java - 更新作为 JFrame 组件的 java Canvas

Question

如何从属于 JFrame 组件的 Canvas 的任何类更新屏幕上的图形？我相信它是 JFrame.update(Graphics g)。但我不确定。如果是 JFrame.update(Graphics g)，正确的调用方式是什么，从哪里获取参数的 Graphics 对象？

Answer 1

只需调用repaint()即可。

您想要进行的任何自定义绘画都应该在 paintComponent(Graphics) 中完成，并且一定要调用 super.paintComponent(g);

一般来说，将 Graphics 实例的处理留给 Swing。它将提供符合自己标准的自己的产品。

This Oracle tutorial shows just that

<小时/>

下面是一个使用 Swing Timer 每 15 毫秒从 JFrame 请求重绘更新的示例。

import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.SwingUtilities;
import javax.swing.Timer;
import javax.swing.WindowConstants;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;

/**
 * @author Obicere
 */
public class ShapeDrawer {

    public ShapeDrawer() {
        final JFrame frame = new JFrame("Shape Drawer");
        final MyPanel panel = new MyPanel();

        frame.add(panel);

        frame.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
        frame.pack();
        frame.setLocationByPlatform(true);
        frame.setVisible(true);

        createRepaintTimer(frame);
    }

    // Just makes calls for the frame to paint every 15 milliseconds
    private void createRepaintTimer(final JFrame frame) {
        final Timer timer = new Timer(15, null);

        timer.addActionListener(e -> {
            if (!frame.isVisible()) {
                timer.stop();
            } else {
                frame.repaint();
            }
        });

        timer.start();
    }

    public static void main(final String[] args) {
        SwingUtilities.invokeLater(ShapeDrawer::new);
    }

    public class MyPanel extends JPanel {

        private long start;

        public MyPanel() {
            start = System.currentTimeMillis();
        }

        @Override
        protected void paintComponent(final Graphics g) {
            // Calling to clear the artifacts!
            super.paintComponent(g);

            g.setColor(Color.RED);

            // Difference between when the current time and when we started
            final long difference = System.currentTimeMillis() - start;

            // Difference in seconds
            final double seconds = difference / 1000d;

            // 1 rotation per second
            final double rotation = seconds * 1440 / 1000;

            final int x = (int) (Math.cos(rotation) * 100);
            final int y = (int) (Math.sin(rotation) * 100);

            g.drawOval(200 + x - 5, 200 + y - 5, 10, 10);
        }

        // Just setting the default size of the panel
        @Override
        public Dimension getPreferredSize() {
            return new Dimension(400, 400);
        }

    }

}

如您所见，createRepaintTimer(JFrame) 方法进行调用以请求更新图形。我们不必提供 Graphics 实例，Swing 会为我们处理这个问题。然后，我们在调用 super.paintComponent(Graphics) 后渲染到 MyPanel#paintComponent(Graphics) 方法中为我们提供的 Graphics swing。