Swing/RearrangeOrderOfPanels のバックアップ(No.1)

class RearrangingHandler extends MouseAdapter {
  private static final int xoffset = 16;
  private static final Rectangle R1 = new Rectangle();
  private static final Rectangle R2 = new Rectangle();
  private static Rectangle prevRect;
  private final int gestureMotionThreshold = DragSource.getDragThreshold();
  private final JWindow window = new JWindow();
  private int index = -1;
  private Component draggingComonent;
  private Component gap;
  private Point startPt;
  private Point dragOffset;

  public RearrangingHandler() {
    super();
    window.setBackground(new Color(0, true));
  }
  @Override public void mousePressed(MouseEvent e) {
    if (((JComponent) e.getComponent()).getComponentCount() <= 1) {
      startPt = null;
    } else {
      startPt = e.getPoint();
    }
  }
  private void startDragging(JComponent parent, Point pt) {
    Component c = parent.getComponentAt(pt);
    index = parent.getComponentZOrder(c);
    if (Objects.equals(c, parent) || index < 0) {
      return;
    }
    draggingComonent = c;
    Dimension d = draggingComonent.getSize();

    Point dp = draggingComonent.getLocation();
    dragOffset = new Point(pt.x - dp.x, pt.y - dp.y);

    gap = Box.createRigidArea(d);
    swapComponentLocation(parent, c, gap, index);

    window.add(draggingComonent);
    //window.setSize(d);
    window.pack();

    updateWindowLocation(pt, parent);
    window.setVisible(true);
  }
  private void updateWindowLocation(Point pt, JComponent parent) {
    Point p = new Point(pt.x - dragOffset.x, pt.y - dragOffset.y);
    SwingUtilities.convertPointToScreen(p, parent);
    window.setLocation(p);
  }
  private static int getTargetIndex(Rectangle r, Point pt, int i) {
    int ht2 = (int)(.5 + r.height * .5);
    R1.setBounds(r.x, r.y,       r.width, ht2);
    R2.setBounds(r.x, r.y + ht2, r.width, ht2);
    if (R1.contains(pt)) {
      prevRect = R1;
      return i - 1 > 0 ? i : 0;
    } else if (R2.contains(pt)) {
      prevRect = R2;
      return i;
    }
    return -1;
  }
  private static void swapComponentLocation(
      Container parent, Component remove, Component add, int idx) {
    parent.remove(remove);
    parent.add(add, idx);
    parent.revalidate();
    parent.repaint();
  }
  @Override public void mouseDragged(MouseEvent e) {
    Point pt = e.getPoint();
    JComponent parent = (JComponent) e.getComponent();
    double a = Math.pow(pt.x - startPt.x, 2);
    double b = Math.pow(pt.y - startPt.y, 2);
    if (draggingComonent == null &&
        Math.sqrt(a + b) > gestureMotionThreshold) {
      startDragging(parent, pt);
      return;
    }
    if (!window.isVisible() || draggingComonent == null) {
      return;
    }
    updateWindowLocation(pt, parent);
    if (prevRect != null && prevRect.contains(pt)) {
      return;
    }

    for (int i = 0; i < parent.getComponentCount(); i++) {
      Component c = parent.getComponent(i);
      Rectangle r = c.getBounds();
      if (Objects.equals(c, gap) && r.contains(pt)) {
        return;
      }
      int tgt = getTargetIndex(r, pt, i);
      if (tgt >= 0) {
        swapComponentLocation(parent, gap, gap, tgt);
        return;
      }
    }
    //System.out.println("outer");
    parent.remove(gap);
    parent.revalidate();
  }

  @Override public void mouseReleased(MouseEvent e) {
    startPt = null;
    if (!window.isVisible() || draggingComonent == null) {
      return;
    }
    Point pt = e.getPoint();
    JComponent parent = (JComponent) e.getComponent();

    Component cmp = draggingComonent;
    draggingComonent = null;
    prevRect = null;
    startPt = null;
    dragOffset = null;
    window.setVisible(false);

    for (int i = 0; i < parent.getComponentCount(); i++) {
      Component c = parent.getComponent(i);
      if (Objects.equals(c, gap)) {
        swapComponentLocation(parent, gap, cmp, i);
        return;
      }
      int tgt = getTargetIndex(c.getBounds(), pt, i);
      if (tgt >= 0) {
        swapComponentLocation(parent, gap, cmp, tgt);
        return;
      }
    }
    if (parent.getParent().getBounds().contains(pt)) {
      swapComponentLocation(parent, gap, cmp, parent.getComponentCount());
    } else {
      swapComponentLocation(parent, gap, cmp, index);
    }
  }
}