A DESCRIPTION OF THE FIX :
The new Swing Drag'n'Drop API enhancements in Mustang are usefull but I find them to limiting when trying to use them with custom components. Here I propose a minimum addition to make them even more usefull.

Swing's TransferManager has 2 new methods that are called from SwingDropTarget instead of the old ones:

    public boolean canImport(TransferHandler.TransferSupport support);

    public boolean importData(TransferHandler.TransferSupport support);

By default they delegate to the old methods so that old programs still work. These methods and mechanisms underneath are very handy and useful for various d'n'd scenarios. I very much would like to use these mechanisms for implementing d'n'd in my custom components too.

There's also a new class called TransferHandler.DropLocation that has some specific subclasses for particular Swing components. The mechanism for creating a custom DropLocation and setting the "dropLocation" property on a Swing component is not exposed to custom subclasses of Swing components. If one wants to create a custom Swing component and use new TransferHandler's capabilities of managing the drop location, it is a no go. The particular methods in JComponent and some subclasses are package protected. There are even some reflection tricks to access the JTextComponent's package protected methods from other package.

I can understand that making these methods public and overridable could hurt compatibility since the names of the methods could already be used in subclasses arround the world. The tricks to use package private methods for standard Swing components is understandable. But why not allowing custom components to support drop location exchange?

I propose the following:

Add an interface to the javax.swing package (or as an inner interface of javax.swing.TransferHandler) that standard swing components would not implement but would be available as an optional interface for custom swing components:

public interface DropLocationSupport
{
   TransferHandler.DropLocation dropLocationForPoint(Point p);
   
   Object setDropLocation(TransferHandler.DropLocation location, Object state, boolean forDrop);
 
   void dndDone();
}


Then add to the TransferHandler.DropHandler some code to call these methods whenever a custom component implements this interface. For example in TransferHandler.DropHandler.setComponentDropLocation():

           if (component instanceof DropLocationSupport) {
                state = ((DropLocationSupport)component).setDropLocation(dropLocation, state, forDrop);
           }
           else if (component instanceof JTextComponent) {
                try {
                    AccessibleMethod method =
                        new AccessibleMethod(JTextComponent.class,
                                             "setDropLocation",
                                             DropLocation.class,
                                             Object.class,
                                             Boolean.TYPE);

                    state =
                        method.invokeNoChecked(component, dropLocation,
                                               state, forDrop);
                } catch (NoSuchMethodException e) {
                    throw new AssertionError(
                        "Couldn't locate method JTextComponet.setDropLocation");
                }
            } else if (component instanceof JComponent) {
                state = ((JComponent)component).setDropLocation(dropLocation, state, forDrop);
            }


That's one way of doing it. And the most straightforward I think. The other possibility would be to delegate drop location interchange to an intermediary that would be associated with:

possibility 1: a TransferHandler
possiblility 2: a Component


possibility 1: add an interface and a constructor to TransferHandler that would take an instance implementing this interface:

public interface DropLocationDelegate
{
   TransferHandler.DropLocation dropLocationForPoint(Component c, Point p);
   
   Object setDropLocation(Component c, TransferHandler.DropLocation location, Object state, boolean forDrop);
 
   void dndDone(Component c);
}


possibility 2: add a single public overridable method to JComponent:

   public DropLocationSupport getDropLocationSupport();

this method would return an instance implementing DropLocationSupport interface described above.


I think that possibility 2 is more suitable since it allows for overriding/delegating on the JComponent level.


Is any of these acceptable?

Regards, Peter


JUnit TESTCASE :
No code to demonstrate.