设计模式-命令模式

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命令模式

定义

命令模式（Command Pattern）是一种数据驱动的设计模式。请求以命令的形式包裹在对象中，并传给调用对象。调用对象寻找可以处理该命令的合适的对象，并把该命令传给相应的对象，该对象执行命令。

类图

命令模式类图

Command（命令）

负责定义命令的接口

ConcreteCommand（具体的命令）

负责实现在Command中定义的接口。

Receiver（接收者）

是Command执行命令时的对象，也可以称之为命令接收者。

Client（请求者）

负责生成ConcreteCommand并分配Receiver。

Invoke（发动者）

是开始执行命令的角色，它会调用在Command中定义的接口。

示例

类图

命令模式示例类图

Command

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

MacroCommand

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public class MacroCommand implements Command {
    private Stack commands = new Stack();
    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.empty()) {
            commands.pop();
        }
    }
    public void clear() {
        commands.clear();
    }
}

Drawable

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

DrawCanvas

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

DrawCommand

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public class DrawCommand implements Command {
    protected Drawable drawable;
    private Point position;
    public DrawCommand(Drawable drawable, Point position) {
        this.drawable = drawable;
        this.position = position;
    }
    public void execute() {
        drawable.draw(position.x, position.y);
    }
}

Main

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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();
    }

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

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

    public void windowClosing(WindowEvent e) {
        System.exit(0);
    }
    public void windowActivated(WindowEvent e) {}
    public void windowClosed(WindowEvent e) {}
    public void windowDeactivated(WindowEvent e) {}
    public void windowDeiconified(WindowEvent e) {}
    public void windowIconified(WindowEvent e) {}
    public void windowOpened(WindowEvent e) {}

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

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