拖动将禁用其他MouseMotionListener

Question

我有两个组件，每个组件都有自己的MouseMotionListener。当我在拖动第一个组件的同时将鼠标从第一个组件移动到第二个组件时，第二个组件的MouseMotionListener似乎被禁用，即，尽管我将鼠标移到第二个组件上，但mouseMoved根本没有被调用。我如何避免这种“致残”？

例子：

import java.awt.Color;
import java.awt.GridLayout;
import java.awt.event.MouseEvent;
import java.awt.event.MouseMotionListener;
import javax.swing.JFrame;
import javax.swing.JPanel;

public class DragTest implements MouseMotionListener {

    private static JPanel p1 = new JPanel();
    private static JPanel p2 = new JPanel();

    public DragTest() {

    }

    public static void main(String[] args) {
        p1.setBackground(Color.RED);
        p1.addMouseMotionListener(new DragTest());
        p2.setBackground(Color.BLUE);
        p2.addMouseMotionListener(new DragTest());

        JFrame frame = new JFrame("Test");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setBounds(100, 100, 600, 300);
        frame.setLayout(new GridLayout(1, 2));

        frame.add(p1);
        frame.add(p2);

        frame.setVisible(true);
    }

    @Override
    public void mouseMoved(MouseEvent e) {
        if (e.getSource() == p1) {
            System.out.println("mouse movement in p1");
        } else if (e.getSource() == p2) {
            System.out.println("mouse movement in p2");
        }
    }

    @Override
    public void mouseDragged(MouseEvent e) {
        if (e.getSource() == p1) {
            System.out.println("mouse drag in p1");
        } else if (e.getSource() == p2) {
            System.out.println("mouse drag in p2");
        }
    }
}

Answer 1

免责声明：这是core drag'n'drop API的一个非常基本的示例，基于此示例、此示例和此示例，旨在简单演示确定指定放置位置的可能性

因此，在拖动东西时，尤其是跨组件拖动时，通常最好使用拖放API，而不是简单地使用MouseMotionListener和MouseListener。这就是API的设计目的，并向两个目标提供有关操作性质的通知

下面的示例简单地存储了您放置某物的位置，并在那里绘制了一个漂亮的小点

import java.awt.Color;
import java.awt.Dimension;
import java.awt.EventQueue;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.GridBagLayout;
import java.awt.GridLayout;
import java.awt.Point;
import java.awt.datatransfer.DataFlavor;
import java.awt.datatransfer.Transferable;
import java.awt.datatransfer.UnsupportedFlavorException;
import java.awt.dnd.DnDConstants;
import java.awt.dnd.DragGestureEvent;
import java.awt.dnd.DragGestureListener;
import java.awt.dnd.DragSource;
import java.awt.dnd.DragSourceDragEvent;
import java.awt.dnd.DragSourceDropEvent;
import java.awt.dnd.DragSourceEvent;
import java.awt.dnd.DragSourceListener;
import java.awt.dnd.DropTarget;
import java.awt.dnd.DropTargetDragEvent;
import java.awt.dnd.DropTargetDropEvent;
import java.awt.dnd.DropTargetEvent;
import java.awt.dnd.DropTargetListener;
import java.io.IOException;
import java.util.ArrayList;
import java.util.List;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.UIManager;
import javax.swing.UnsupportedLookAndFeelException;
import javax.swing.border.LineBorder;
import javax.swing.border.MatteBorder;

public class DragAndDropTest {

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

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

                JFrame frame = new JFrame("Testing");
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                frame.add(new TestPane());
                frame.pack();
                frame.setLocationRelativeTo(null);
                frame.setVisible(true);
            }
        });
    }

    public class TestPane extends JPanel {

        public TestPane() {
            setLayout(new GridLayout(1, 2));
            add(new DropPane());
            add(new DragPane());
        }

        @Override
        public Dimension getPreferredSize() {
            return new Dimension(200, 200);
        }

        protected void paintComponent(Graphics g) {
            super.paintComponent(g);
            Graphics2D g2d = (Graphics2D) g.create();
            g2d.dispose();
        }

    }

    public class DragPane extends JPanel {

        private DragSource ds;
        private Transferable transferable;

        public DragPane() {
            ds = new DragSource();
            transferable = new Transferable() {

                @Override
                public DataFlavor[] getTransferDataFlavors() {
                    return new DataFlavor[]{DataFlavor.stringFlavor};
                }

                @Override
                public boolean isDataFlavorSupported(DataFlavor flavor) {
                    return DataFlavor.stringFlavor.equals(flavor);
                }

                @Override
                public Object getTransferData(DataFlavor flavor) throws UnsupportedFlavorException, IOException {
                    return "This is a test";
                }
            };
            ds.createDefaultDragGestureRecognizer(this, DnDConstants.ACTION_COPY_OR_MOVE, new DragGestureListener() {
                @Override
                public void dragGestureRecognized(DragGestureEvent dge) {                   
                    // This is where you would export the data you want
                    // to transfer
                    ds.startDrag(dge, DragSource.DefaultCopyDrop, transferable, new DragSourceListener() {

                        @Override
                        public void dragEnter(DragSourceDragEvent dsde) {
                        }

                        @Override
                        public void dragOver(DragSourceDragEvent dsde) {
                        }

                        @Override
                        public void dropActionChanged(DragSourceDragEvent dsde) {
                        }

                        @Override
                        public void dragExit(DragSourceEvent dse) {
                        }

                        @Override
                        public void dragDropEnd(DragSourceDropEvent dsde) {
                        }

                    });
                }
            });

            setLayout(new GridBagLayout());
            add(new JLabel("Drag from here"));
            setBorder(new LineBorder(Color.RED));
        }

    }

    public class DropPane extends JPanel {

        private List<Point> dropPoints;

        public DropPane() {
            dropPoints = new ArrayList<>(25);
            setDropTarget(new DropTarget(this, new DropTargetListener() {

                @Override
                public void dragEnter(DropTargetDragEvent dtde) {
                }

                @Override
                public void dragOver(DropTargetDragEvent dtde) {
                }

                @Override
                public void dropActionChanged(DropTargetDragEvent dtde) {
                }

                @Override
                public void dragExit(DropTargetEvent dte) {
                }

                @Override
                public void drop(DropTargetDropEvent dtde) {
                    // Normally here, I'd inspect the Transferable and make sure
                    // what is been dropped and can be imported, I'd then go through
                    // the process of unwrapping the data from the Transferable and 
                    // processing it appropriatly, but in this example, I really don't
                    // care, I just care about WHERE the event occured
                    dropPoints.add(dtde.getLocation());
                    dtde.dropComplete(true);
                    repaint();
                }
            }));
            setLayout(new GridBagLayout());
            add(new JLabel("Drop to here"));
            setBorder(new MatteBorder(1, 1, 1, 0, Color.RED));
        }

        @Override
        protected void paintComponent(Graphics g) {
            super.paintComponent(g);
            g.setColor(Color.RED);
            for (Point p : dropPoints) {
                g.fillOval(p.x - 2, p.y - 2, 5, 5);
            }
        }

    }

}