Java KeyListener未检测到键盘输入

Question

Java KeyListener未检测到键盘输入

Dav*_*wen 5 java swing image keylistener scale

我正在尝试修改此程序,以便绘制城堡的图像,并且我可以使用向上和向下箭头键缩放此图像.我无法让keylistener工作,程序运行但它没有响应按键.任何帮助将不胜感激,谢谢.

import java.awt.*;
import java.awt.geom.*;
import java.util.*;
import javax.swing.*;
import java.net.*;
import java.awt.event.*;

public class DrawImage extends JFrame implements KeyListener {
int scale = 1;
    private Image image;
    enter code here
    public static void main(String[] args) {
        new DrawImage();
    }

    public DrawImage() {
        super("DrawImage");
        setSize(600,600);
        setVisible(true);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        Toolkit tk = Toolkit.getDefaultToolkit();
        image = tk.getImage(getURL("castle.png"));
        addKeyListener(this);
    }

    private URL getURL(String filename) {
        URL url = null;
        try {
            url = this.getClass().getResource(filename);
        }
        catch (Exception e) { }
        return url;
    }

    public void paint(Graphics g) {
        Graphics2D g2d = (Graphics2D) g;
        AffineTransform trans = new AffineTransform();
        trans.scale(scale, scale);
        System.out.println("scale: " + scale);
        g2d.setColor(Color.BLACK);
        g2d.fillRect(0, 0, getSize().width, getSize().height);
        g2d.setTransform(trans);
        g2d.drawImage(image, 0, 40, this);
        addKeyListener(this);
    }

    public void keyReleased(KeyEvent e) { }
    public void keyTyped(KeyEvent e) { }
    public void keyPressed(KeyEvent e) {
        int key = e.getKeyCode();
        if (key == KeyEvent.VK_UP) {
            scale++;
        }
        if (key == KeyEvent.VK_DOWN) {
            scale--;
        }
    }
}

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Answer 1

Hov*_*els 8

我无法让keylistener工作,程序运行但它没有响应按键.

你的问题是经常被问到的一个非常常见的问题,几乎总是问题是由于缺乏关注 - 被收听的组件没有当前的焦点,并且焦点对于KeyListener的工作至关重要.

简短的回答是给予正在听取焦点的组件.

更好的答案是不使用KeyListeners,而是使用Key Bindings来实现这样的项目.

编辑
其他问题:

在上面的代码中,您将KeyListener添加到JFrame,即使您有充分的理由使用KeyListeners,也不应该这样做.
你也直接在JFrame中绘图,在我看来这是一个更大的问题,因为你冒着不必要的副作用.
你在paint方法中添加了一个监听器,这是一个更大的问题.不应该覆盖paint方法(通常),如果你必须覆盖它,则永远不应该用于程序逻辑,添加或删除侦听器,或者进行任何非绘制活动.它应该只用于绘画和绘画.
相反,您应该直接在JPanel或JComponent中绘制.
相反,您应该paintComponent(Graphics g)在绘画JPanel或JComponent 中的覆盖中进行绘制.
如上所述,此方法应仅用于绘画和绘画,并且应尽可能快.
你应该调用覆盖的super.paintComponent(g)内部paintComponent(Graphics g).
同样,绘图组件应该使用Key Bindings来监听击键(这里的教程).本教程将解释为什么这种区别(KeyListener与Key Bindings)很重要.

编辑2
你永远不想忽略异常,因为你的代码在这里显示,因为这是编程相当于驾驶盲人:

  try {
     url = this.getClass().getResource(filename);
  } catch (Exception e) {
  }

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希望这不是你的生产代码看起来如何,你只是为了简洁而忽略了你发布的代码中的异常,但我们很多人都明白这就像在粉笔板上听到钉子一样.

编辑3
关于KeyListeners与Key Bindings的更多信息:假设您的代码与KeyListener一起使用,然后假设您向GUI添加了任何其他可聚焦组件,并且他们通过用户交互以某种方式获得了焦点,那么您的KeyBindings将不再工作.如果您使用Key Bindings正确完成了这一操作,那么这不是问题.

编辑4
您真的希望您的比例字段是双精度数,而不是整数.并且你真的不想增加和减少它,而是你想要乘以它并将它除以一些乘数常数,比如1.2作为例子.repaint()每当您更改比例时,您也会想要打电话.

编辑5
请查看两个示例程序,第一个名为DrawImagePanelKeyListener.java,使用KeyListener,而第二个名为DrawImagePanelKeyBindings,使用Key Bindings.它们都应按预期编译,运行和运行:当您按向上或向下箭头键时,显示的图像会缩小并增大.当按下JButton时,可以看到它们的行为差异.按下按钮,查看您的密钥响应会发生什么.当具有KeyListener的组件失去焦点时,其KeyListener将停止工作,但对于使用键绑定的组件则不同.

可以解决这个问题的一个问题可能是阻止所有其他组件获得焦点,但对于大多数GUI来说这是不实际或不可取的.

DrawImagePanelKeyListener.java

import java.awt.*;
import javax.imageio.ImageIO;
import javax.swing.*;
import java.io.IOException;
import java.net.*;
import java.awt.event.*;

@SuppressWarnings("serial")
public class DrawImagePanelKeyListener extends JPanel implements KeyListener {
   public static final String IMAGE_PATH = "https://duke.kenai.com/"
         + "nyanya/.Midsize/NyaNya.jpg.png";
   private static final double MULTIPLIER = 1.2;
   double scale = 1.0;
   private Image image;
   private Dimension initialSize = new Dimension(0, 0);

   public static void main(String[] args) {
      SwingUtilities.invokeLater(new Runnable() {
         public void run() {
            DrawImagePanelKeyListener drawImage = new DrawImagePanelKeyListener();
            drawImage.add(new JButton("Foo"));
            JFrame frame = new JFrame("Draw Image");
            frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
            frame.add(drawImage);
            frame.pack();
            frame.setLocationRelativeTo(null);
            frame.setVisible(true);
         }
      });
   }

   public DrawImagePanelKeyListener() {
      setFocusable(true);
      requestFocusInWindow();
      URL imgURL;
      try {
         imgURL = new URL(IMAGE_PATH);
         image = ImageIO.read(imgURL);
         initialSize = new Dimension(image.getWidth(this),
               image.getHeight(this));
         addKeyListener(this);
         setVisible(true);
      } catch (MalformedURLException e) {
         e.printStackTrace();
      } catch (IOException e) {
         e.printStackTrace();
      }
   }

   @Override
   public Dimension getPreferredSize() {
      return initialSize;
   }

   @Override
   protected void paintComponent(Graphics g) {
      super.paintComponent(g);
      g.setColor(Color.BLACK);
      g.fillRect(0, 0, getSize().width, getSize().height);
      Graphics2D g2d = (Graphics2D) g.create();
      g2d.scale(scale, scale);
      g2d.drawImage(image, 0, 0, this);
   }

   public void keyReleased(KeyEvent e) {
   }

   public void keyTyped(KeyEvent e) {
   }

   public void keyPressed(KeyEvent e) {
      int key = e.getKeyCode();
      if (key == KeyEvent.VK_UP) {
         scale *= MULTIPLIER;
      }
      if (key == KeyEvent.VK_DOWN) {
         scale /= MULTIPLIER;
      }
      repaint();
   }
}

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DrawImagePanelKeyBindings.java

import java.awt.*;
import javax.imageio.ImageIO;
import javax.swing.*;
import java.io.IOException;
import java.net.*;
import java.awt.event.*;

@SuppressWarnings("serial")
public class DrawImagePanelKeyBindings extends JPanel {
   public static final String IMAGE_PATH = "https://duke.kenai.com/"
         + "nyanya/.Midsize/NyaNya.jpg.png";
   private static final double MULTIPLIER = 1.2;
   double scale = 1.0;
   private Image image;
   private Dimension initialSize = new Dimension(0, 0);

   public static void main(String[] args) {
      SwingUtilities.invokeLater(new Runnable() {
         public void run() {
            DrawImagePanelKeyBindings drawImage = new DrawImagePanelKeyBindings();
            drawImage.add(new JButton("Foo"));
            JFrame frame = new JFrame("Draw Image");
            frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
            frame.add(drawImage);
            frame.pack();
            frame.setLocationRelativeTo(null);
            frame.setVisible(true);
         }
      });
   }

   public DrawImagePanelKeyBindings() {
      setBindings();
      URL imgURL;
      try {
         imgURL = new URL(IMAGE_PATH);
         image = ImageIO.read(imgURL);
         initialSize = new Dimension(image.getWidth(this),
               image.getHeight(this));
         setVisible(true);
      } catch (MalformedURLException e) {
         e.printStackTrace();
      } catch (IOException e) {
         e.printStackTrace();
      }
   }

   private void setBindings() {
      int condition = WHEN_IN_FOCUSED_WINDOW;
      InputMap inputMap = getInputMap(condition);
      ActionMap actionMap = getActionMap();

      final KeyStroke[] keyStrokes = {
            KeyStroke.getKeyStroke(KeyEvent.VK_UP, 0),
            KeyStroke.getKeyStroke(KeyEvent.VK_DOWN, 0)
      };
      for (final KeyStroke keyStroke : keyStrokes) {
         inputMap.put(keyStroke, keyStroke.toString());
         actionMap.put(keyStroke.toString(), new AbstractAction() {
            @Override
            public void actionPerformed(ActionEvent evt) {
               myKeyPressed(keyStroke.getKeyCode());
            }
         });
      }
   }

   @Override
   public Dimension getPreferredSize() {
      return initialSize;
   }

   @Override
   protected void paintComponent(Graphics g) {
      super.paintComponent(g);
      g.setColor(Color.BLACK);
      g.fillRect(0, 0, getSize().width, getSize().height);
      Graphics2D g2d = (Graphics2D) g.create();
      g2d.scale(scale, scale);
      g2d.drawImage(image, 0, 0, this);
   }

   public void myKeyPressed(int key) {
      if (key == KeyEvent.VK_UP) {
         scale *= MULTIPLIER;
      }
      if (key == KeyEvent.VK_DOWN) {
         scale /= MULTIPLIER;
      }
      repaint();
   }
}

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