import javax.swing.*;

import java.awt.BorderLayout;

import java.awt.Color;

import java.awt.Component;

import java.awt.Graphics;

import java.awt.Graphics2D;

import java.awt.HeadlessException;

import java.awt.RenderingHints;

import java.awt.event.HierarchyEvent;

import java.awt.event.HierarchyListener;

import java.awt.event.MouseAdapter;

import java.awt.event.MouseEvent;

import java.awt.image.BufferedImage;



/**

 * 简单的 Java 铅笔画图程序。程序不是直接将笔划画到控件上，而是

 * 先画到 Image 对象，然后将 Image 对象画到控件上，这样画出来

 * 的图不会被抹去。

 */

public class JPencil extends JFrame {



    // 程序执行入口

    public static void main(String[] args) throws Exception {

        try {

            UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());

        } catch (Exception e) {

            e.printStackTrace();

        }



        new JPencil("JPencil").setVisible(true);

    }



    public JPencil(String title) throws HeadlessException {

        super(title);

        setupWindow();      // 设置窗体

        setupControls();    // 设置控件

    }



    private void setupControls() {

        this.setLayout(new BorderLayout());



        final DrawPanel drawPanel = new DrawPanel();

        this.add(drawPanel, BorderLayout.CENTER);



        final JPencil frame = this;

        // 当窗口打开的时候初始化 drawPanel（否则无法获取正确的大小）

        this.addHierarchyListener(new HierarchyListener() {



            public void hierarchyChanged(HierarchyEvent e) {

                boolean isShowingUp = frame.isVisible() &&

                        e.getChangeFlags() == HierarchyEvent.SHOWING_CHANGED;



                if (isShowingUp) {

                    drawPanel.init();

                    frame.removeHierarchyListener(this);  // 只初始化一次

                }

            }

        });

    }



    private void setupWindow() {

        this.setDefaultCloseOperation(EXIT_ON_CLOSE);

        this.setResizable(false);

        this.setSize(800, 600);

        this.setLocation(50, 50);

    }



    /////////////////////////////////////////



    /**

     * 绘图板

     */

    private class DrawPanel extends JPanel {



        private BufferedImage image;    // 缓存的 Image 对象



        // 初始化

        public void init() {

            this.image = new BufferedImage(getWidth(), getHeight(), BufferedImage.TYPE_INT_ARGB);



            new Pencil(this, this.image.createGraphics());

        }



        @Override

        protected void paintComponent(Graphics g) {

            super.paintComponent(g);

            g.drawImage(this.image, 0, 0, null);

        }

    }



    /////////////////////////////////////////



    /**

     * 铅笔工具

     */

    private class Pencil extends MouseAdapter {



        private Graphics2D graphics;



        private Component component;



        private int lastX = -1, lastY = -1;



        /**

         * 构造函数

         *

         * @param component 可绘图的 Component 对象

         * @param graphics  绘图对象

         */

        public Pencil(Component component, Graphics2D graphics) {

            this.graphics = graphics;

            this.component = component;

            this.clear();



            component.addMouseListener(this);       // MouseListener 负责处理鼠标键按下和松开事件

            component.addMouseMotionListener(this); // MouseMotionListener 负责处理鼠标拖拽事件

        }



        private void clear() {

            Graphics2D graphics = this.graphics;

            graphics.setColor(Color.white);

            graphics.fillRect(0, 0, getWidth(), getHeight());

            graphics.setColor(Color.black);

        }



        private void draw(int x, int y) {

            Graphics2D graphics = this.graphics;

            graphics.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);

            graphics.setColor(Color.black);

            

            // 从上次记录的鼠标位置到当前鼠标位置画一条直线。当不停地

            // 获取到鼠标位置时，画出来的就是一条鼠标移动的轨迹了。

            graphics.drawLine(lastX, lastY, x, y);

            

            this.component.repaint();

        }

        

        /////////////////////////////////////////



        @Override

        public void mouseDragged(MouseEvent e) {

            if (this.lastX != -1 && this.lastY != -1) {

                draw(e.getX(), e.getY());

            }



            this.lastX = e.getX();

            this.lastY = e.getY();

        }



        @Override

        public void mousePressed(MouseEvent e) {

            // 设置绘图的起始点

            this.lastX = e.getX();

            this.lastY = e.getY();

        }



        @Override

        public void mouseReleased(MouseEvent e) {

            // 结束绘图

            this.lastX = -1;

            this.lastY = -1;

        }

    }

}