import javax.swing.*;

import java.awt.BorderLayout;

import java.awt.Color;

import java.awt.Component;

import java.awt.Graphics;

import java.awt.HeadlessException;

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

 */

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();

        setupControls();

    }



    private void setupControls() {

        this.setLayout(new BorderLayout());



        final DrawPanel drawPanel = new DrawPanel();

        this.add(drawPanel, BorderLayout.CENTER);



        final JPencil jPencil = this;



        // 当窗口打开的时候初始化 drawPanel（否则无法获取 width 和 height）

        this.addHierarchyListener(new HierarchyListener() {



            public void hierarchyChanged(HierarchyEvent e) {

                boolean isShowingUp = jPencil.isVisible() &&

                        e.getChangeFlags() == HierarchyEvent.SHOWING_CHANGED;



                if (isShowingUp) {

                    drawPanel.init();

                }

            }

        });

    }



    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 Image 对象



        private Pencil pencil;



        public void init() {

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

            this.pencil = new Pencil(this, this.image);

        }



        @Override

        protected void paintComponent(Graphics g) {

            super.paintComponent(g);

            g.drawImage(this.image, 0, 0, null);  // 在这里绘图

        }

    }



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



    /**

     * 铅笔工具

     */

    private class Pencil extends MouseAdapter {



        private BufferedImage image;

        private java.awt.Graphics imageGraphics;



        private Component component;



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



        /**

         * 构造函数

         *

         * @param component 可绘图的 Component 对象

         * @param image     要绘制的 Image 对象

         */

        public Pencil(Component component, BufferedImage image) {

            this.image = image;

            imageGraphics = this.image.getGraphics();

            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() {

        	imageGraphics.setColor(Color.white);

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

        	imageGraphics.setColor(Color.black);

        }



        private void draw(int x, int y) {

            imageGraphics.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;

        }

    }

}