命令模式--撤销恢复

古城微笑少年丶 2022-06-17 00:41 165阅读 0赞

原文：[http://abstractforever.iteye.com/blog/962219][http_abstractforever.iteye.com_blog_962219]

该例子来自阎宏提供的例子程序，以画线为例：  
![7b5b6558-28e0-3b5e-adb9-c1a9b6772760.png][]

命令接口Command：

package com.javapatterns.command.drawlines;
    
    //命令接口
    public interface Command {
    	abstract public void execute();// 执行操作
    
    	abstract public void unexecute();// 取消操作
    
    	abstract public boolean canExecute();//是否允许执行操作
    
    	abstract public boolean canUnexecute();//是否允许取消操作
    }

添加线命令类AddLineCommand：

/* Generated by Together */
    
    package com.javapatterns.command.drawlines;
    
    //添加线命令类
    public class AddLineCommand implements Command {
    	private Drawing drawing;// 画布
    	private Line line;//直线
    
    	public AddLineCommand(Drawing targetDrawing, Line newLine) {
    		drawing = targetDrawing;
    		line = newLine;
    	}
    
    	//执行将直线添加到画布的操作
    	public void execute() {
    		drawing.add(line);
    	}
    
    	//执行将直线从画布中移除的操作
    	public void unexecute() {
    		drawing.remove(line);
    	}
    
    	public boolean canExecute() {
    		return true;
    	}
    
    	public boolean canUnexecute() {
    		return true;
    	}
    }

命令集合类CommandList：

package com.javapatterns.command.drawlines;
    
    import java.util.Stack;
    
    //命令集合类，负责存储执行过的命令和撤销执行的命令
    public class CommandList {
    	private Stack<Command> executedCommands = new Stack<Command>();
    	private Stack<Command> unexecutedCommands = new Stack<Command>();
    
    	private void _execute(Command command) {
    		command.execute();//执行具体的命令
    		executedCommands.push(command);//记录执行的命令
    	}
    
    	//执行命令
    	public void execute(Command command) {
    		unexecutedCommands.removeAllElements();
    		_execute(command);
    	}
    
    	//撤销执行,每执行一次该方法，将从最近的一次操作开始撤销
    	public void unexecute() {
    		Command command = (Command) executedCommands.pop();//出栈最后一次执行命令
    		command.unexecute();//撤销该命令的执行
    		unexecutedCommands.push(command);//记录撤销的命令
    	}
    
    	//恢复执行,每执行一次该方法，将从最近的一次操作开始撤销
    	public void reexecute() {
    		Command command = (Command) unexecutedCommands.pop();//出栈最后一次撤销命令
    		_execute(command);//重新执行
    	}
    
    	//清空撤销和恢复的缓存
    	public void reset() {
    		executedCommands.removeAllElements();
    		unexecutedCommands.removeAllElements();
    	}
    
    	//是否允许撤销命令,没有执行过命令就不存在撤销
    	public boolean canUnexecuteCommand() {
    		return !executedCommands.empty();
    	}
    
    	//是否允许恢复命令,没有撤销过命令就不存在恢复
    	public boolean canReexecuteCommand() {
    		return !unexecutedCommands.empty();
    	}
    }

画布Drawing：

/* Generated by Together */
    
    package com.javapatterns.command.drawlines;
    
    import java.awt.Canvas;
    import java.awt.Color;
    import java.awt.Graphics;
    import java.awt.Point;
    import java.awt.event.MouseEvent;
    import java.awt.event.MouseListener;
    import java.awt.event.MouseMotionListener;
    import java.util.Enumeration;
    import java.util.Vector;
    
    //(接收者，负责具体实施和执行一个请求)画布，可以在该画布上画任意条直线
    public class Drawing extends Canvas implements MouseListener,
    		MouseMotionListener {
    
    	private static final long serialVersionUID = -4887753465176351050L;
    	private SimpleDraw applet;//应用程序
    	private Vector<Line> lines;//线集合
    	private Point startPosition;//画线开始位置
    	private Point mousePosition;//鼠标位置
    	private boolean mousePressed;//鼠标是否按下
    
    	//构造方法，初始化属性信息
    	public Drawing(SimpleDraw owner) {
    		applet = owner;
    		lines = new Vector<Line>();
    		mousePressed = false;//默认鼠标未按下
    		setBackground(Color.GREEN);//设置画布背景色
    		addMouseListener(this);//添加鼠标监听器,自己实现了该接口
    		addMouseMotionListener(this);//添加鼠标移动事件的侦听器，自己实现了该接口
    	}
    
    	//添加一条线（行动方法）
    	public void add(Line line) {
    		lines.addElement(line);
    		repaint();
    	}
    
    	//删除一条线（行动方法）
    	public void remove(Line line) {
    		lines.removeElement(line);
    		repaint();
    	}
    
    	public void paint(Graphics graphics) {
    		Enumeration<Line> enumeration = lines.elements();
    		Line currentLine;
    
    		while (enumeration.hasMoreElements()) {
    			currentLine = (Line) (enumeration.nextElement());
    			currentLine.paint(graphics);
    		}
    		if (mousePressed) {
    			graphics.drawLine(startPosition.x, startPosition.y,
    					mousePosition.x, mousePosition.y);
    		}
    	}
    
    	//鼠标按键在组件上单击（按下并释放）时调用
    	public void mouseClicked(MouseEvent event) {
    	}
    
    	//鼠标进入到组件上时调用
    	public void mouseEntered(MouseEvent event) {
    	}
    
    	//鼠标离开组件时调用
    	public void mouseExited(MouseEvent event) {
    	}
    
    	//鼠标按键在组件上按下时调用
    	public void mousePressed(MouseEvent event) {
    		mousePressed = true;//设置鼠标为按下状态
    		startPosition = event.getPoint();//记录直线的开始位置
    	}
    
    	//鼠标按钮在组件上释放时调用
    	public void mouseReleased(MouseEvent event) {
    		if (!event.getPoint().equals(startPosition)) {//两点不重合
    			Line line = new Line(startPosition, event.getPoint());//创建直线
    			AddLineCommand command = new AddLineCommand(this, line);//创建添加直线的一个命令
    			applet.execute(command);//请求者请求命令的执行
    		}
    		mousePressed = false;
    	}
    
    	public void mouseDragged(MouseEvent event) {
    		mousePosition = event.getPoint();
    		repaint();
    	}
    
    	public void mouseMoved(MouseEvent event) {
    	}
    }

直线Line：

package com.javapatterns.command.drawlines;
    
    import java.awt.Point;
    import java.awt.Graphics;
    
    public class Line {
       private Point  start;
       private Point  end;
    
       public Line( Point startPos, Point endPos ) {
          start = startPos;
          end = endPos;
       }
    
       public void paint( Graphics graphics ) {
          graphics.drawLine( start.x, start.y, end.x, end.y );
       }
    }

容器SimpleDraw：

/* Generated by Together */
    
    package com.javapatterns.command.drawlines;
    
    import java.applet.Applet;
    import java.awt.BorderLayout;
    import java.awt.Button;
    import java.awt.Panel;
    import java.awt.event.ActionEvent;
    import java.awt.event.ActionListener;
    
    // 请求者(负责调用命令对象执行请求)
    public class SimpleDraw extends Applet implements ActionListener {
    
    	private static final long serialVersionUID = 2317696438230972428L;
    
    	private Drawing drawing;// 画布
    	private CommandList commands;// 命令集合
    	private Button undoButton;// 撤销按钮
    	private Button redoButton;// 恢复按钮
    	private Button resetButton;// 清空撤销和恢复按钮
    
    	public void init() {
    		Panel panel = new Panel();// 面板
    		commands = new CommandList();// 初始化命令集合
    		setLayout(new BorderLayout());// 设置布局
    		drawing = new Drawing(this);// 初始化画布
    		add("Center", drawing);
    		undoButton = new Button("撤销");// 初始化撤销按钮
    		redoButton = new Button("恢复");// 初始化恢复按钮
    		resetButton = new Button("重置");
    		undoButton.addActionListener(this);// 添加按钮监听器
    		redoButton.addActionListener(this);// 添加按钮监听器
    		resetButton.addActionListener(this);// 添加按钮监听器
    		panel.add(undoButton);
    		panel.add(redoButton);
    		panel.add(resetButton);
    		add("South", panel);
    		updateButtons();// 初始化按钮状态
    	}
    
    	public void execute(Command command) {
    		commands.execute(command);// 执行命令
    		updateButtons();
    	}
    
    	// 更新按钮状态(根据是否能够undo撤销或者是否能够redo恢复)
    	private void updateButtons() {
    		undoButton.setEnabled(commands.canUnexecuteCommand());
    		redoButton.setEnabled(commands.canReexecuteCommand());
    	}
    
    	// 当点击撤销按钮或者恢复按钮时出发该方法执行
    	public void actionPerformed(ActionEvent event) {
    		if (event.getSource() == undoButton) {
    			commands.unexecute();
    			updateButtons();
    		}else if (event.getSource() == redoButton) {
    			commands.reexecute();
    			updateButtons();
    		}else if (event.getSource() == resetButton) {
    			commands.reset();
    			updateButtons();
    		}
    	}
    }

**实例下载：**

[http://download.csdn.net/detail/yang\_hui1986527/3840789][http_download.csdn.net_detail_yang_hui1986527_3840789]

[http_abstractforever.iteye.com_blog_962219]: http://abstractforever.iteye.com/blog/962219
[7b5b6558-28e0-3b5e-adb9-c1a9b6772760.png]: /images/20220617/d27c9ae3af9d49e7bdb246f31eecf279.png
[http_download.csdn.net_detail_yang_hui1986527_3840789]: http://download.csdn.net/detail/yang_hui1986527/3840789