java - 为什么我们需要重新绘制 JPanel？

Question

问题是当你像这样绘制 JPanel 时:

public void paintComponent(Graphics g) {
    super.paintComponent(g);
    g.setColor (Color.CYAN);
    g.fillRect(10,10,200,50);
    g.setColor(Color.RED);
    g.fillOval(50,50,50,50);
}

因此，如果创建 JPanel 时，它会绘制一个青色矩形和一个红色椭圆形。问题是，例如，如果您想移动椭圆形，则必须重新绘制整个面板!

如何制作 2D 对象并使其可移动且可更改，而无需重新绘制整个面板？

Answer 1

首先查看 Painting in AWT and Swing和 Performing Custom Painting了解绘画过程。

如果您不愿意，您不必“必须”重新绘制整个面板，可以使用 JPanel#repaint(int, int, int, int)它允许您指定要重新绘制的区域。

现在，请记住，如果您需要绘制用于包含椭圆形的区域，然后绘制它现在所在的区域，这可以确保以前的位置也被更新，否则您将出现重影。 .

/*
 * To change this license header, choose License Headers in Project Properties.
 * To change this template file, choose Tools | Templates
 * and open the template in the editor.
 */
package testball;

import java.awt.Color;
import java.awt.Dimension;
import java.awt.EventQueue;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.Timer;
import javax.swing.UIManager;
import javax.swing.UnsupportedLookAndFeelException;

public class TestBall {

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

    public TestBall() {
        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 int x = 100 - 10;
        private int y = 100 - 10;
        private int delta = 4;

        public TestPane() {
            Timer timer = new Timer(40, new ActionListener() {
                @Override
                public void actionPerformed(ActionEvent e) {
                    // Replace existing area...
                    repaint(x, y, 21, 21);

                    x += delta;
                    if (x + 20 >= getWidth()) {
                        x = getWidth() - 20;
                        delta *= -1;
                    } else if (x < 0) {
                        x = 0;
                        delta *= -1;
                    }
                    repaint(x, y, 21, 21);
                }
            });
            timer.start();
        }

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

        @Override
        protected void paintComponent(Graphics g) {
            super.paintComponent(g);
            Graphics2D g2d = (Graphics2D) g.create();
            g2d.setColor(Color.RED);
            g2d.drawOval(x, y, 20, 20);
            g2d.dispose();
        }

    }

}