为什么我的方法不能正确重绘？

Question

I've just recently picked up Java and I'm aiming to make a simple graphical game with it so feel free to point out any style mistakes.

In transition from my main title screen to the main screen, my old title screen is not refreshed, the button used to click to get to the main screen is frozen, basically, the image is frozen and the main screen paintComponent is not called and the program just goes into an infinite loop and will not close (have to shut down via task manager).

Interesting thing to note is that, without the while loop it works just fine, the paintComponent is called and everything works as it should, when reintroducing the while loop, same problem persists.

public class Game {

private static final int HEIGHT = 650;
private static final int WIDTH = 820;
private static final int FRAMES_PER_SEC = 60;
private JFrame frame = new JFrame("Game");
private boolean inIntroScreen = true;
private boolean game_running = false;
private int x  = 1;
private int y  = 1;
private int dx = 1;
private int dy = 1;

/* method to set up GUI for the game. */
public void initGUI () {
    //Build Frame
    frame.setSize(WIDTH, HEIGHT);
    frame.setVisible(true);
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.setResizable(false);
    //End Build Frame

    /* Intro screen build */
    class drawIntro extends JPanel {

        public void paintComponent(Graphics g) {
            if (inIntroScreen) {
            Graphics2D g2d = (Graphics2D) g;
            //Background
            g2d.setPaint(Color.BLACK);
            g2d.fillRect(0, 0, 820, 650);
            //Title 
            BufferedImage img = null;
            try { img = ImageIO.read(new File("game.png")); }
            catch (IOException e) { System.out.println("Error image"); }
            g2d.drawImage(img, 180, 52, null);

            g2d.setPaint(Color.WHITE);
            g2d.fillOval(550, 60, 40, 40);
            g2d.fillOval(195, 60, 40, 40);
            System.out.println("Intro screen painted");
            }

        } //end paint
    } //end draw inner class

    final drawIntro introScreen = new drawIntro();
    final JPanel introPanel = new JPanel();
    final JButton startButton = new JButton("Start");

    frame.getContentPane().add(introPanel,BorderLayout.SOUTH);
    introPanel.setBackground(Color.BLACK);
    frame.getContentPane().add(introScreen, BorderLayout.CENTER);
    startButton.setPreferredSize(new Dimension(100,50));
    startButton.setBackground(Color.BLACK);
    startButton.setForeground(Color.WHITE);
    introPanel.add(startButton);
    introScreen.repaint();
    //End intro screen build
    startButton.addActionListener(new ActionListener() {
        public void actionPerformed(ActionEvent e) {
            introPanel.removeAll();
            introPanel.revalidate();
            inIntroScreen = false;
            game_running = true;
            System.out.println("button clicked");
            Start();
        }
    });

} //End initGUI

/* Level building class */
class Level extends JPanel {
    @Override 
    public void paintComponent(Graphics g) {
        super.paintComponent(g);
        Graphics2D g2d = (Graphics2D) g;
        //Background
        g2d.setPaint(Color.BLACK);
        g2d.fillRect(0, 0, 820, 650);
        //Anti-aliasing 
        g2d.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);
        g2d.setRenderingHint(RenderingHints.KEY_INTERPOLATION, RenderingHints.VALUE_INTERPOLATION_BILINEAR);

        g2d.setPaint(Color.BLUE);
        g2d.fillOval(x, y, 70, 70);
        x += dx;
        y += dy;


        System.out.println("Main screen painted");
    } //End paint component
}


/* Game loop */
public void Start () {
    Level player = new Level();
    frame.add(player);
    player.repaint();

    int FPS = 1000 / FRAMES_PER_SEC;

        while(game_running) {   /* PROBLEM HERE, if while loop is removed everything works as intended */
        frame.repaint();
        try { Thread.sleep(FPS); } 
        catch (InterruptedException e) {}
        }


}


public static void main(String[] args) {
    Game game = new Game();
    game.initGUI();
    System.out.println("Program terminated");

}

} //end game class

Answer 1

Yours is a classic Swing threading issue where you perform a long-running task on the Swing event thread. You appear in fact to be doing long-running code in the painting method, something that should absolutely never be done since this will repeatedly perform this task every time a repaint is performed, slowing your painting to a crawl.

Suggestions:

• Do long-running tasks, such as reading files, in a background thread such as provided by a SwingWorker.
• Do only painting in paint methods, and nothing else.
• Call the super's paintComponent method in your override to allow the JPanel to do its house-keeping painting.
• If you are going to be swapping views , use a CardLayout to make it easy and safe to do so.
• The while (game_running) { loop is doing the same thing -- tying up the Swing event thread, freezing your GUI. Use a Swing Timer instead for this.
• You've got game logic in your painting method (the paintComponent method), where you're setting the x and y variables. Don't do this, but instead change them in your Swing Timer's code. You never have complete control over whether or if the paintComponent method is called, and so you want to have no program logic, no code that changes fields, within this method.

For example:

// start method name should start with a lower-case letter
public void start() {
  final Level player = new Level();
  frame.add(player);
  player.repaint();

  int fps = 1000 / FRAMES_PER_SEC;

  // use a field called timer
  timer = new Timer(fps, new ActionListener() {

     @Override
     public void actionPerformed(ActionEvent e) {
        // get this out of the paintComponent method
        x += dx;
        y += dy;
        player.repaint();
     }
  });
  timer.start();
}

---

For example:

import java.awt.BorderLayout;
import java.awt.CardLayout;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.RenderingHints;
import java.awt.event.*;
import java.awt.image.BufferedImage;
import java.io.IOException;
import java.net.URL;

import javax.imageio.ImageIO;
import javax.swing.*;

@SuppressWarnings("serial")
public class Game2 extends JPanel {
   public static final String INTRO = "intro";
   public static final String GAME = "game";
   public static final int FPS = 15;
   private CardLayout cardLayout = new CardLayout();

   public Game2() throws IOException {
      URL imgUrl = new URL(IntroScreen.IMAGE_PATH);
      BufferedImage img = ImageIO.read(imgUrl);
      IntroScreen introScreen = new IntroScreen(img);
      introScreen.setLayout(new BorderLayout());

      JButton startButton = new JButton(new StartAction("Start"));
      JPanel bottomPanel = new JPanel();
      bottomPanel.setOpaque(false);
      bottomPanel.add(startButton);
      introScreen.add(bottomPanel, BorderLayout.PAGE_END);

      setLayout(cardLayout);
      add(introScreen, INTRO);
   }

   private class StartAction extends AbstractAction {
      public StartAction(String name) {
         super(name);
         int mnemonic = (int) name.charAt(0);
         putValue(MNEMONIC_KEY, mnemonic);
      }

      @Override
      public void actionPerformed(ActionEvent e) {
         GamePanel gamePanel = new GamePanel(FPS);
         Game2.this.add(gamePanel, GAME);
         cardLayout.show(Game2.this, GAME);
         gamePanel.start();
      }
   }

   private static void createAndShowGui() {
      Game2 game2 = null;
      try {
         game2 = new Game2();
      } catch (IOException e) {
         e.printStackTrace();
         System.exit(-1);
      }

      JFrame frame = new JFrame("Game");
      frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
      frame.getContentPane().add(game2);
      frame.pack();
      frame.setLocationRelativeTo(null);
      frame.setVisible(true);
   }

   public static void main(String[] args) {
      SwingUtilities.invokeLater(new Runnable() {
         public void run() {
            createAndShowGui();
         }
      });
   }
}

@SuppressWarnings("serial")
class IntroScreen extends JPanel {
   public static final String IMAGE_PATH = "https://duke.kenai.com/"
         + "glassfish/GlassFishMedium.jpg";
   private BufferedImage img;

   public IntroScreen(BufferedImage img) {
      this.img = img;
   }

   @Override
   protected void paintComponent(Graphics g) {
      super.paintComponent(g);
      if (img != null) {
         g.drawImage(img, 0, 0, this);
      }
   }

   @Override
   public Dimension getPreferredSize() {
      if (img != null) {
         int width = img.getWidth();
         int height = img.getHeight();
         return new Dimension(width, height);
      }
      return super.getPreferredSize();
   }
}

@SuppressWarnings("serial")
class GamePanel extends JPanel {
   protected static final int DX = 2;
   protected static final int DY = DX;
   private int x;
   private int y;
   private Timer timer;
   private int fps = 0;

   public GamePanel(int fps) {
      this.fps = fps;
   }

   @Override 
   public void paintComponent(Graphics g) {
       super.paintComponent(g);
       Graphics2D g2d = (Graphics2D) g;
       //Background
       g2d.setPaint(Color.BLACK);
       g2d.fillRect(0, 0, 820, 650);
       //Anti-aliasing 
       g2d.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);
       g2d.setRenderingHint(RenderingHints.KEY_INTERPOLATION, RenderingHints.VALUE_INTERPOLATION_BILINEAR);

       g2d.setPaint(Color.BLUE);
       g2d.fillOval(x, y, 70, 70);
   }

   public void start() {
      // use a field called timer
      timer = new Timer(fps, new ActionListener() {

         @Override
         public void actionPerformed(ActionEvent e) {
            // get this out of the paintComponent method
            x += DX;
            y += DY;
            repaint();
         }
      });
      timer.start();
   }
}