如何在Linux上拖放错误期间实现autoscroll的变通方法？

Question

I have a list with many elements inside a scroll pane and I've implemented drag and drop on the list. When I select an item from the list and drag it to the bottom of the list, the list should automatically scroll down as long as I keep the mouse close to the edge. This works ok on Windows, but on Linux the list scrolls one element and then stops.

Here is a short program which reveals this bug:

import java.awt.BorderLayout;
import java.awt.Container;
import java.awt.Dimension;
import java.awt.datatransfer.DataFlavor;
import java.awt.datatransfer.Transferable;
import java.awt.datatransfer.UnsupportedFlavorException;
import java.io.IOException;

import javax.swing.DropMode;
import javax.swing.JComponent;
import javax.swing.JFrame;
import javax.swing.JList;
import javax.swing.JPanel;
import javax.swing.JScrollPane;
import javax.swing.SwingUtilities;
import javax.swing.TransferHandler;
import javax.swing.WindowConstants;


public class JListAutoscroll {

    protected static Container createUI() {
        JList<String> jlist = new JList<>(generateData(100));
        setDragAndDrop(jlist);
        JScrollPane scrollPane = new JScrollPane(jlist);
        JPanel panel = new JPanel(new BorderLayout());
        panel.add(scrollPane, BorderLayout.CENTER);
        return panel;
    }

    private static void setDragAndDrop(JList<String> jlist) {
        jlist.setDragEnabled(true);
        jlist.setDropMode(DropMode.INSERT);
        jlist.setTransferHandler(new ListTransferHandler());
    }

    private static String[] generateData(int nRows) {
        String rows[] = new String[nRows];
        for (int i = 0; i < rows.length; i++) {
            rows[i] = "element " + i;
        }
        return rows;
    }

    private static class ListTransferHandler extends TransferHandler {

        @Override
        public int getSourceActions(JComponent component) {
            return COPY_OR_MOVE;
        }

        @Override
        protected Transferable createTransferable(JComponent component) {
            return new ListItemTransferable((JList)component);
        }

        @Override
        public boolean canImport(TransferHandler.TransferSupport support) {
            return true;
        }

        @Override
        public boolean importData(TransferHandler.TransferSupport support) {
            return true;
        }
    }

    private static class ListItemTransferable implements Transferable {

        private String item;

        public ListItemTransferable(JList<String> jlist) {
            item = jlist.getSelectedValue();
        }

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

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

        @Override
        public Object getTransferData(DataFlavor flavor) throws UnsupportedFlavorException, IOException {
            if(!isDataFlavorSupported(flavor)) {
                throw new UnsupportedFlavorException(flavor);
            }
            return item;
        }

    }

    public static void main(String args[]) {
        SwingUtilities.invokeLater(new Runnable() {

            @Override
            public void run() {
                JFrame frame = new JFrame("JList Autoscroll");
                frame.setDefaultCloseOperation(WindowConstants.DISPOSE_ON_CLOSE);
                frame.setContentPane(createUI());
                frame.setPreferredSize(new Dimension(400, 600));
                frame.pack();
                frame.setVisible(true);
            }

        });
    }

}

I've implemented a simple TransferHandler, which does nothing on drop, but is enough to show the problem while dragging to the edge of the list.

It seems this is a known bug in the JDK, which is best described in this report . I've seen some suggested workarounds, like this one or this one , but it's not clear to me how I can implement them. It looks to me like I have to create a DropTarget subclass and the component that I use with it should implement the Autoscroll interface. But JList does not implement it! Also, if I set a DropTarget on the list, instead of the TransferHandler, won't I lose all the default drag and drop behaviour implemented by the TransferHandler?

So how can I modify my program to workaround this bug?

Answer 1

As mentioned in the bug description , there are two classes that handle drag and drop:

• DropTargetAutoScroller , a member class of java.awt.dnd.DropTarget , responsible of supporting components implementing the Autoscroll interface;
• DropHandler , a member class of javax.swing.TransferHandler , that automates d&d autoscrolling on components implementing the Scrollable interface.

So, indeed, the workaround is not suitable for JList , which implements Scrollable and not Autoscroll . But, if you look in the source code for DropTarget and TransferHandler , you'll notice that the autoscroll code is basically the same, and in both cases wrong. The workaround is also very similar to the DropTarget code, with only a few lines added. Basically, the solution is to convert the location of the mouse cursor from the component coordinate system to the screen coordinate system. That way, when checking whether the mouse has moved, absolute coordinates are used. So we can copy the code from TransferHandler instead and add these few lines.

That's great... but where do we put this code and how do we get it called?

If we look in setTransferHandler() we see that it actually sets a DropTarget , which is a package-private static class called SwingDropTarget from the TransferHandler class. It delegates drag and drop events to a private static DropTargetListener called DropHandler . This class does all of the magic that happens during drag and drop, and of course it uses other private methods from TransferHandler . This means we can't just set our own DropTarget without losing everything already implemented in TransferHandler . We could rewrite TransferHandler (about 1800 lines) with our few lines added to fix the bug, but that's not very realistic.

A simpler solution is to write a DropTargetListener , in which we simply copy the autoscroll-related code from DropHandler (which also implements this interface), with our lines added. This is the class:

import java.awt.Insets;
import java.awt.Point;
import java.awt.Rectangle;
import java.awt.Toolkit;
import java.awt.dnd.DropTargetDragEvent;
import java.awt.dnd.DropTargetDropEvent;
import java.awt.dnd.DropTargetEvent;
import java.awt.dnd.DropTargetListener;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.util.TooManyListenersException;

import javax.swing.JComponent;
import javax.swing.Scrollable;
import javax.swing.SwingConstants;
import javax.swing.SwingUtilities;
import javax.swing.Timer;


public class AutoscrollWorkaround implements DropTargetListener, ActionListener {

    private JComponent component;

    private Point lastPosition;

    private Rectangle outer;
    private Rectangle inner;

    private Timer timer;
    private int hysteresis = 10;

    private static final int AUTOSCROLL_INSET = 10;

    public AutoscrollWorkaround(JComponent component) {
        if (!(component instanceof Scrollable)) {
            throw new IllegalArgumentException("Component must be Scrollable for autoscroll to work!");
        }
        this.component = component;
        outer = new Rectangle();
        inner = new Rectangle();

        Toolkit t = Toolkit.getDefaultToolkit();
        Integer prop;

        prop = (Integer)t.getDesktopProperty("DnD.Autoscroll.interval");
        timer = new Timer(prop == null ? 100 : prop.intValue(), this);

        prop = (Integer)t.getDesktopProperty("DnD.Autoscroll.initialDelay");
        timer.setInitialDelay(prop == null ? 100 : prop.intValue());

        prop = (Integer)t.getDesktopProperty("DnD.Autoscroll.cursorHysteresis");
        if (prop != null) {
            hysteresis = prop.intValue();
        }
    }

    @Override
    public void dragEnter(DropTargetDragEvent e) {
        lastPosition = e.getLocation();
        SwingUtilities.convertPointToScreen(lastPosition, component);
        updateRegion();
    }

    @Override
    public void dragOver(DropTargetDragEvent e) {
        Point p = e.getLocation();
        SwingUtilities.convertPointToScreen(p, component);

        if (Math.abs(p.x - lastPosition.x) > hysteresis
                || Math.abs(p.y - lastPosition.y) > hysteresis) {
            // no autoscroll
            if (timer.isRunning()) timer.stop();
        } else {
            if (!timer.isRunning()) timer.start();
        }

        lastPosition = p;
    }

    @Override
    public void dragExit(DropTargetEvent dte) {
        cleanup();
    }

    @Override
    public void drop(DropTargetDropEvent dtde) {
        cleanup();
    }

    @Override
    public void dropActionChanged(DropTargetDragEvent e) {
    }

    private void updateRegion() {
        // compute the outer
        Rectangle visible = component.getVisibleRect();
        outer.setBounds(visible.x, visible.y, visible.width, visible.height);

        // compute the insets
        Insets i = new Insets(0, 0, 0, 0);
        if (component instanceof Scrollable) {
            int minSize = 2 * AUTOSCROLL_INSET;

            if (visible.width >= minSize) {
                i.left = i.right = AUTOSCROLL_INSET;
            }

            if (visible.height >= minSize) {
                i.top = i.bottom = AUTOSCROLL_INSET;
            }
        }

        // set the inner from the insets
        inner.setBounds(visible.x + i.left,
                      visible.y + i.top,
                      visible.width - (i.left + i.right),
                      visible.height - (i.top  + i.bottom));
    }

    @Override
    public void actionPerformed(ActionEvent e) {
        updateRegion();
        Point componentPosition = new Point(lastPosition);
        SwingUtilities.convertPointFromScreen(componentPosition, component);
        if (outer.contains(componentPosition) && !inner.contains(componentPosition)) {
            autoscroll(componentPosition);
        }
    }

    private void autoscroll(Point position) {
        Scrollable s = (Scrollable) component;
        if (position.y < inner.y) {
            // scroll upward
            int dy = s.getScrollableUnitIncrement(outer, SwingConstants.VERTICAL, -1);
            Rectangle r = new Rectangle(inner.x, outer.y - dy, inner.width, dy);
            component.scrollRectToVisible(r);
        } else if (position.y > (inner.y + inner.height)) {
            // scroll downard
            int dy = s.getScrollableUnitIncrement(outer, SwingConstants.VERTICAL, 1);
            Rectangle r = new Rectangle(inner.x, outer.y + outer.height, inner.width, dy);
            component.scrollRectToVisible(r);
        }

        if (position.x < inner.x) {
            // scroll left
            int dx = s.getScrollableUnitIncrement(outer, SwingConstants.HORIZONTAL, -1);
            Rectangle r = new Rectangle(outer.x - dx, inner.y, dx, inner.height);
            component.scrollRectToVisible(r);
        } else if (position.x > (inner.x + inner.width)) {
            // scroll right
            int dx = s.getScrollableUnitIncrement(outer, SwingConstants.HORIZONTAL, 1);
            Rectangle r = new Rectangle(outer.x + outer.width, inner.y, dx, inner.height);
            component.scrollRectToVisible(r);
        }
    }

    private void cleanup() {
        timer.stop();
    }
}

(You'll notice that basically only the SwingUtilities.convertXYZ() calls are extra from the TransferHandler code)

Next, we can add this listener to the DropTarget installed when setting the TransferHandler . (Note that a regular DropTarget only accepts one listener and will throw an exception if another one is added. SwingDropTarget uses DropHandler , but fortunately it also adds support for other listeners as well)

So let's just add this static factory method to the AutoscrollWorkaround class, which does this for us:

    public static void applyTo(JComponent component) {
        if (component.getTransferHandler() == null) {
            throw new IllegalStateException("A TransferHandler must be set before calling this method!");
        }
        try {
            component.getDropTarget().addDropTargetListener(new AutoscrollWorkaround(component));
        } catch (TooManyListenersException e) {
            throw new IllegalStateException("Something went wrong! DropTarget should have been " +
                    "SwingDropTarget which accepts multiple listeners", e);
        }
    }

This provides an easy and very convenient way to apply the workaround to any component that suffers from this bug, by only calling this one method. Just make sure to call it after having setTransferHandler() on the component. So, we only have to add one line to the original program:

private static void setDragAndDrop(JList<String> jlist) {
    jlist.setDragEnabled(true);
    jlist.setDropMode(DropMode.INSERT);
    jlist.setTransferHandler(new ListTransferHandler());
    AutoscrollWorkaround.applyTo(jlist); // <--- just this line added
}

The autoscroll now works OK on both Windows and Linux. (Although on Linux the line for the drop location is not repainted until autoscroll works, but oh well.)

This workaround should work also for JTable (I tested), JTree and probably any components that implement Scrollable .