仅在鼠标悬停时显示滚动条

Question

我只想在鼠标悬停时在 JScrollPane 中显示滚动条。我尝试使用下面显示的方法将 MouseAdapter 作为 MouseListener 添加到 JScrollbar，但效果不佳。当鼠标移到滚动条上方时，滚动条会闪烁。

有什么建议吗？

@Override
public void mouseEntered(java.awt.event.MouseEvent evt) {

    Runnable runner = new Runnable()
    {
        public void run() {

            JScrollPane sp = (JScrollPane) evt.getSource();
            sp.setVerticalScrollBarPolicy(JScrollPane.VERTICAL_SCROLLBAR_ALWAYS);
            sp.setHorizontalScrollBarPolicy(JScrollPane.HORIZONTAL_SCROLLBAR_ALWAYS);


        }

    };   

    Thread t = new Thread(runner, "Enter Thread");
    t.start();

}
@Override
public void mouseExited(java.awt.event.MouseEvent evt) {

    Runnable runner = new Runnable()
    {
        public void run() {

            JScrollPane sp = (JScrollPane) evt.getSource();
            sp.setVerticalScrollBarPolicy(JScrollPane.VERTICAL_SCROLLBAR_NEVER);
            sp.setHorizontalScrollBarPolicy(JScrollPane.HORIZONTAL_SCROLLBAR_NEVER);

        }

    }; 
    Thread t = new Thread(runner, "Exit Thread");
    t.start();}

Answer 1

• 在这种情况下，使用MouseListener 的方式很麻烦，所以我不太推荐它。
  • 例如，鼠标光标在JScrollBar 上闪烁。
• 您也许可以使用JLayer：
  • Responding to Events - How to Decorate Components with the JLayer Class (The Java™ Tutorials > Creating a GUI With JFC/Swing > Using Other Swing Features)

import java.awt.*;
import java.awt.event.*;
import java.util.Collections;
import javax.swing.*;
import javax.swing.plaf.LayerUI;

public class ScrollPaneMouseOverTest {
  public JComponent makeUI() {
    String text = String.join("\n", Collections.nCopies(100, "aaaaa"));
    JTextArea ta = new JTextArea(
        "Mouse cursor flickers over the JScrollBar.\n" + text);
    ta.addMouseListener(new MouseAdapter() {
      @Override public void mouseEntered(MouseEvent e) {
        JScrollPane sp = (JScrollPane) SwingUtilities.getAncestorOfClass(
            JScrollPane.class, (Component) e.getSource());
        sp.setVerticalScrollBarPolicy(JScrollPane.VERTICAL_SCROLLBAR_ALWAYS);
      }
      @Override public void mouseExited(MouseEvent e) {
        JScrollPane sp = (JScrollPane) SwingUtilities.getAncestorOfClass(
            JScrollPane.class, (Component) e.getSource());
        sp.setVerticalScrollBarPolicy(JScrollPane.VERTICAL_SCROLLBAR_NEVER);
      }
    });

    JScrollPane scroll = new JScrollPane(new JTextArea(text));
    scroll.setVerticalScrollBarPolicy(JScrollPane.VERTICAL_SCROLLBAR_NEVER);

    JPanel p = new JPanel(new GridLayout(2, 1));
    p.add(new JScrollPane(ta));
    p.add(new JLayer<>(scroll, new LayerUI<JScrollPane>() {
      @Override public void installUI(JComponent c) {
        super.installUI(c);
        if (c instanceof JLayer) {
          ((JLayer) c).setLayerEventMask(AWTEvent.MOUSE_EVENT_MASK);
        }
      }
      @Override public void uninstallUI(JComponent c) {
        if (c instanceof JLayer) {
          ((JLayer) c).setLayerEventMask(0);
        }
        super.uninstallUI(c);
      }
      @Override protected void processMouseEvent(
            MouseEvent e, JLayer<? extends JScrollPane> l) {
        JScrollPane sp = l.getView();
        switch (e.getID()) {
          case MouseEvent.MOUSE_ENTERED:
            sp.setVerticalScrollBarPolicy(JScrollPane.VERTICAL_SCROLLBAR_ALWAYS);
            break;
          case MouseEvent.MOUSE_EXITED:
            sp.setVerticalScrollBarPolicy(JScrollPane.VERTICAL_SCROLLBAR_NEVER);
            break;
          default:
            break;
        }
        //super.processMouseEvent(e, l);
      }
    }));
    return p;
  }
  public static void main(String[] args) {
    EventQueue.invokeLater(() -> {
      JFrame f = new JFrame();
      f.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
      f.getContentPane().add(new ScrollPaneMouseOverTest().makeUI());
      f.setSize(320, 240);
      f.setLocationRelativeTo(null);
      f.setVisible(true);
    });
  }
}

Answer 2

我已编辑我的答案以包含以下评论。

    JScrollPane scrollPane = new JScrollPane();
    scrollPane.setPreferredSize(new Dimension(600, 600));
    scrollPane.addMouseListener(new MouseAdapter() {
        @Override
        public void mouseEntered(final java.awt.event.MouseEvent evt) {

            EventQueue.invokeLater(new Runnable() {
                @Override
                public void run() {
                    JScrollPane sp = (JScrollPane) evt.getSource();
                    sp.setVerticalScrollBarPolicy(JScrollPane.VERTICAL_SCROLLBAR_ALWAYS);
                    sp.setHorizontalScrollBarPolicy(JScrollPane.HORIZONTAL_SCROLLBAR_ALWAYS);
                }
            });

        }

        @Override
        public void mouseExited(final java.awt.event.MouseEvent evt) {

            EventQueue.invokeLater(new Runnable() {

                @Override
                public void run() {

                    JScrollPane sp = (JScrollPane) evt.getSource();
                    sp.setVerticalScrollBarPolicy(JScrollPane.VERTICAL_SCROLLBAR_NEVER);
                    sp.setHorizontalScrollBarPolicy(JScrollPane.HORIZONTAL_SCROLLBAR_NEVER);

                }
            });
        }
    });

    JPanel panel = new JPanel(new BorderLayout());
    add(panel);
    JTextArea ta = new JTextArea();
    ta.setPreferredSize(new Dimension(500, 500));
    scrollPane.add(ta);
    panel.add(scrollPane);