src/java.desktop/share/classes/java/awt/dnd/DragGestureRecognizer.java

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package java.awt.dnd;

import java.awt.Component;
import java.awt.Point;
import java.awt.event.InputEvent;
import java.io.IOException;
import java.io.InvalidObjectException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.Serial;
import java.io.Serializable;
import java.util.ArrayList;
import java.util.TooManyListenersException;

/**
 * The {@code DragGestureRecognizer} is an
 * abstract base class for the specification
 * of a platform-dependent listener that can be associated with a particular
 * {@code Component} in order to
 * identify platform-dependent drag initiating gestures.
 * <p>
 * The appropriate {@code DragGestureRecognizer}
 * subclass instance is obtained from the
 * {@link DragSource} associated with
 * a particular {@code Component}, or from the {@code Toolkit} object via its
 * {@link java.awt.Toolkit#createDragGestureRecognizer createDragGestureRecognizer()}
 * method.
 * <p>
 * Once the {@code DragGestureRecognizer}
 * is associated with a particular {@code Component}
 * it will register the appropriate listener interfaces on that
 * {@code Component}
 * in order to track the input events delivered to the {@code Component}.
 * <p>
 * Once the {@code DragGestureRecognizer} identifies a sequence of events
 * on the {@code Component} as a drag initiating gesture, it will notify
 * its unicast {@code DragGestureListener} by
 * invoking its
 * {@link java.awt.dnd.DragGestureListener#dragGestureRecognized gestureRecognized()}
 * method.
 * <P>
 * When a concrete {@code DragGestureRecognizer}
 * instance detects a drag initiating
 * gesture on the {@code Component} it is associated with,
 * it fires a {@link DragGestureEvent} to
 * the {@code DragGestureListener} registered on
 * its unicast event source for {@code DragGestureListener}
 * events. This {@code DragGestureListener} is responsible
 * for causing the associated
 * {@code DragSource} to start the Drag and Drop operation (if
 * appropriate).
 *
 * @author Laurence P. G. Cable
 * @see java.awt.dnd.DragGestureListener
 * @see java.awt.dnd.DragGestureEvent
 * @see java.awt.dnd.DragSource
 */

public abstract class DragGestureRecognizer implements Serializable {

    /**
     * Use serialVersionUID from JDK 1.4 for interoperability.
     */
    @Serial
    private static final long serialVersionUID = 8996673345831063337L;

    /**
     * Construct a new {@code DragGestureRecognizer}
     * given the {@code DragSource} to be used
     * in this Drag and Drop operation, the {@code Component}
     * this {@code DragGestureRecognizer} should "observe"
     * for drag initiating gestures, the action(s) supported
     * for this Drag and Drop operation, and the
     * {@code DragGestureListener} to notify
     * once a drag initiating gesture has been detected.
     *
     * @param ds  the {@code DragSource} this
     * {@code DragGestureRecognizer}
     * will use to process the Drag and Drop operation
     *
     * @param c the {@code Component}
     * this {@code DragGestureRecognizer}
     * should "observe" the event stream to,
     * in order to detect a drag initiating gesture.
     * If this value is {@code null}, the
     * {@code DragGestureRecognizer}
     * is not associated with any {@code Component}.
     *
     * @param sa  the set (logical OR) of the
     * {@code DnDConstants}
     * that this Drag and Drop operation will support
     *
     * @param dgl the {@code DragGestureRecognizer}
     * to notify when a drag gesture is detected
     *
     * @throws IllegalArgumentException
     * if ds is {@code null}.
     */

    protected DragGestureRecognizer(DragSource ds, Component c, int sa, DragGestureListener dgl) {
        super();

        if (ds == null) throw new IllegalArgumentException("null DragSource");

        dragSource    = ds;
        component     = c;
        sourceActions = sa & (DnDConstants.ACTION_COPY_OR_MOVE | DnDConstants.ACTION_LINK);

        try {
            if (dgl != null) addDragGestureListener(dgl);
        } catch (TooManyListenersException tmle) {
            // can't happen ...
        }
    }

    /**
     * Construct a new {@code DragGestureRecognizer}
     * given the {@code DragSource} to be used in this
     * Drag and Drop
     * operation, the {@code Component} this
     * {@code DragGestureRecognizer} should "observe"
     * for drag initiating gestures, and the action(s)
     * supported for this Drag and Drop operation.
     *
     * @param ds  the {@code DragSource} this
     * {@code DragGestureRecognizer} will use to
     * process the Drag and Drop operation
     *
     * @param c   the {@code Component} this
     * {@code DragGestureRecognizer} should "observe" the event
     * stream to, in order to detect a drag initiating gesture.
     * If this value is {@code null}, the
     * {@code DragGestureRecognizer}
     * is not associated with any {@code Component}.
     *
     * @param sa the set (logical OR) of the {@code DnDConstants}
     * that this Drag and Drop operation will support
     *
     * @throws IllegalArgumentException
     * if ds is {@code null}.
     */

    protected DragGestureRecognizer(DragSource ds, Component c, int sa) {
        this(ds, c, sa, null);
    }

    /**
     * Construct a new {@code DragGestureRecognizer}
     * given the {@code DragSource} to be used
     * in this Drag and Drop operation, and
     * the {@code Component} this
     * {@code DragGestureRecognizer}
     * should "observe" for drag initiating gestures.
     *
     * @param ds the {@code DragSource} this
     * {@code DragGestureRecognizer}
     * will use to process the Drag and Drop operation
     *
     * @param c the {@code Component}
     * this {@code DragGestureRecognizer}
     * should "observe" the event stream to,
     * in order to detect a drag initiating gesture.
     * If this value is {@code null},
     * the {@code DragGestureRecognizer}
     * is not associated with any {@code Component}.
     *
     * @throws IllegalArgumentException
     * if ds is {@code null}.
     */

    protected DragGestureRecognizer(DragSource ds, Component c) {
        this(ds, c, DnDConstants.ACTION_NONE);
    }

    /**
     * Construct a new {@code DragGestureRecognizer}
     * given the {@code DragSource} to be used in this
     * Drag and Drop operation.
     *
     * @param ds the {@code DragSource} this
     * {@code DragGestureRecognizer} will
     * use to process the Drag and Drop operation
     *
     * @throws IllegalArgumentException
     * if ds is {@code null}.
     */

    protected DragGestureRecognizer(DragSource ds) {
        this(ds, null);
    }

    /**
     * register this DragGestureRecognizer's Listeners with the Component
     *
     * subclasses must override this method
     */

    protected abstract void registerListeners();

    /**
     * unregister this DragGestureRecognizer's Listeners with the Component
     *
     * subclasses must override this method
     */

    protected abstract void unregisterListeners();

    /**
     * This method returns the {@code DragSource}
     * this {@code DragGestureRecognizer}
     * will use in order to process the Drag and Drop
     * operation.
     *
     * @return the DragSource
     */

    public DragSource getDragSource() { return dragSource; }

    /**
     * This method returns the {@code Component}
     * that is to be "observed" by the
     * {@code DragGestureRecognizer}
     * for drag initiating gestures.
     *
     * @return The Component this DragGestureRecognizer
     * is associated with
     */

    public synchronized Component getComponent() { return component; }

    /**
     * set the Component that the DragGestureRecognizer is associated with
     *
     * registerListeners() and unregisterListeners() are called as a side
     * effect as appropriate.
     *
     * @param c The {@code Component} or {@code null}
     */

    public synchronized void setComponent(Component c) {
        if (component != null && dragGestureListener != null)
            unregisterListeners();

        component = c;

        if (component != null && dragGestureListener != null)
            registerListeners();
    }

    /**
     * This method returns an int representing the
     * type of action(s) this Drag and Drop
     * operation will support.
     *
     * @return the currently permitted source action(s)
     */

    public synchronized int getSourceActions() { return sourceActions; }

    /**
     * This method sets the permitted source drag action(s)
     * for this Drag and Drop operation.
     *
     * @param actions the permitted source drag action(s)
     */

    public synchronized void setSourceActions(int actions) {
        sourceActions = actions & (DnDConstants.ACTION_COPY_OR_MOVE | DnDConstants.ACTION_LINK);
    }

    /**
     * This method returns the first event in the
     * series of events that initiated
     * the Drag and Drop operation.
     *
     * @return the initial event that triggered the drag gesture
     */

    public InputEvent getTriggerEvent() { return events.isEmpty() ? null : events.get(0); }

    /**
     * Reset the Recognizer, if its currently recognizing a gesture, ignore
     * it.
     */

    public void resetRecognizer() { events.clear(); }

    /**
     * Register a new {@code DragGestureListener}.
     *
     * @param dgl the {@code DragGestureListener} to register
     * with this {@code DragGestureRecognizer}.
     *
     * @throws java.util.TooManyListenersException if a
     * {@code DragGestureListener} has already been added.
     */

    public synchronized void addDragGestureListener(DragGestureListener dgl) throws TooManyListenersException {
        if (dragGestureListener != null)
            throw new TooManyListenersException();
        else {
            dragGestureListener = dgl;

            if (component != null) registerListeners();
        }
    }

    /**
     * unregister the current DragGestureListener
     *
     * @param dgl the {@code DragGestureListener} to unregister
     * from this {@code DragGestureRecognizer}
     *
     * @throws IllegalArgumentException if
     * dgl is not (equal to) the currently registered {@code DragGestureListener}.
     */

    public synchronized void removeDragGestureListener(DragGestureListener dgl) {
        if (dragGestureListener == null || !dragGestureListener.equals(dgl))
            throw new IllegalArgumentException();
        else {
            dragGestureListener = null;

            if (component != null) unregisterListeners();
        }
    }

    /**
     * Notify the DragGestureListener that a Drag and Drop initiating
     * gesture has occurred. Then reset the state of the Recognizer.
     *
     * @param dragAction The action initially selected by the users gesture
     * @param p          The point (in Component coords) where the gesture originated
     */
    protected synchronized void fireDragGestureRecognized(int dragAction, Point p) {
        try {
            if (dragGestureListener != null) {
                dragGestureListener.dragGestureRecognized(new DragGestureEvent(this, dragAction, p, events));
            }
        } finally {
            events.clear();
        }
    }

    /**
     * Listeners registered on the Component by this Recognizer shall record
     * all Events that are recognized as part of the series of Events that go
     * to comprise a Drag and Drop initiating gesture via this API.
     * <P>
     * This method is used by a {@code DragGestureRecognizer}
     * implementation to add an {@code InputEvent}
     * subclass (that it believes is one in a series
     * of events that comprise a Drag and Drop operation)
     * to the array of events that this
     * {@code DragGestureRecognizer} maintains internally.
     *
     * @param awtie the {@code InputEvent}
     * to add to this {@code DragGestureRecognizer}'s
     * internal array of events. Note that {@code null}
     * is not a valid value, and will be ignored.
     */

    protected synchronized void appendEvent(InputEvent awtie) {
        events.add(awtie);
    }

    /**
     * Serializes this {@code DragGestureRecognizer}. This method first
     * performs default serialization. Then, this object's
     * {@code DragGestureListener} is written out if and only if it can be
     * serialized. If not, {@code null} is written instead.
     *
     * @param  s the {@code ObjectOutputStream} to write
     * @throws IOException if an I/O error occurs
     * @serialData The default serializable fields, in alphabetical order,
     *             followed by either a {@code DragGestureListener}, or
     *             {@code null}.
     * @since 1.4
     */
    @Serial
    private void writeObject(ObjectOutputStream s) throws IOException {
        s.defaultWriteObject();

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

    /**
     * Deserializes this {@code DragGestureRecognizer}. This method first
     * performs default deserialization for all non-{@code transient}
     * fields. This object's {@code DragGestureListener} is then
     * deserialized as well by using the next object in the stream.
     *
     * @param  s the {@code ObjectInputStream} to read
     * @throws ClassNotFoundException if the class of a serialized object could
     *         not be found
     * @throws IOException if an I/O error occurs
     * @since 1.4
     */
    @Serial
    @SuppressWarnings("unchecked")
    private void readObject(ObjectInputStream s)
        throws ClassNotFoundException, IOException
    {
        ObjectInputStream.GetField f = s.readFields();

        DragSource newDragSource = (DragSource)f.get("dragSource", null);
        if (newDragSource == null) {
            throw new InvalidObjectException("null DragSource");
        }
        dragSource = newDragSource;

        component = (Component)f.get("component", null);
        sourceActions = f.get("sourceActions", 0) & (DnDConstants.ACTION_COPY_OR_MOVE | DnDConstants.ACTION_LINK);
        events = (ArrayList<InputEvent>)f.get("events", new ArrayList<>(1));

        dragGestureListener = (DragGestureListener)s.readObject();
    }

    /*
     * fields
     */

    /**
     * The {@code DragSource}
     * associated with this
     * {@code DragGestureRecognizer}.
     *
     * @serial
     */
    protected DragSource          dragSource;

    /**
     * The {@code Component}
     * associated with this {@code DragGestureRecognizer}.
     *
     * @serial
     */
    protected Component           component;

    /**
     * The {@code DragGestureListener}
     * associated with this {@code DragGestureRecognizer}.
     */
    protected transient DragGestureListener dragGestureListener;

  /**
   * An {@code int} representing
   * the type(s) of action(s) used
   * in this Drag and Drop operation.
   *
   * @serial
   */
  protected int  sourceActions;

   /**
    * The list of events (in order) that
    * the {@code DragGestureRecognizer}
    * "recognized" as a "gesture" that triggers a drag.
    *
    * @serial
    */
   protected ArrayList<InputEvent> events = new ArrayList<InputEvent>(1);
}