java/11 : java.desktop/java/awt/dnd/DragSourceContext.java

DragSourceContext
https://openjdk.java.net/
GPLv2 + Classpath Exception
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package java.awt.dnd;

import java.awt.AWTError;
import java.awt.Component;
import java.awt.Cursor;
import java.awt.Image;
import java.awt.Point;
import java.awt.Toolkit;
import java.awt.datatransfer.DataFlavor;
import java.awt.datatransfer.Transferable;
import java.awt.datatransfer.UnsupportedFlavorException;
import java.awt.dnd.peer.DragSourceContextPeer;
import java.io.IOException;
import java.io.InvalidObjectException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.Serializable;
import java.util.TooManyListenersException;

import sun.awt.AWTAccessor;
import sun.awt.ComponentFactory;

The DragSourceContext class is responsible for managing the initiator side of the Drag and Drop protocol. In particular, it is responsible for managing drag event notifications to the DragSourceListeners and DragSourceMotionListeners, and providing the Transferable representing the source data for the drag operation.  Note that the DragSourceContext itself implements the DragSourceListener and DragSourceMotionListener interfaces. This is to allow the platform peer (the DragSourceContextPeer instance) created by the DragSource to notify the DragSourceContext of state changes in the ongoing operation. This allows the DragSourceContext object to interpose itself between the platform and the listeners provided by the initiator of the drag operation. 

 By default, DragSourceContext sets the cursor as appropriate for the current state of the drag and drop operation. For example, if the user has chosen the move action, and the pointer is over a target that accepts the move action, the default move cursor is shown. When the pointer is over an area that does not accept the transfer, the default "no drop" cursor is shown. 
 This default handling mechanism is disabled when a custom cursor is set by the setCursor method. When the default handling is disabled, it becomes the responsibility of the developer to keep the cursor up to date, by listening to the DragSource events and calling the setCursor() method. Alternatively, you can provide custom cursor behavior by providing custom implementations of the DragSource and the DragSourceContext classes. 
See Also: DragSourceListener
DragSourceMotionListener
DnDConstants
Since: 1.2/**
 * The {@code DragSourceContext} class is responsible for managing the
 * initiator side of the Drag and Drop protocol. In particular, it is responsible
 * for managing drag event notifications to the
 * {@linkplain DragSourceListener DragSourceListeners}
 * and {@linkplain DragSourceMotionListener DragSourceMotionListeners}, and providing the
 * {@link Transferable} representing the source data for the drag operation.
 * <p>
 * Note that the {@code DragSourceContext} itself
 * implements the {@code DragSourceListener} and
 * {@code DragSourceMotionListener} interfaces.
 * This is to allow the platform peer
 * (the {@link DragSourceContextPeer} instance)
 * created by the {@link DragSource} to notify
 * the {@code DragSourceContext} of
 * state changes in the ongoing operation. This allows the
 * {@code DragSourceContext} object to interpose
 * itself between the platform and the
 * listeners provided by the initiator of the drag operation.
 * <p>
 * <a id="defaultCursor"></a>
 * By default, {@code DragSourceContext} sets the cursor as appropriate
 * for the current state of the drag and drop operation. For example, if
 * the user has chosen {@linkplain DnDConstants#ACTION_MOVE the move action},
 * and the pointer is over a target that accepts
 * the move action, the default move cursor is shown. When
 * the pointer is over an area that does not accept the transfer,
 * the default "no drop" cursor is shown.
 * <p>
 * This default handling mechanism is disabled when a custom cursor is set
 * by the {@link #setCursor} method. When the default handling is disabled,
 * it becomes the responsibility
 * of the developer to keep the cursor up to date, by listening
 * to the {@code DragSource} events and calling the {@code setCursor()} method.
 * Alternatively, you can provide custom cursor behavior by providing
 * custom implementations of the {@code DragSource}
 * and the {@code DragSourceContext} classes.
 *
 * @see DragSourceListener
 * @see DragSourceMotionListener
 * @see DnDConstants
 * @since 1.2
 */

public class DragSourceContext
    implements DragSourceListener, DragSourceMotionListener, Serializable {

    private static final long serialVersionUID = -115407898692194719L;

    // used by updateCurrentCursor

    An int used by updateCurrentCursor() indicating that the Cursor should change to the default (no drop) Cursor. /**
     * An {@code int} used by updateCurrentCursor()
     * indicating that the {@code Cursor} should change
     * to the default (no drop) {@code Cursor}.
     */
    protected static final int DEFAULT = 0;

    An int used by updateCurrentCursor() indicating that the Cursor has entered a DropTarget. /**
     * An {@code int} used by updateCurrentCursor()
     * indicating that the {@code Cursor}
     * has entered a {@code DropTarget}.
     */
    protected static final int ENTER   = 1;

    An int used by updateCurrentCursor() indicating that the Cursor is over a DropTarget. /**
     * An {@code int} used by updateCurrentCursor()
     * indicating that the {@code Cursor} is
     * over a {@code DropTarget}.
     */
    protected static final int OVER    = 2;

    An int used by updateCurrentCursor() indicating that the user operation has changed. /**
     * An {@code int} used by updateCurrentCursor()
     * indicating that the user operation has changed.
     */

    protected static final int CHANGED = 3;

    static {
        AWTAccessor.setDragSourceContextAccessor(dsc -> dsc.peer);
    }

    Called from DragSource, this constructor creates a new DragSourceContext given the DragSourceContextPeer for this Drag, the DragGestureEvent that triggered the Drag, the initial Cursor to use for the Drag, an (optional) Image to display while the Drag is taking place, the offset of the Image origin from the hotspot at the instant of the triggering event, the Transferable subject data, and the DragSourceListener to use during the Drag and Drop operation. 
 If DragSourceContextPeer is null NullPointerException is thrown. 
 If DragGestureEvent is null NullPointerException is thrown. 
 If Cursor is null no exception is thrown and the default drag cursor behavior is activated for this drag operation. 
 If Image is null no exception is thrown. 
 If Image is not null and the offset is null NullPointerException is thrown. 
 If Transferable is null NullPointerException is thrown. 
 If DragSourceListener is null no exception is thrown. 
Params: trigger –    the triggering event
dragCursor –  the initial Cursor for this drag operation or null for the default cursor handling; see class level documentation
                      for more details on the cursor handling mechanism during drag and drop
dragImage –  the Image to drag (or null)
offset –     the offset of the image origin from the hotspot at the
                  instant of the triggering event
t –  the Transferable
dsl –  the DragSourceListener
Throws: IllegalArgumentException – if the Component associated with the trigger event is null.
IllegalArgumentException – if the DragSource for the trigger event is null.
IllegalArgumentException – if the drag action for the trigger event is DnDConstants.ACTION_NONE.
IllegalArgumentException – if the source actions for the DragGestureRecognizer associated with the trigger event are equal to DnDConstants.ACTION_NONE.
NullPointerException – if dscp, trigger, or t are null, or
        if dragImage is non-null and offset is null/**
     * Called from {@code DragSource}, this constructor creates a new
     * {@code DragSourceContext} given the
     * {@code DragSourceContextPeer} for this Drag, the
     * {@code DragGestureEvent} that triggered the Drag, the initial
     * {@code Cursor} to use for the Drag, an (optional)
     * {@code Image} to display while the Drag is taking place, the offset
     * of the {@code Image} origin from the hotspot at the instant of the
     * triggering event, the {@code Transferable} subject data, and the
     * {@code DragSourceListener} to use during the Drag and Drop
     * operation.
     * <br>
     * If {@code DragSourceContextPeer} is {@code null}
     * {@code NullPointerException} is thrown.
     * <br>
     * If {@code DragGestureEvent} is {@code null}
     * {@code NullPointerException} is thrown.
     * <br>
     * If {@code Cursor} is {@code null} no exception is thrown and
     * the default drag cursor behavior is activated for this drag operation.
     * <br>
     * If {@code Image} is {@code null} no exception is thrown.
     * <br>
     * If {@code Image} is not {@code null} and the offset is
     * {@code null NullPointerException} is thrown.
     * <br>
     * If {@code Transferable} is {@code null}
     * {@code NullPointerException} is thrown.
     * <br>
     * If {@code DragSourceListener} is {@code null} no exception
     * is thrown.
     *
     * @param trigger    the triggering event
     * @param dragCursor     the initial {@code Cursor} for this drag operation
     *                       or {@code null} for the default cursor handling;
     *                       see <a href="DragSourceContext.html#defaultCursor">class level documentation</a>
     *                       for more details on the cursor handling mechanism during drag and drop
     * @param dragImage  the {@code Image} to drag (or {@code null})
     * @param offset     the offset of the image origin from the hotspot at the
     *                   instant of the triggering event
     * @param t          the {@code Transferable}
     * @param dsl        the {@code DragSourceListener}
     *
     * @throws IllegalArgumentException if the {@code Component} associated
     *         with the trigger event is {@code null}.
     * @throws IllegalArgumentException if the {@code DragSource} for the
     *         trigger event is {@code null}.
     * @throws IllegalArgumentException if the drag action for the
     *         trigger event is {@code DnDConstants.ACTION_NONE}.
     * @throws IllegalArgumentException if the source actions for the
     *         {@code DragGestureRecognizer} associated with the trigger
     *         event are equal to {@code DnDConstants.ACTION_NONE}.
     * @throws NullPointerException if dscp, trigger, or t are null, or
     *         if dragImage is non-null and offset is null
     */
    public DragSourceContext(DragGestureEvent trigger, Cursor dragCursor,
                             Image dragImage, Point offset, Transferable t,
                             DragSourceListener dsl) {
        Toolkit toolkit = Toolkit.getDefaultToolkit();
        if (!(toolkit instanceof ComponentFactory)) {
            throw new AWTError("Unsupported toolkit: " + toolkit);
        }
        DragSourceContextPeer dscp = ((ComponentFactory) toolkit).
                createDragSourceContextPeer(trigger);

        if (dscp == null) {
            throw new NullPointerException("DragSourceContextPeer");
        }

        if (trigger == null) {
            throw new NullPointerException("Trigger");
        }

        if (trigger.getDragSource() == null) {
            throw new IllegalArgumentException("DragSource");
        }

        if (trigger.getComponent() == null) {
            throw new IllegalArgumentException("Component");
        }

        if (trigger.getSourceAsDragGestureRecognizer().getSourceActions() ==
                 DnDConstants.ACTION_NONE) {
            throw new IllegalArgumentException("source actions");
        }

        if (trigger.getDragAction() == DnDConstants.ACTION_NONE) {
            throw new IllegalArgumentException("no drag action");
        }

        if (t == null) {
            throw new NullPointerException("Transferable");
        }

        if (dragImage != null && offset == null) {
            throw new NullPointerException("offset");
        }

        peer         = dscp;
        this.trigger = trigger;
        cursor       = dragCursor;
        transferable = t;
        listener     = dsl;
        sourceActions =
            trigger.getSourceAsDragGestureRecognizer().getSourceActions();

        useCustomCursor = (dragCursor != null);

        updateCurrentCursor(trigger.getDragAction(), getSourceActions(), DEFAULT);
    }

    Returns the DragSource that instantiated this DragSourceContext. 
Returns: the DragSource that instantiated this DragSourceContext/**
     * Returns the {@code DragSource}
     * that instantiated this {@code DragSourceContext}.
     *
     * @return the {@code DragSource} that
     *   instantiated this {@code DragSourceContext}
     */

    public DragSource   getDragSource() { return trigger.getDragSource(); }

    Returns the Component associated with this DragSourceContext. 
Returns: the Component that started the drag/**
     * Returns the {@code Component} associated with this
     * {@code DragSourceContext}.
     *
     * @return the {@code Component} that started the drag
     */

    public Component    getComponent() { return trigger.getComponent(); }

    Returns the DragGestureEvent that initially triggered the drag. 
Returns: the Event that triggered the drag/**
     * Returns the {@code DragGestureEvent}
     * that initially triggered the drag.
     *
     * @return the Event that triggered the drag
     */

    public DragGestureEvent getTrigger() { return trigger; }

    Returns a bitwise mask of DnDConstants that represent the set of drop actions supported by the drag source for the drag operation associated with this DragSourceContext. 
Returns: the drop actions supported by the drag source/**
     * Returns a bitwise mask of {@code DnDConstants} that
     * represent the set of drop actions supported by the drag source for the
     * drag operation associated with this {@code DragSourceContext}.
     *
     * @return the drop actions supported by the drag source
     */
    public int  getSourceActions() {
        return sourceActions;
    }

    Sets the custom cursor for this drag operation to the specified Cursor. If the specified Cursor is null, the default drag cursor behavior is activated for this drag operation, otherwise it is deactivated. 
Params: c –  the initial Cursor for this drag operation, or null for the default cursor handling; see class
                      level documentation for more details on the cursor handling during drag and drop/**
     * Sets the custom cursor for this drag operation to the specified
     * {@code Cursor}.  If the specified {@code Cursor}
     * is {@code null}, the default drag cursor behavior is
     * activated for this drag operation, otherwise it is deactivated.
     *
     * @param c     the initial {@code Cursor} for this drag operation,
     *                       or {@code null} for the default cursor handling;
     *                       see {@linkplain Cursor class
     *                       level documentation} for more details
     *                       on the cursor handling during drag and drop
     *
     */

    public synchronized void setCursor(Cursor c) {
        useCustomCursor = (c != null);
        setCursorImpl(c);
    }

    Returns the current custom drag Cursor. 
See Also: setCursor
Returns: the current custom drag Cursor, if it was set otherwise returns null./**
     * Returns the current custom drag {@code Cursor}.
     *
     * @return the current custom drag {@code Cursor}, if it was set
     *         otherwise returns {@code null}.
     * @see #setCursor
     */

    public Cursor getCursor() { return cursor; }

    Add a DragSourceListener to this DragSourceContext if one has not already been added. If a DragSourceListener already exists, this method throws a TooManyListenersException. 
Params: dsl – the DragSourceListener to add. Note that while null is not prohibited, it is not acceptable as a parameter.
Throws: TooManyListenersException – if a DragSourceListener has already been added/**
     * Add a {@code DragSourceListener} to this
     * {@code DragSourceContext} if one has not already been added.
     * If a {@code DragSourceListener} already exists,
     * this method throws a {@code TooManyListenersException}.
     *
     * @param dsl the {@code DragSourceListener} to add.
     * Note that while {@code null} is not prohibited,
     * it is not acceptable as a parameter.
     *
     * @throws TooManyListenersException if
     * a {@code DragSourceListener} has already been added
     */

    public synchronized void addDragSourceListener(DragSourceListener dsl) throws TooManyListenersException {
        if (dsl == null) return;

        if (equals(dsl)) throw new IllegalArgumentException("DragSourceContext may not be its own listener");

        if (listener != null)
            throw new TooManyListenersException();
        else
            listener = dsl;
    }

    Removes the specified DragSourceListener from this DragSourceContext. 
Params: dsl – the DragSourceListener to remove; note that while null is not prohibited, it is not acceptable as a parameter/**
     * Removes the specified {@code DragSourceListener}
     * from  this {@code DragSourceContext}.
     *
     * @param dsl the {@code DragSourceListener} to remove;
     *     note that while {@code null} is not prohibited,
     *     it is not acceptable as a parameter
     */

    public synchronized void removeDragSourceListener(DragSourceListener dsl) {
        if (listener != null && listener.equals(dsl)) {
            listener = null;
        } else
            throw new IllegalArgumentException();
    }

    Notifies the peer that the Transferable's DataFlavors have changed. /**
     * Notifies the peer that the {@code Transferable}'s
     * {@code DataFlavor}s have changed.
     */

    public void transferablesFlavorsChanged() {
        if (peer != null) peer.transferablesFlavorsChanged();
    }

    Calls dragEnter on the DragSourceListeners registered with this DragSourceContext and with the associated DragSource, and passes them the specified DragSourceDragEvent. 
Params: dsde – the DragSourceDragEvent/**
     * Calls {@code dragEnter} on the
     * {@code DragSourceListener}s registered with this
     * {@code DragSourceContext} and with the associated
     * {@code DragSource}, and passes them the specified
     * {@code DragSourceDragEvent}.
     *
     * @param dsde the {@code DragSourceDragEvent}
     */
    public void dragEnter(DragSourceDragEvent dsde) {
        DragSourceListener dsl = listener;
        if (dsl != null) {
            dsl.dragEnter(dsde);
        }
        getDragSource().processDragEnter(dsde);

        updateCurrentCursor(getSourceActions(), dsde.getTargetActions(), ENTER);
    }

    Calls dragOver on the DragSourceListeners registered with this DragSourceContext and with the associated DragSource, and passes them the specified DragSourceDragEvent. 
Params: dsde – the DragSourceDragEvent/**
     * Calls {@code dragOver} on the
     * {@code DragSourceListener}s registered with this
     * {@code DragSourceContext} and with the associated
     * {@code DragSource}, and passes them the specified
     * {@code DragSourceDragEvent}.
     *
     * @param dsde the {@code DragSourceDragEvent}
     */
    public void dragOver(DragSourceDragEvent dsde) {
        DragSourceListener dsl = listener;
        if (dsl != null) {
            dsl.dragOver(dsde);
        }
        getDragSource().processDragOver(dsde);

        updateCurrentCursor(getSourceActions(), dsde.getTargetActions(), OVER);
    }

    Calls dragExit on the DragSourceListeners registered with this DragSourceContext and with the associated DragSource, and passes them the specified DragSourceEvent. 
Params: dse – the DragSourceEvent/**
     * Calls {@code dragExit} on the
     * {@code DragSourceListener}s registered with this
     * {@code DragSourceContext} and with the associated
     * {@code DragSource}, and passes them the specified
     * {@code DragSourceEvent}.
     *
     * @param dse the {@code DragSourceEvent}
     */
    public void dragExit(DragSourceEvent dse) {
        DragSourceListener dsl = listener;
        if (dsl != null) {
            dsl.dragExit(dse);
        }
        getDragSource().processDragExit(dse);

        updateCurrentCursor(DnDConstants.ACTION_NONE, DnDConstants.ACTION_NONE, DEFAULT);
    }

    Calls dropActionChanged on the DragSourceListeners registered with this DragSourceContext and with the associated DragSource, and passes them the specified DragSourceDragEvent. 
Params: dsde – the DragSourceDragEvent/**
     * Calls {@code dropActionChanged} on the
     * {@code DragSourceListener}s registered with this
     * {@code DragSourceContext} and with the associated
     * {@code DragSource}, and passes them the specified
     * {@code DragSourceDragEvent}.
     *
     * @param dsde the {@code DragSourceDragEvent}
     */
    public void dropActionChanged(DragSourceDragEvent dsde) {
        DragSourceListener dsl = listener;
        if (dsl != null) {
            dsl.dropActionChanged(dsde);
        }
        getDragSource().processDropActionChanged(dsde);

        updateCurrentCursor(getSourceActions(), dsde.getTargetActions(), CHANGED);
    }

    Calls dragDropEnd on the DragSourceListeners registered with this DragSourceContext and with the associated DragSource, and passes them the specified DragSourceDropEvent. 
Params: dsde – the DragSourceDropEvent/**
     * Calls {@code dragDropEnd} on the
     * {@code DragSourceListener}s registered with this
     * {@code DragSourceContext} and with the associated
     * {@code DragSource}, and passes them the specified
     * {@code DragSourceDropEvent}.
     *
     * @param dsde the {@code DragSourceDropEvent}
     */
    public void dragDropEnd(DragSourceDropEvent dsde) {
        DragSourceListener dsl = listener;
        if (dsl != null) {
            dsl.dragDropEnd(dsde);
        }
        getDragSource().processDragDropEnd(dsde);
    }

    Calls dragMouseMoved on the DragSourceMotionListeners registered with the DragSource associated with this DragSourceContext, and them passes the specified DragSourceDragEvent. 
Params: dsde – the DragSourceDragEvent
Since: 1.4/**
     * Calls {@code dragMouseMoved} on the
     * {@code DragSourceMotionListener}s registered with the
     * {@code DragSource} associated with this
     * {@code DragSourceContext}, and them passes the specified
     * {@code DragSourceDragEvent}.
     *
     * @param dsde the {@code DragSourceDragEvent}
     * @since 1.4
     */
    public void dragMouseMoved(DragSourceDragEvent dsde) {
        getDragSource().processDragMouseMoved(dsde);
    }

    Returns the Transferable associated with this DragSourceContext. 
Returns: the Transferable/**
     * Returns the {@code Transferable} associated with
     * this {@code DragSourceContext}.
     *
     * @return the {@code Transferable}
     */
    public Transferable getTransferable() { return transferable; }

    If the default drag cursor behavior is active, this method
sets the default drag cursor for the specified actions
supported by the drag source, the drop target action,
and status, otherwise this method does nothing.
Params: sourceAct – the actions supported by the drag source
targetAct – the drop target action
status – one of the fields DEFAULT, ENTER, OVER, CHANGED/**
     * If the default drag cursor behavior is active, this method
     * sets the default drag cursor for the specified actions
     * supported by the drag source, the drop target action,
     * and status, otherwise this method does nothing.
     *
     * @param sourceAct the actions supported by the drag source
     * @param targetAct the drop target action
     * @param status one of the fields {@code DEFAULT},
     *               {@code ENTER}, {@code OVER},
     *               {@code CHANGED}
     */
    @SuppressWarnings("fallthrough")
    protected synchronized void updateCurrentCursor(int sourceAct, int targetAct, int status) {

        // if the cursor has been previously set then don't do any defaults
        // processing.

        if (useCustomCursor) {
            return;
        }

        // do defaults processing

        Cursor c = null;

        switch (status) {
            default:
                targetAct = DnDConstants.ACTION_NONE;
            case ENTER:
            case OVER:
            case CHANGED:
                int    ra = sourceAct & targetAct;

                if (ra == DnDConstants.ACTION_NONE) { // no drop possible
                    if ((sourceAct & DnDConstants.ACTION_LINK) == DnDConstants.ACTION_LINK)
                        c = DragSource.DefaultLinkNoDrop;
                    else if ((sourceAct & DnDConstants.ACTION_MOVE) == DnDConstants.ACTION_MOVE)
                        c = DragSource.DefaultMoveNoDrop;
                    else
                        c = DragSource.DefaultCopyNoDrop;
                } else { // drop possible
                    if ((ra & DnDConstants.ACTION_LINK) == DnDConstants.ACTION_LINK)
                        c = DragSource.DefaultLinkDrop;
                    else if ((ra & DnDConstants.ACTION_MOVE) == DnDConstants.ACTION_MOVE)
                        c = DragSource.DefaultMoveDrop;
                    else
                        c = DragSource.DefaultCopyDrop;
                }
        }

        setCursorImpl(c);
    }

    private void setCursorImpl(Cursor c) {
        if (cursor == null || !cursor.equals(c)) {
            cursor = c;
            if (peer != null) peer.setCursor(cursor);
        }
    }

    Serializes this DragSourceContext. This method first performs default serialization. Next, this object's Transferable is written out if and only if it can be serialized. If not, null is written instead. In this case, a DragSourceContext created from the resulting deserialized stream will contain a dummy Transferable which supports no DataFlavors. Finally, this object's DragSourceListener is written out if and only if it can be serialized. If not, null is written instead. 
@serialData The default serializable fields, in alphabetical order, followed by either a Transferable instance, or null, followed by either a DragSourceListener instance, or null.
Since: 1.4/**
     * Serializes this {@code DragSourceContext}. This method first
     * performs default serialization. Next, this object's
     * {@code Transferable} is written out if and only if it can be
     * serialized. If not, {@code null} is written instead. In this case,
     * a {@code DragSourceContext} created from the resulting deserialized
     * stream will contain a dummy {@code Transferable} which supports no
     * {@code DataFlavor}s. Finally, this object's
     * {@code DragSourceListener} is written out if and only if it can be
     * serialized. If not, {@code null} is written instead.
     *
     * @serialData The default serializable fields, in alphabetical order,
     *             followed by either a {@code Transferable} instance, or
     *             {@code null}, followed by either a
     *             {@code DragSourceListener} instance, or
     *             {@code null}.
     * @since 1.4
     */
    private void writeObject(ObjectOutputStream s) throws IOException {
        s.defaultWriteObject();

        s.writeObject(SerializationTester.test(transferable)
                      ? transferable : null);
        s.writeObject(SerializationTester.test(listener)
                      ? listener : null);
    }

    Deserializes this DragSourceContext. This method first performs default deserialization for all non-transient fields. This object's Transferable and DragSourceListener are then deserialized as well by using the next two objects in the stream. If the resulting Transferable is null, this object's Transferable is set to a dummy Transferable which supports no DataFlavors. 
Since: 1.4/**
     * Deserializes this {@code DragSourceContext}. This method first
     * performs default deserialization for all non-{@code transient}
     * fields. This object's {@code Transferable} and
     * {@code DragSourceListener} are then deserialized as well by using
     * the next two objects in the stream. If the resulting
     * {@code Transferable} is {@code null}, this object's
     * {@code Transferable} is set to a dummy {@code Transferable}
     * which supports no {@code DataFlavor}s.
     *
     * @since 1.4
     */
    private void readObject(ObjectInputStream s)
        throws ClassNotFoundException, IOException
    {
        ObjectInputStream.GetField f = s.readFields();

        DragGestureEvent newTrigger = (DragGestureEvent)f.get("trigger", null);
        if (newTrigger == null) {
            throw new InvalidObjectException("Null trigger");
        }
        if (newTrigger.getDragSource() == null) {
            throw new InvalidObjectException("Null DragSource");
        }
        if (newTrigger.getComponent() == null) {
            throw new InvalidObjectException("Null trigger component");
        }

        int newSourceActions = f.get("sourceActions", 0)
                & (DnDConstants.ACTION_COPY_OR_MOVE | DnDConstants.ACTION_LINK);
        if (newSourceActions == DnDConstants.ACTION_NONE) {
            throw new InvalidObjectException("Invalid source actions");
        }
        int triggerActions = newTrigger.getDragAction();
        if (triggerActions != DnDConstants.ACTION_COPY &&
                triggerActions != DnDConstants.ACTION_MOVE &&
                triggerActions != DnDConstants.ACTION_LINK) {
            throw new InvalidObjectException("No drag action");
        }
        trigger = newTrigger;

        cursor = (Cursor)f.get("cursor", null);
        useCustomCursor = f.get("useCustomCursor", false);
        sourceActions = newSourceActions;

        transferable = (Transferable)s.readObject();
        listener = (DragSourceListener)s.readObject();

        // Implementation assumes 'transferable' is never null.
        if (transferable == null) {
            if (emptyTransferable == null) {
                emptyTransferable = new Transferable() {
                        public DataFlavor[] getTransferDataFlavors() {
                            return new DataFlavor[0];
                        }
                        public boolean isDataFlavorSupported(DataFlavor flavor)
                        {
                            return false;
                        }
                        public Object getTransferData(DataFlavor flavor)
                            throws UnsupportedFlavorException
                        {
                            throw new UnsupportedFlavorException(flavor);
                        }
                    };
            }
            transferable = emptyTransferable;
        }
    }

    private static Transferable emptyTransferable;

    /*
     * fields
     */
    private final transient DragSourceContextPeer peer;

    The event which triggered the start of the drag.
@serial /**
     * The event which triggered the start of the drag.
     *
     * @serial
     */
    private DragGestureEvent    trigger;

    The current drag cursor.
@serial /**
     * The current drag cursor.
     *
     * @serial
     */
    private Cursor              cursor;

    private transient Transferable      transferable;

    private transient DragSourceListener    listener;

    true if the custom drag cursor is used instead of the default one. 
@serial /**
     * {@code true} if the custom drag cursor is used instead of the
     * default one.
     *
     * @serial
     */
    private boolean useCustomCursor;

    A bitwise mask of DnDConstants that represents the set of drop actions supported by the drag source for the drag operation associated with this DragSourceContext. 
@serial /**
     * A bitwise mask of {@code DnDConstants} that represents the set of
     * drop actions supported by the drag source for the drag operation associated
     * with this {@code DragSourceContext.}
     *
     * @serial
     */
    private int sourceActions;
}
See Also:	DragSourceListener DragSourceMotionListener DnDConstants
Since:	1.2
See Also:	setCursor
Returns:	the current custom drag `Cursor`, if it was set otherwise returns `null`.
Params:	dsl – the `DragSourceListener` to add. Note that while `null` is not prohibited, it is not acceptable as a parameter.
Throws:	TooManyListenersException – if a `DragSourceListener` has already been added
@serialData	The default serializable fields, in alphabetical order, followed by either a `Transferable` instance, or `null`, followed by either a `DragSourceListener` instance, or `null`.
Since:	1.4
/

java/ 11/ java.desktop/java/awt/dnd/DragSourceContext.java
