                return false;            }            return true;        }        public Object next(){            return items[position++];        }    }    class PancakeHouseMenuIterator : Iterator{        private ArrayList items;        int position = 0;        public PancakeHouseMenuIterator(ArrayList items){            this.items = items;        }        public Boolean hasNext(){            if (position >= items.Count || items[position] == null){                return false;            }            return true;        }        public Object next(){            return items[position++];        }    }    class MenuItem{        private String name;        private String description;        private Boolean vegetarian;        private Double price;        public MenuItem(String name, String description, Boolean vegetarian, Double price){            this.name = name;            this.description = description;            this.vegetarian = vegetarian;            this.price = price;        }        public String getName{            get { return name; }        }        public String getDescription{            get { return description; }        }        public Boolean getVegetatian{            get { return vegetarian; }        }        public Double getPrice{            get { return price; }        }    }

    class PancakeHouseMenu{        private ArrayList menuItems;        public PancakeHouseMenu(){            menuItems = new ArrayList();            addItem("11", "11", true, 2);        }        public void addItem(String name, String description, Boolean vegetarian, Double price){            menuItems.Add(new MenuItem(name, description, vegetarian, price));        }        //关键部分        public Iterator createIterator(){            return new PancakeHouseMenuIterator(menuItems);        }    }    class DinerMenu{        public const int MAX_ITEMS = 6;        int numberOfItems = 0;        MenuItem[] menuItems;        public DinerMenu(){            menuItems = new MenuItem[MAX_ITEMS];            addItem("21", "21", false, 2);        }        public void addItem(String name, String description, Boolean vegetarian, Double price){            if (numberOfItems < MAX_ITEMS - 1){                menuItems[numberOfItems++] = new MenuItem(name, description, vegetarian, price);            }        }        //关键部分        public Iterator createIterator(){            return new DinerMenuIterator(menuItems);        }    }

三：迭代器模式：提供一种方法顺序访问一个聚合对象中的各个元素，而又不暴露其内部的表示。    迭代器模式把元素之间游走的责任交给迭代器，而不是聚合对象。这样不仅让聚合的接口和实现变得更简洁，也可以让聚合更专注在它所应该专注的事情上面，而不必去理会遍历的事情。

四：设计原则：单一责任：一个类应该只有一个引起变化的原因。    类的每个责任都由改变的潜在区域。超过一个责任，就意味着超过一个改变的区域。    当一个模块或一个类被设计成只支持一组相关的功能时，我们说它具有高内聚。