Java Swing重绘延迟

Question

I have a simple paint program:

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

public class CanvasPanel extends JFrame implements MouseMotionListener
{
    private int x1, y1, x2, y2;
    public CanvasPanel()
    {
        addMouseMotionListener(this);
        setBounds(50,50,400,250);
        setVisible(true);
    }

    public static void main(String[] argv)
    {
        new CanvasPanel();
    }

    public void update(Graphics g)
    {
        paint(g);
    }

    public void paint(Graphics g)
    {
        g.setColor(Color.black);
        g.drawLine(x1, y1, x2, y2);
    }

    public void mouseDragged(MouseEvent mouseEvent)
    {
        mouseEvent.consume();
        int x = mouseEvent.getX();
        int y = mouseEvent.getY();

        if ( x1 == 0 )
        {
            x1 = x;
        }

        if ( y1 == 0 )
        {
            y1 = y;
        }

        x2 = x;
        y2 = y;
        repaint();
        x1 = x2;
        y1 = y2;
    }

    public void mouseMoved(MouseEvent me)
    {

    }
}

If you run it, you will see that there is a lag when painting with your mouse and the drawing will disintegrate into dots the faster you go. How do I fix this? I was thinking if the repaint is super fast, then the dots should form into lines and curves.

Answer 1

There are quite a few things to learn when painting. I have done a fair amount of graphics so I modified your program to illustrate.

1. Don't extend JFrame unless you plan to override something. It is seldom necessary.
2. Do your painting in a JPanel that overrides paintComponent() .
3. Collect your points in an array or list to run thru them each time. This is because the call to super.paintComponent() will clear the screen (but it is necessary to do so).
4. Not required but I find it convenient to use MouseAdapter (or any *-Adapter) in a private class to handle events.
5. Since Swing is not thread safe it is a good idea to start it in the EDT. Just make certain that no long processing is done while in the EDT or your program will lock up and become unresponsive.

import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.Point;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;

import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.SwingUtilities;

public class CanvasPanel extends JPanel {
   List<Point> points = new ArrayList<>();
   // Use compositon over inheritance.
   // Don't extend JFrame unless you plan to override something.
   JFrame      frame  = new JFrame();

   public static void main(String[] args) {
      SwingUtilities.invokeLater(() -> new CanvasPanel());
   }
   public CanvasPanel() {
      MyMouseListener ml = new MyMouseListener();
      addMouseMotionListener(ml);
      addMouseListener(ml);
      frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
      setPreferredSize(new Dimension(500, 500));
      frame.add(this);
      frame.pack();
      frame.setLocationRelativeTo(null); // center on screen
      frame.setVisible(true);
   }

   public void paintComponent(Graphics g) {
      super.paintComponent(g);
      g.setColor(Color.black);
      if (points.size() >= 2) {
         Iterator<Point> it = points.iterator();
         Point p1 = it.next();
         while (it.hasNext()) {
            Point p2 = it.next();
            g.drawLine(p1.x, p1.y, p2.x, p2.y);
            p1 = p2;
         }
      }
   }

   // MouseAdapter provides dummy implementations
   private class MyMouseListener extends MouseAdapter {
      public void mouseDragged(MouseEvent me) {
         points.add(me.getPoint());
         repaint();

      }
      public void mousePressed(MouseEvent me) {
         // initialize first point in list.
         points.add(me.getPoint());
      }
   }
}

This example still has problems. For example, if you release the mouse button and position to a new location and then click, it will continue from the last point. There are probably many ways to fix this but the simplest would be to have a list of lists, where each list is a separate set of points with detached origins.