JLabels, that store ImageIcons, are returned back

问题:

I've added this listener to a JLabel, and I am able to drag the image around perfectly, However, as soon as I click on the panel(in an area where a JLabel is not present) the label returns back to it's original location. I can't figure out why that would happen. Please help me out, I've spent hours working on this. Thanks!

public class CardLabelListener extends MouseAdapter {

    private MouseEvent initiateEvent = null;

    @Override
    public void mouseReleased(MouseEvent me) {
        System.err.println("mouse release");
        int dx = me.getX() - initiateEvent.getX();
        int dy = me.getY() - initiateEvent.getY();
        if (Math.abs(dx) > 5 || Math.abs(dy) > 5) {
            Rectangle oldBound = me.getComponent().getBounds();
            int newX = oldBound.x + dx;
            int newY = oldBound.y + dy;
            me.getComponent().setBounds(newX, newY, oldBound.width, oldBound.height);
        }
        initiateEvent = null;
    }

    public void mousePressed(MouseEvent me) {
        GreetingCard.setBackground.findComponentAt(me.getX(), me.getY());
        System.err.println("mouse pressed");
        initiateEvent = me;
        me.consume();
    }

    public void mouseDragged(MouseEvent me) {
        System.err.println(me.getSource());
        if (initiateEvent == null) return;

        me.consume();
        JComponent jc = (JComponent) me.getSource();
        TransferHandler handler = jc.getTransferHandler();
        handler.exportAsDrag(jc, me, TransferHandler.MOVE);

        initiateEvent = null;
    }
}

回答1:

Firstly, you're not dragging the icon, your dragging the component itself.

Secondly, you're fighting against the layout manager which contains the label component. When ever the panel is invalidated, the labels are returned to their original layout positions.

Trying using something like a JLayeredPane to put your labels in

UPDATED with example based on feedback

public class MoveMe {

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

    public MoveMe() {
        EventQueue.invokeLater(new Runnable() {
            @Override
            public void run() {
                try {
                    UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
                } catch (ClassNotFoundException ex) {
                } catch (InstantiationException ex) {
                } catch (IllegalAccessException ex) {
                } catch (UnsupportedLookAndFeelException ex) {
                }

                JFrame frame = new JFrame();
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                frame.setLayout(new BorderLayout());
                frame.add(new MoveMePane());
                frame.pack();
                frame.setLocationRelativeTo(null);
                frame.setVisible(true);
            }
        });
    }

    public class MoveMePane extends JLayeredPane {

        public MoveMePane() {
            int width = 400;
            int height = 400;
            for (int index = 0; index < 10; index++) {
                String text = "Label " + index;
                JLabel label = new JLabel(text);
                label.setSize(label.getPreferredSize());

                int x = (int) Math.round(Math.random() * width);
                int y = (int) Math.round(Math.random() * height);
                if (x + label.getWidth() > width) {
                    x = width - label.getWidth();
                }
                if (y + label.getHeight() > width) {
                    y = width - label.getHeight();
                }
                label.setLocation(x, y);
                add(label);
            }

            MoveMeMouseHandler handler = new MoveMeMouseHandler();
            addMouseListener(handler);
            addMouseMotionListener(handler);
        }

        @Override
        public Dimension getPreferredSize() {
            return new Dimension(400, 400);
        }
    }

    public class MoveMeMouseHandler extends MouseAdapter {

        private int xOffset;
        private int yOffset;
        private JLabel draggy;
        private String oldText;

        @Override
        public void mouseReleased(MouseEvent me) {
            if (draggy != null) {
                draggy.setText(oldText);
                draggy.setSize(draggy.getPreferredSize());
                draggy = null;
            }
        }

        public void mousePressed(MouseEvent me) {
            JComponent comp = (JComponent) me.getComponent();
            Component child = comp.findComponentAt(me.getPoint());
            if (child instanceof JLabel) {
                xOffset = me.getX() - child.getX();
                yOffset = me.getY() - child.getY();

                draggy = (JLabel) child;
                oldText = draggy.getText();
                draggy.setText("What a drag");
                draggy.setSize(draggy.getPreferredSize());
            }
        }

        public void mouseDragged(MouseEvent me) {
            if (draggy != null) {
                draggy.setLocation(me.getX() - xOffset, me.getY() - yOffset);
            }
        }
    }
}

回答2:

For this to work you would need use a null layout on the container where the JLabel is located, otherwise the position will reset to that where the current layout manager positions the label.

A better approach would be to use DragLayout

DragLayout was designed to replace a null layout. It will respect the location of a component. By default it will use the preferred size of the component to determines its size. Finally, it will automatically calculate the preferred size of the Container.