Re: Drag & Drop

[Pete Wyckoff]

Niels Hilbrink writes:
> I have been fiddling unsuccessfully with the Drag and Drop (dnd)
> features in Java.

Me too.  No fix here, but a simpler test case that points to a problem
with Swing.  The code below only works if class Source extends TextArea,
but not JTextArea  (search for "blows").  The target of the drop can
be a Swing component with no errors.

Wandering through the source shows that native function
Java_sun_awt_motif_MDragSourceContextPeer_startDrag
returns null when it gets a NULL XEvent from the passed-in trigger
object (awt_XmDnD.c:843 unmodified jdk1.2 src).  No idea why this is
different for JComponents versus Components.

                -- Pete

/*
 * Test DnD.
 */
import java.awt.*;
import java.awt.dnd.*;
import java.awt.datatransfer.*;
import java.awt.event.*;
import java.io.*;
import javax.swing.*;

class Target extends JTextArea implements DropTargetListener {
    private DropTarget target;

    public Target(String text) {
        super(text);
        target = new DropTarget(this, DnDConstants.ACTION_COPY, this);
        setDropTarget(target);
    }

    // Interface DropTargetListner
    public void dragEnter(DropTargetDragEvent dtde) {
        System.out.println("Target: dragEnter");

        if (dtde.isDataFlavorSupported(DataFlavor.plainTextFlavor))
            dtde.acceptDrag(DnDConstants.ACTION_COPY);
        else {
            dtde.rejectDrag();
        }
    }

    public void dragOver(DropTargetDragEvent dtde) {
        System.out.println("Target: dragOver");
    }

    public void dropActionChanged(DropTargetDragEvent dtde) {
        System.out.println("Target: dropActionChanged");
    }

    public void dragScroll(DropTargetDragEvent dtde) {
        System.out.println("Target: dragScroll");
    }

    public void dragExit(DropTargetEvent dte) {
        System.out.println("Target: dragExit");
    }

    public void drop(DropTargetDropEvent dtde) {
        System.out.println("Target: dropped");

        dtde.acceptDrop(DnDConstants.ACTION_COPY);
        StringReader r = null;

        try {
            r = (StringReader) dtde.getTransferable()
              .getTransferData(DataFlavor.plainTextFlavor);
        } catch (UnsupportedFlavorException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }

        StringBuffer sb = new StringBuffer();
        char[] buf = new char[64];
        int i;

        try {
            do {
                if ((i = r.read(buf)) > 0)
                    sb.append(new String(buf, 0, i));
            } while (i >= 0);
        } catch (Exception e) {
            e.printStackTrace();
        }

        setText(sb.toString());
        target.getDropTargetContext().dropComplete(true);
    }
}

/*
 * Source can't be JTextArea or it blows
 */
class Source extends TextArea
  implements DragSourceListener, DragGestureListener {

    private DragSource source;

    public Source(String text) {
        super(text);
        source = DragSource.getDefaultDragSource();
        source.createDefaultDragGestureRecognizer(this,
          DnDConstants.ACTION_COPY, this);
    }

    // Interface DragGestureListener
    public void dragGestureRecognized(DragGestureEvent dge) {
        System.out.println("Source: dragGestureRecognized");
        try {
            source.startDrag(dge, DragSource.DefaultCopyDrop,
                null, new Point(0,0), new StringSelection(getText()), this);
        } catch (InvalidDnDOperationException e) {
            System.err.println("Source.dragGestureRecognized: " + e);
            e.printStackTrace();
        }
    }

    // Interface DragSourceListener
    public void dragEnter(DragSourceDragEvent dsde) {
        System.out.println("Source: dragEnter");
    }

    public void dragOver(DragSourceDragEvent dsde) {
        System.out.println("Source: dragOver");
    }

    public void dropActionChanged(DragSourceDragEvent dsde) {
        System.out.println("Source: dropActionChanged");
    }

    public void dragGestureChanged(DragSourceDragEvent dsde) {
        System.out.println("Source: dragGestureChanged");
    }

    public void dragExit(DragSourceEvent dse) {
        System.out.println("Source: dragExit");
    }

    public void dragDropEnd(DragSourceDropEvent dsde) {
        System.out.println("Source: dragDropEnd");
    }
}

public class dnd extends JFrame {
    public dnd() {
        super("DnD test");
        Container c = getContentPane();
        c.setLayout(new GridLayout(2, 1));
        c.add(new Target("Target: drag the source here"));
        c.add(new Source("Source: drag this to the target"));
        addWindowListener(new WindowAdapter() {
            public void windowClosing(WindowEvent e) {
                System.exit(0);
            }
        });
    }
    public static void main(String args[]) {
        dnd f = new dnd();
        f.setSize(600, 400);
        f.setVisible(true);
        while (true) {
            try {
                Thread.sleep(1000 * 1000);
            } catch(InterruptedException e) { }
        }
    }
}


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