【图解设计模式】Command模式

用对象表示“命令”来保存命令历史记录和重复执行命令。

示例

一个画图软件，用户拖动鼠标时程序会绘制出红色圆点，点击clear按钮后会清除所有的圆点。

类图

时序图

Command接口

package command;

public interface Command {
    public abstract void execute();
}

MacroCommand类

package command;

import java.util.Iterator;
import java.util.Stack;

public class MacroCommand implements Command {
    private Stack commands = new Stack();

    @Override
    public void execute() {
        Iterator it = commands.iterator();

        while (it.hasNext())
            ((Command) it.next()).execute();
    }

    public void append(Command cmd) {
        if (cmd != this)
            commands.push(cmd);
    }

    public void undo() {
        if (!commands.isEmpty())
            commands.pop();
    }

    public void clear() {
        commands.clear();
    }
}

DrawCommand类

package drawer;

import command.Command;

import java.awt.*;

public class DrawCommand implements Command {
    private Drawable drawable;
    private Point position;

    public DrawCommand(Drawable drawable, Point position) {
        this.drawable = drawable;
        this.position = position;
    }

    @Override
    public void execute() {
        drawable.draw(position.x, position.y);
    }
}

Drawable接口

package drawer;

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

DrawCanvas类

package drawer;

import command.MacroCommand;

import java.awt.*;

public class DrawCanvas extends Canvas implements Drawable {
    private Color color = Color.red;
    private int radius = 6;
    private MacroCommand history;

    public DrawCanvas(int width, int height, MacroCommand history) {
        setSize(width, height);
        setBackground(Color.white);
        this.history = history;
    }

    public void paint(Graphics g) {
        history.execute();
    }

    @Override
    public void draw(int x, int y) {
        Graphics g = getGraphics();
        g.setColor(color);
        g.fillOval(x - radius, y - radius, radius * 2, radius * 2);
    }
}

Main类

import command.Command;
import command.MacroCommand;
import drawer.DrawCanvas;
import drawer.DrawCommand;

import javax.swing.*;
import java.awt.event.*;

public class Main extends JFrame implements ActionListener, MouseMotionListener, WindowListener {
    private MacroCommand history = new MacroCommand();
    private DrawCanvas canvas = new DrawCanvas(400, 400, history);
    private JButton clearButton = new JButton("clear");

    public Main(String title) {
        super(title);

        this.addWindowListener(this);
        canvas.addMouseMotionListener(this);
        clearButton.addActionListener(this);

        Box buttonBox = new Box(BoxLayout.X_AXIS);
        buttonBox.add(clearButton);
        Box mainBox = new Box(BoxLayout.Y_AXIS);
        mainBox.add(buttonBox);
        mainBox.add(canvas);
        getContentPane().add(mainBox);

        pack();
        show();
    }

    @Override
    public void actionPerformed(ActionEvent e) {
        if (e.getSource() == clearButton) {
            history.clear();
            canvas.repaint();
        }
    }

    @Override
    public void mouseDragged(MouseEvent e) {
        Command cmd = new DrawCommand(canvas, e.getPoint());
        history.append(cmd);
        cmd.execute();
    }

    @Override
    public void mouseMoved(MouseEvent e) {}

    @Override
    public void windowOpened(WindowEvent e) {}

    @Override
    public void windowClosing(WindowEvent e) {
        System.exit(0);
    }

    @Override
    public void windowClosed(WindowEvent e) {}

    @Override
    public void windowIconified(WindowEvent e) {}

    @Override
    public void windowDeiconified(WindowEvent e) {}

    @Override
    public void windowActivated(WindowEvent e) {}

    @Override
    public void windowDeactivated(WindowEvent e) {}

    public static void main(String[] args) {
        new Main("Command Pattern Sample");
    }
}

运行结果

登场角色

Command（命令）

Command角色负责定义命令的接口（API）。在示例程序中，由Command接口扮演此角色。

ConcreteCommand（具体的命令）

ConcreteCommand角色负责实现在Command角色中定义的接口（API）。在示例程序中，由MacroCommand类和DrawCommand类扮演此角色。

Receiver（接受者）

Receiver角色是Command角色执行命令时的对象，也可以称其为命令接收者。在示例程序中，由DrawCanvas类接收DrawCommand的命令。

Client（请求者）

Client角色负责生成ConcreteCommand角色并分配Receiver角色。在示例程序中，由Main类扮演此角色。在相应鼠标拖拽事件时，它生成了DrawCommand类的实例，并将扮演Receiver角色的DrawCanvas类的实例传递给了DrawCommand类的构造函数。

Invoker（发动者）

Invoker角色是开始执行命令的角色，它会调用在Command角色中定义的接口（API）。在示例程序中，由Main类和DrawCanvas类扮演此角色。这两个类都调用了Command接口中的execute方法。Main类同时扮演了Client角色和Invoker角色。

类图

时序图