java - 为什么画的线会自动删除？

Question

我正在构建一个带有 Canvas 的系统，用户可以通过在Java中拖动鼠标来绘制线条。我想要存储并显示所有笔画。但是，当我按下鼠标绘制新线时，之前的线会自动删除，这不是我所期望的。我该如何解决这个问题？

这是我的代码:

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

public class FreeDrawing extends JPanel implements MouseListener,   MouseMotionListener {

private int indexOfStroke = 0;
private int indexOfPoint = 0;

private Stroke[] strokes = new Stroke[50];
private Point[] currentPoints = new Point[500];

public FreeDrawing(String name) {
    super();

    this.addMouseListener(this);
    this.addMouseMotionListener(this);

    JFrame fr = new JFrame(name);

    fr.add(this);
    fr.setSize(500, 500);

    setBackground(Color.GRAY);

    fr.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    fr.setVisible(true);
}

public void paintComponent(Graphics g) {
    System.out.println("Paint");

    super.paintComponents(g);

    strokes[indexOfStroke] = new Stroke();
    for (int i = 0; i < indexOfPoint - 1; i++) {
        System.out.println("Really draw");

        g.drawLine(currentPoints[i].x, currentPoints[i].y, currentPoints[i + 1].x, currentPoints[i + 1].y);
    }
}

public void mouseDragged(MouseEvent e) {
    System.out.println("Drag");

    currentPoints[indexOfPoint] = new Point(e.getX(), e.getY());
    indexOfPoint++;

    repaint();
}

public void mousePressed(MouseEvent e) {
    System.out.println("Press");

    currentPoints[indexOfPoint] = new Point(e.getX(), e.getY());
    indexOfPoint = 0;
    repaint();
}

public void mouseReleased(MouseEvent e) {
    indexOfPoint = 0;
    indexOfStroke++;
}

public void mouseExited(MouseEvent e) {
}

public void mouseClicked(MouseEvent e) {
}

public void mouseEntered(MouseEvent e) {
}

public void mouseMoved(MouseEvent e) {
}

public static void main(String[] args) {
    FreeDrawing canvas = new FreeDrawing("Mouse");
}
}

class Stroke {

    public Stroke() {
        System.out.println("Stroke initiated");
        points = new Point[500];
    }

    Point[] points;
}

Answer 1

因为，这就是绘画的原理，请参阅Painting in AWT and Swing和 Performing Custom Painting更多细节。

基本上，绘制是破坏性的，这意味着每次调用 PaintComponent 时，您都需要从头开始重新绘制组件的整个状态

您可能会考虑做的一件事是创建一个类，其中包含每条线所需的笔画、颜色和点，并在每次单击时将其存储在列表中

import java.awt.BasicStroke;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.EventQueue;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Point;
import java.awt.Stroke;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import java.awt.event.MouseMotionAdapter;
import java.awt.geom.Line2D;
import java.util.ArrayList;
import java.util.List;
import java.util.Random;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.UIManager;
import javax.swing.UnsupportedLookAndFeelException;

public class Test {

    public static void main(String[] args) {
        new Test();
    }

    public Test() {
        EventQueue.invokeLater(new Runnable() {
            @Override
            public void run() {
                try {
                    UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
                } catch (ClassNotFoundException | InstantiationException | IllegalAccessException | UnsupportedLookAndFeelException ex) {
                    ex.printStackTrace();
                }

                JFrame frame = new JFrame("Testing");
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                frame.add(new TestPane());
                frame.pack();
                frame.setLocationRelativeTo(null);
                frame.setVisible(true);
            }
        });
    }

    public class TestPane extends JPanel {

        private List<Drawing> drawings;
        private Drawing current;

        private Random rnd = new Random();

        public TestPane() {
            drawings = new ArrayList<>(25);
            addMouseListener(new MouseAdapter() {
                @Override
                public void mousePressed(MouseEvent e) {
                    Stroke stroke = new BasicStroke(rnd.nextInt(9) + 1, BasicStroke.CAP_ROUND, BasicStroke.JOIN_ROUND);
                    Color color = new Color(rnd.nextInt(255), rnd.nextInt(255), rnd.nextInt(255));
                    current = new Drawing(stroke, color);
                    current.addPoint(e.getPoint());
                    drawings.add(current);
                }
            });

            addMouseMotionListener(new MouseMotionAdapter() {
                @Override
                public void mouseDragged(MouseEvent e) {
                    current.addPoint(e.getPoint());
                    repaint();
                }
            });
        }

        @Override
        public Dimension getPreferredSize() {
            return new Dimension(200, 200);
        }

        @Override
        protected void paintComponent(Graphics g) {
            super.paintComponent(g);
            Graphics2D g2d = (Graphics2D) g.create();
            for (Drawing drawing : drawings) {
                drawing.paint(g2d);
            }
            g2d.dispose();
        }

    }

    public class Drawing {

        private Stroke stroke;
        private Color color;
        private List<Point> points;

        public Drawing(Stroke stroke, Color color) {
            this.stroke = stroke;
            this.color = color;
            this.points = new ArrayList<>(25);
        }

        public void addPoint(Point p) {
            points.add(p);
        }

        public void paint(Graphics2D g) {
            Graphics2D g2d = (Graphics2D) g.create();
            g2d.setColor(color);
            g2d.setStroke(stroke);
            if (!points.isEmpty()) {
                Point from = points.get(0);
                for (Point to : points.subList(1, points.size())) {
                    g2d.draw(new Line2D.Double(from, to));
                    from = to;
                }
            }
            g2d.dispose();
        }

    }

}