设计模式读书笔记：装饰器模式

“装饰器”，顾名思义 就是在一个现有的对象上附加新的东西。作为例子，可以考虑在屏幕上显示一张图片。在图片上附加新东西有多种方式，比如加上一个边框，或者打上

一个跟图片内容有关的标签。

装饰器模式的美妙之处在于：

1 原始对象对装饰一无所知；

2 不需要引入一个包含各种装饰选项的特性类；

3 多个装饰类之间彼此独立；

4 多个装饰类可以以一种“混搭”的方式组合到一起。

示例代码：

using System;
using System.Drawing;
using System.Drawing.Drawing2D;
using System.Windows.Forms;
using System.Collections.Generic;
using Given;

namespace Given {

public class Photo : Form {
Image image;
public Photo () {
image = new Bitmap("jug.jpg");
this.Text = "Lemonade";
this.Paint += new PaintEventHandler(Drawer);
}

public virtual void Drawer(Object source, PaintEventArgs e) {
e.Graphics.DrawImage(image,30,20);
}
}
}

class DecoratorPatternExample {

class BorderedPhoto : Photo {
Photo photo;
Color color;

public BorderedPhoto (Photo p, Color c) {
photo = p;
color=c;
}

public override void Drawer(Object source, PaintEventArgs e) {
photo.Drawer(source, e);
e.Graphics.DrawRectangle(new Pen(color, 10),25,15,215,225);
}
}

class TaggedPhoto : Photo {
Photo photo;
string tag;
int number;
static int count;
List <string> tags = new List <string> ();

public TaggedPhoto(Photo p, string t) {
photo = p;
tag = t;
tags.Add(t);
number = ++count;
}

public override void Drawer(Object source, PaintEventArgs e) {
photo.Drawer(source,e);
e.Graphics.DrawString(tag,
new Font("Arial", 16),
new SolidBrush(Color.Black),
new PointF(80,100+number*20));
}

public string ListTaggedPhotos() {
string s = "Tags are: ";
foreach (string t in tags) s +=t+" ";
return s;
}
}

static void Main () {
Photo photo;
TaggedPhoto foodTaggedPhoto, colorTaggedPhoto, tag;
BorderedPhoto composition;

// Compose a photo with two TaggedPhotos and a blue BorderedPhoto
photo = new Photo();
Application.Run(photo);
foodTaggedPhoto = new TaggedPhoto (photo,"Food");
colorTaggedPhoto = new TaggedPhoto (foodTaggedPhoto,"Yellow");
composition = new BorderedPhoto(colorTaggedPhoto, Color.Blue);
Application.Run(composition);
Console.WriteLine(colorTaggedPhoto.ListTaggedPhotos());

// Compose a photo with one TaggedPhoto and a yellow BorderedPhoto
photo = new Photo();
tag = new TaggedPhoto (photo,"Jug");
composition = new BorderedPhoto(tag, Color.Yellow);
Application.Run(composition);
Console.WriteLine(tag.ListTaggedPhotos());
}
}