如果run（）方法中的if语句为true，为什么我不能再使用keyPressed？

Question

I'm trying to make a kind of brickbreaker game but once I press the start button ("starten" = true). I can't use the keyPressed()-utility anymore but before i pressed the start button I could use the keyPressed()-utility. Could somebody please tell me why I suddenly can't use the keyPressed()-utility anymore and give me a possible solution too?

import java.util.*;
import java.awt.*;
import java.awt.event.*;

import javax.swing.*;

public class VGKernel extends JPanel implements KeyListener {

public Rectangle screen, ball, block; // The screen area and ball location/size.
public Rectangle bounds;  // The boundaries of the drawing area.
public JFrame frame; // A JFrame to put the graphics into.
public VGTimerTask vgTask; // The TimerTask that runs the game.
public boolean down, right, starten = false; // Direction of ball's travel.
public JButton start;

public VGKernel(){
    super();
    screen = new Rectangle(0, 0, 600, 400);
    ball   = new Rectangle(0, 0, 20, 20);
    block = new Rectangle(260, 350, 40, 10);
    bounds = new Rectangle(0, 0, 600, 400); // Give some starter values.
    frame = new JFrame("VGKernel");
    vgTask = new VGTimerTask();
    addKeyListener(this);
    setFocusable(true);
    start = new JButton("Start");
    start.addActionListener(new ActionListener() {
        @Override
        public void actionPerformed(ActionEvent e) {
            starten = true;
        }
    });
    add(start);
}

class VGTimerTask extends TimerTask{
    public void run(){
        repaint();

        if(starten){
        moveBall();
        frame.repaint();
        }
    }
  }

// Now the instance methods:
  public void paintComponent(Graphics g){
    // Get the drawing area bounds for game logic.
    bounds = g.getClipBounds();
    // Clear the drawing area, then draw the ball.
    g.clearRect(screen.x, screen.y, screen.width, screen.height);
    g.fillRect(ball.x, ball.y, ball.width, ball.height);
    g.fillRect(block.x, block.y, block.width, block.height);
  }

  public void moveBall(){
  // Ball should really be its own class with this as a method.
    if (right) ball.x+=1; // If right is true, move ball right,
    else ball.x-=1;       // otherwise move left.
    if (down)  ball.y+=1; // Same for up/down.
    else ball.y-=1;
    if (ball.x > (bounds.width - ball.width)) // Detect edges and bounce.
      { right = false; ball.x = bounds.width -  ball.width; }
    if (ball.y > (bounds.height - ball.height))
      { down  = false; ball.y = bounds.height - ball.height;}
    if (ball.x == 0) { right = true; ball.x = 0; }
    if (ball.y == 0) { down  = true; ball.y = 0; }
  }

  public void keyPressed(KeyEvent evt) {
      if(evt.getKeyCode() == KeyEvent.VK_G && block.x > 0) {
          block.x -= 20;
      }

      if(evt.getKeyCode() == KeyEvent.VK_H && block.x < 540) {
          block.x += 20;
      }
  }

  public void keyTyped(KeyEvent evt){  }
  public void keyReleased(KeyEvent evt){ }

  public void startActionPerformed(java.awt.event.ActionEvent evt) {
      starten = true;
  }

  public static void main(String arg[]){
    java.util.Timer vgTimer = new java.util.Timer();  // Create a Timer.
    VGKernel panel = new VGKernel(); // Create and instance of our kernel.

    // Set the intial ball movement direction.
    panel.down = true;
    panel.right = true;

    // Set up our JFRame
    panel.frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    panel.frame.setSize(panel.screen.width, panel.screen.height);
    panel.frame.setContentPane(panel); 
    panel.frame.setVisible(true);

    // Set up a timer to do the vgTask regularly.
    vgTimer.schedule(panel.vgTask, 0, 10);
  }

}

Answer 1

You need to make sure whatever component you're listening for KeyEvents on has the focus.

Once you click the button, that button has focus instead of your JPanel. More info here: http://docs.oracle.com/javase/tutorial/uiswing/misc/focus.html

You can use the requestFocusInWindow() method on your JPanel, or you can call setFocusable(false) on your JButton.

Or you could use key bindings instead: http://docs.oracle.com/javase/tutorial/uiswing/misc/keybinding.html

Answer 2

Add VGKernel.this.grabFocus(); to the ActionListener to return focus to the VGKernel panel after clicking the button.

start.addActionListener(new ActionListener() {
    @Override
    public void actionPerformed(ActionEvent e) {
        starten = true;
        VGKernel.this.grabFocus();
    }
});