享元模式

Flyweight Pattern 对象结构型模式

意图

运用共享技术有效地支持大量细粒度的对象

结构

其中：

• Flyweight 描述一个接口，通过这个接口 Flyweight 可以接受并作用于外部状态

• ConcreteFlyweight 实现 Flyweight 接口，并为内部状态 (如果有) 增加存储空间。ConcreteFlyweight 对象必项是可共享的。它所存储的状态必须是内部的，即它必须独立于 ConcreteFlyweight 对象的场景

• 并非所有的 Flyweight 子类都需要被共享。Flyweight 接口使共享成为可能，但它并不强制共亨。在 Flyweight 对象结构的某些层次，UnsharedConcreteFlyweight 对象通常将 ConcreteFlyweight 对象作为子结点

• FlyweightFactory 创建并管理 Flyweight 对象；确保合理地共享 Flyweight，当用户请求一个 Flyweight 时，FlyweightFactory 对象提供一个已创建的实例或者不存在时创建一个实例

• Client 维持一个对 Flyweight 的引用；计算或存储一个或多个 Flyweight 的外部状态

适用性

Flyweight 模式适用于：

• 一个应用程序使用了大量的对象
• 完全由于使用大量的对象，造成很大的存储开销
• 对象的大多数状态都可变为外部状态
• 如果删除对象的外部状态，那么可以用相对较少的共享对象取代很多组对象
• 应用程序不依赖于对象标识。由于 Flyweight 对象可以被共享，所以对于概念上明显有别的对象，标识测试将返回真值

例子 1

现要开发一个网络围棋程序，允许多个玩家联机下棋。由于只有一台服务器，为节省内存空间，采用享元模式实现该程序，类图如下

import java.util.ArrayList;

enum PieceColor {BLACK, WHITE} // 棋子颜色

class PiecePods{ // 棋子位置
    private int x;
    private int y;

    public PiecePods(int a, int b){
        x = a;
        y = b;
    }

    public int getX(){
        return x;
    }

    public int getY() {
        return y;
    }
}

abstract class Piece{ // 棋子定义
    protected PieceColor m_color; // 颜色
    protected PiecePods m_pos; // 位置

    public Piece(PieceColor color, PiecePods pos){
        this.m_color = color;
        this.m_pos = pos;
    }

    public abstract void draw();
}

class BlackPiece extends Piece{
    public BlackPiece(PieceColor color, PiecePods pos){
        super(color, pos);
    }

    @Override
    public void draw(){
        System.out.println("Draw a black piece");
    }
}

class WhitePiece extends Piece{
    public WhitePiece(PieceColor color, PiecePods pos){
        super(color, pos);
    }

    @Override
    public void draw(){
        System.out.println("Draw a white piece");
    }
}

class PieceBoard{ // 棋盘上已有的棋子
    private static final ArrayList<Piece> m_arrayPiece = new ArrayList<>();
    private String m_blackName; // 黑方名称
    private String m_whiteName; // 白方名称

    public PieceBoard(String black, String white){
        m_blackName = black;
        m_whiteName = white;
    }

    // 一步棋，在棋盘上放一颗棋子
    public void setPiece(PieceColor color, PiecePods pos){
        Piece piece = null;

        if(color == PieceColor.BLACK){ // 放黑子
            piece = new BlackPiece(color, pos);
            System.out.println(m_blackName + pos.getX() + pos.getY());
            piece.draw();
        }else{ // 放白子
            piece = new WhitePiece(color, pos);
            System.out.println(m_whiteName + pos.getX() + pos.getY());
            piece.draw();
        }
        m_arrayPiece.add(piece);
    }
}

例子 2

五子棋

public class FlyWeightPattern {
    public static void main(String[] args) {
        PieceFactory factory = new PieceFactory();

        Piece wp1 = factory.getPiece(0);
        wp1.draw(2023, 0527);
    }
}

class PieceFactory{
    private Piece[] pieces = {new WhitePiece(), new BlackPiece()};

    public Piece getPiece(int key){
        if(key == 0) return pieces[0];
        else return pieces[1];
    }
}

abstract class Piece{
    protected String color;

    public abstract void draw(int x, int y);
}

class WhitePiece extends Piece{
    public WhitePiece(){
        this.color = "white";
    }

    @Override
    public void draw(int x, int y){
        System.out.println("draw a " + this.color + " piece x: " + x + " y: " + y);
    }
}

class BlackPiece extends Piece{
    public BlackPiece(){
        this.color = "Black";
    }

    @Override
    public void draw(int x, int y){
        System.out.println("draw a " + this.color + " piece x: " + x + " y: " + y);
    }
}

例子 3

import java.util.HashMap;
import java.util.Map;
import java.util.Random;

public class FlyWeightPattern {
    public static void main(String[] args) {
        ShapeFactory sf = new ShapeFactory();

        Random r = new Random();
        String[] colors = {"red", "blue", "green", "white", "black"};

        for (int i = 0; i < 10; i++) {
            int x = r.nextInt(colors.length);
            Shape s = sf.getShape(colors[x]);
            s.draw(r.nextInt(2023), r.nextInt(527));
        }
    }
}

class ShapeFactory{
    private Map<String, Shape> map = new HashMap<>();

    public Shape getShape(String key){
        if (!map.containsKey(key)) {
            map.put(key, new Circle(key));
            System.out.println("create new circle, color: " + key);
        }
        return map.get(key);
    }
}

abstract class Shape{
    protected String color;

    public abstract void draw(int x, int y);
}

class Circle extends Shape{
    public Circle(String color){
        this.color = color;
    }

    @Override
    public void draw(int x, int y) {
        System.out.println("draw a " + this.color + " circle x: " + x + " y: " + y);
    }
}