JButton KeyPressed - 没有任何反应

Question

我正在努力使按右箭头键的效果与按 JButton 的效果相同。我可以将右箭头键绑定到按钮本身——但这意味着我必须在右键起作用之前按下按钮。现在我想看看是否绑定到实际的 JFrame 是我想要的，但是当我绑定到框架时我根本无法发生任何事情：

private void jButton1ActionPerformed(java.awt.event.ActionEvent evt) {                                         
    onButtonPress();
}                                        

private void formKeyPressed(java.awt.event.KeyEvent evt) {                                
    if (evt.getKeyCode() == KeyEvent.VK_RIGHT){
        onButtonPress();
    }
} 

private void onButtonPress() {
    pressNum++;
    jLabel1.setText("Button has been pressed " + pressNum + " times.");
}

Answer 1

根据一般经验，您应该避免 KeyListener。主要原因是，为了让 KeyListener 生成按键事件，它注册到的组件必须是可聚焦的并且具有键盘焦点。在您的情况下，这可能意味着向 UI 中的每个组件添加一个 KeyListener，从而 "might" 获得键盘焦点，这在现实世界中并不实用。

相反，您应该使用 Key Bindings API，它为您提供了定义触发相关操作所需的焦点级别的方法。

键绑定 API 和示例使用了 Actions API，它允许我定义一个可应用于多个 "actionable" 控件的工作单元

该示例还使用了 delegate/callback/listener（即 CounterListener），它允许我将 "side effects" 与操作本身分离。

这基本上意味着 Action 可以做它需要做的事情，但是 "other" 感兴趣的人可以在它发生变化时执行一些其他的操作。同样，您可以将 ActionListener 附加到 Action，但这只是实施起来更简单、更快捷

import java.awt.GridBagConstraints;
import java.awt.GridBagLayout;
import java.awt.event.ActionEvent;
import java.awt.event.KeyEvent;
import javax.swing.AbstractAction;
import javax.swing.ActionMap;
import javax.swing.InputMap;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.KeyStroke;
import javax.swing.SwingUtilities;

public class Test {

    public static void main(String[] args) {
        new Test();
    }

    public Test() {
        SwingUtilities.invokeLater(new Runnable() {
            @Override
            public void run() {
                JFrame frame = new JFrame();
                frame.add(new TestPane());
                frame.pack();
                frame.setLocationRelativeTo(null);
                frame.setVisible(true);
            }
        });
    }

    public class TestPane extends JPanel {

        public TestPane() {
            JLabel label = new JLabel("...");
            MyAwesomeAction action = new MyAwesomeAction(new CounterListener() {
                @Override
                public void counterChanged(int count) {
                    label.setText("Button has been pressed " + count + " times");
                }
            });

            setLayout(new GridBagLayout());
            GridBagConstraints gbc = new GridBagConstraints();
            gbc.gridwidth = GridBagConstraints.REMAINDER;

            JButton button = new JButton(action);
            add(button, gbc);
            add(label, gbc);

            InputMap im = getInputMap(WHEN_IN_FOCUSED_WINDOW);
            ActionMap am = getActionMap();

            im.put(KeyStroke.getKeyStroke(KeyEvent.VK_RIGHT, 0), "MakeItSo");
            am.put("MakeItSo", action);
        }

    }

    public interface CounterListener {

        public void counterChanged(int count);
    }

    public class MyAwesomeAction extends AbstractAction {

        private int count;
        private CounterListener listener;

        public MyAwesomeAction(CounterListener listener) {
            putValue(NAME, "Make it so");
            this.listener = listener;
        }

        @Override
        public void actionPerformed(ActionEvent e) {
            count++;
            listener.counterChanged(count);
        }

    }

}

Answer 2

这个例子有一个 JButton 和两个侦听器：一个 ActionListener 和一个 KeyListener。关键侦听器是通过 KeyAdapter 抽象 class 实现的。来自 API 文档：

KeyAdapter is for receiving keyboard events. The methods in this class are empty. This class exists as convenience for creating listener objects.

Create a listener object using the extended class and then register it with a component using the component's addKeyListener method. When a key is pressed, released, or typed, the relevant method in the listener object is invoked, and the KeyEvent is passed to it.

示例代码：

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
public class ButtonListeners {
    private JLabel label;
    private int counter;
    public static void main(String [] args) {
        new ButtonListeners().gui();
    }
    private void gui() {
        JFrame frame = new JFrame();
        frame.setTitle("JButton Listeners");
        JButton button = new JButton("jButton1");
        button.addActionListener(actionEvent -> displayLabel());
        button.addKeyListener(new ButtonKeyPressListener());
        label = new JLabel("Press button or -> key");
        frame.add(button, BorderLayout.SOUTH);
        frame.add(label, BorderLayout.CENTER);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setLocationRelativeTo(null);
        frame.setSize(300, 150);        
        frame.setVisible(true);
    }
    private void displayLabel() {
        label.setText("Action count: " + ++counter);
    }
    private class ButtonKeyPressListener extends KeyAdapter {
        @Override public void keyPressed(KeyEvent e) {
            if (e.getKeyCode() == KeyEvent.VK_RIGHT){
                displayLabel();
            }
        }
    }
}

JButton KeyPressed - 没有任何反应

JButton KeyPressed - Nothing Happens

java

swing

keyevent

keylistener

jbutton