java - DnD - 为什么拖动的标签显示在其他组件下方？

Question

我编写了一个基本的 DnD 程序，其中有四个连续的 JLabel。我发觉到 当我向左拖动一个标签时，它会显示在下一个标签的下方。然而， 当我向右拖动标签时，它显示在下一个标签上方。 标签按从左到右的顺序添加。所以拖动的标签正在 显示在被拖动标签之前添加的其他标签下方，并显示 在拖动标签之后添加的其他标签上方。 谁能解释为什么会这样？任何人都可以提供修复程序，以便拖动的标签 显示在其他标签上方？ 谢谢。

源代码:

public class LabelDnd extends JPanel {

    private JLabel[] labels;
    private Color[] colors = { Color.BLUE, Color.BLACK, Color.RED, Color.MAGENTA };

    public LabelDnd() {
        super();
        this.setLayout(new FlowLayout(FlowLayout.CENTER, 0, 0));
        this.setBackground(Color.WHITE);
        this.setBorder(BorderFactory.createEmptyBorder(4, 4, 4, 4));

        JPanel basePanel = new JPanel();
        basePanel.setLayout(new GridLayout(1, 4, 4, 4));
        basePanel.setBackground(Color.CYAN);

        MouseAdapter listener = new MouseAdapter() {

            Point p = null;

            @Override
            public void mousePressed(MouseEvent e) {
              p = e.getLocationOnScreen();
            }

            @Override
            public void mouseDragged(MouseEvent e) {
              JComponent c = (JComponent) e.getSource();
              Point l = c.getLocation();
              Point here = e.getLocationOnScreen();
              c.setLocation(l.x + here.x - p.x, l.y + here.y - p.y);
              p = here;
            }
          };

        this.labels = new JLabel[4];

        for (int i = 0; i < this.labels.length; i++) {
            this.labels[i] = new JLabel(String.valueOf(i), JLabel.CENTER);
            this.labels[i].setOpaque(true);
            this.labels[i].setPreferredSize(new Dimension(100, 100));
            this.labels[i].setBackground(this.colors[i]);
            this.labels[i].setForeground(Color.WHITE);
            this.labels[i].addMouseListener(listener);
            this.labels[i].addMouseMotionListener(listener);
            basePanel.add(this.labels[i]);
        }

        this.add(basePanel);
    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                JFrame frame = new JFrame("LabelDnd");
                frame.getContentPane().setLayout(new BorderLayout());

                LabelDnd panel = new LabelDnd();

                frame.getContentPane().add(panel, BorderLayout.CENTER);

                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

                frame.pack();
                frame.setLocationRelativeTo(null);
                frame.setVisible(true);
            }
        });
    }
}

Answer 1

您不会更改标签在代码中添加或绘制的顺序。考虑在拖动时将 JLabel 提升到玻璃面板(在将其从主容器中移除后)，然后在释放鼠标时将其重新添加到主容器。

例如:

import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Component;
import java.awt.Container;
import java.awt.Dimension;
import java.awt.GridLayout;
import java.awt.Point;
import java.awt.event.*;
import java.util.Comparator;
import java.util.PriorityQueue;

import javax.swing.*;

@SuppressWarnings("serial")
public class LabelDnd extends JPanel {
   private static final String X_POS = "x position";
   private JLabel[] labels;
   private Color[] colors = { Color.BLUE, Color.BLACK, Color.RED, Color.MAGENTA };

   public LabelDnd() {
      super();
      // this.setLayout(new FlowLayout(FlowLayout.CENTER, 0, 0));
      this.setBackground(Color.WHITE);
      this.setBorder(BorderFactory.createEmptyBorder(4, 4, 4, 4));

      JPanel basePanel = new JPanel();
      basePanel.setLayout(new GridLayout(1, 4, 4, 4));
      basePanel.setBackground(Color.CYAN);

      MouseAdapter listener = new MouseAdapter() {
         Point loc = null;
         Container parentContainer = null;
         Container glasspane = null;
         JLabel placeHolder = new JLabel("");
         private Point glassLocOnScn;

         @Override
         public void mousePressed(MouseEvent e) {
            JComponent selectedLabel = (JComponent) e.getSource();

            loc = e.getPoint();
            Point currLocOnScn = e.getLocationOnScreen();

            parentContainer = selectedLabel.getParent();

            JRootPane rootPane = SwingUtilities.getRootPane(selectedLabel);
            glasspane = (Container) rootPane.getGlassPane();
            glasspane.setVisible(true);
            glasspane.setLayout(null);
            glassLocOnScn = glasspane.getLocationOnScreen();

            Component[] comps = parentContainer.getComponents();

            // remove all labels from parent
            parentContainer.removeAll();

            // add labels back except for selected one
            for (Component comp : comps) {
               if (comp != selectedLabel) {
                  parentContainer.add(comp);
               } else {
                  // add placeholder in place of selected component
                  parentContainer.add(placeHolder);
               }
            }

            selectedLabel.setLocation(currLocOnScn.x - loc.x - glassLocOnScn.x,
                  currLocOnScn.y - loc.y - glassLocOnScn.y);
            glasspane.add(selectedLabel);
            glasspane.setVisible(true);

            parentContainer.revalidate();
            parentContainer.repaint();

         }

         @Override
         public void mouseDragged(MouseEvent e) {
            JComponent selectedLabel = (JComponent) e.getSource();

            glassLocOnScn = glasspane.getLocationOnScreen();
            Point currLocOnScn = e.getLocationOnScreen();
            selectedLabel.setLocation(currLocOnScn.x - loc.x - glassLocOnScn.x,
                  currLocOnScn.y - loc.y - glassLocOnScn.y);

            glasspane.repaint();
         }

         @Override
         public void mouseReleased(MouseEvent e) {
            JComponent selectedLabel = (JComponent) e.getSource();

            if (parentContainer == null || glasspane == null) {
               return;
            }

            // sort the labels based on their x position on screen
            PriorityQueue<JComponent> compQueue = new PriorityQueue<JComponent>(
                  4, new Comparator<JComponent>() {

                     @Override
                     public int compare(JComponent o1, JComponent o2) {
                        // sort of a kludge -- checking a client property that
                        // holds the x-position 
                        Integer i1 = (Integer) o1.getClientProperty(X_POS);
                        Integer i2 = (Integer) o2.getClientProperty(X_POS);
                        return i1.compareTo(i2);
                     }
                  });

            // sort of a kludge -- putting x position before removing component
            // into a client property to associate it with the JLabel
            selectedLabel.putClientProperty(X_POS, selectedLabel.getLocationOnScreen().x);
            glasspane.remove(selectedLabel);
            compQueue.add(selectedLabel);
            Component[] comps = parentContainer.getComponents();
            for (Component comp : comps) {
               JLabel label = (JLabel) comp;
               if (!label.getText().trim().isEmpty()) { // if placeholder!
                  label.putClientProperty(X_POS, label.getLocationOnScreen().x);
                  compQueue.add(label); // add to queue
               }
            }
            parentContainer.removeAll();

            // add back labels sorted by x-position on screen
            while (compQueue.size() > 0) {
               parentContainer.add(compQueue.remove());
            }

            parentContainer.revalidate();
            parentContainer.repaint();
            glasspane.repaint();

         }
      };

      this.labels = new JLabel[4];

      for (int i = 0; i < this.labels.length; i++) {
         this.labels[i] = new JLabel(String.valueOf(i), JLabel.CENTER);
         this.labels[i].setOpaque(true);
         this.labels[i].setPreferredSize(new Dimension(100, 100));
         this.labels[i].setBackground(this.colors[i]);
         this.labels[i].setForeground(Color.WHITE);
         this.labels[i].addMouseListener(listener);
         this.labels[i].addMouseMotionListener(listener);
         basePanel.add(this.labels[i]);
      }

      this.add(basePanel);
   }

   public static void main(String[] args) {
      SwingUtilities.invokeLater(new Runnable() {
         public void run() {
            JFrame frame = new JFrame("LabelDnd");
            frame.getContentPane().setLayout(new BorderLayout());

            LabelDnd panel = new LabelDnd();

            frame.getContentPane().add(panel, BorderLayout.CENTER);

            frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

            frame.pack();
            frame.setLocationRelativeTo(null);
            frame.setVisible(true);
         }
      });
   }
}