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A themeable Look and Feel for java swing.
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/*
 * MIT License
 *
 * Copyright (c) 2019 Jannis Weis
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in all
 * copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 * SOFTWARE.
 */
package com.github.weisj.darklaf.ui;

import org.jdesktop.swingx.SwingXUtilities;

import javax.swing.*;
import java.awt.dnd.DragSource;
import java.awt.event.MouseEvent;
//import sun.awt.dnd.SunDragSourceContextPeer;
//import sun.awt.AppContext;

/**
 * Drag gesture recognition support for classes that have a
 * TransferHandler. The gesture for a drag in this class is a mouse
 * press followed by movement by DragSource.getDragThreshold() pixels. An instance of this class is
 * maintained per AppContext, and the public static methods call into the appropriate instance. 
 * 

 * This is a c&p of core (package private) needed for BasicXListUI. It differs from core in that references to sun
 * packages have been replaced.
 * 

 *  a static method of SunDragSourceContextPeer has been copied into SwingXUtilities
 *    and is used here
 * 
 the shared instance of this class is maintained in the UIManager instead of
 *   per appContext.
 * 
 *
 * @author Shannon Hickey
 * @version 1.2 11/17/05
 */
public class DragRecognitionSupport {
    private int motionThreshold;
    private MouseEvent dndArmedEvent;
    private JComponent component;

    /**
     * Returns whether or not the event is potentially part of a drag sequence.
     */
    public static boolean mousePressed(final MouseEvent me) {
        return getDragRecognitionSupport().
                                                  mousePressedImpl(me);
    }

    /**
     * Returns whether or not the event is potentially part of a drag sequence.
     */
    private boolean mousePressedImpl(final MouseEvent me) {
        component = (JComponent) me.getSource();

        if (mapDragOperationFromModifiers(me, component.getTransferHandler())
                != TransferHandler.NONE) {

            motionThreshold = DragSource.getDragThreshold();
            dndArmedEvent = me;
            return true;
        }

        clearState();
        return false;
    }

    /**
     * Returns the DragRecognitionSupport for the caller's AppContext.
     */
    private static DragRecognitionSupport getDragRecognitionSupport() {
//        DragRecognitionSupport support =
//            (DragRecognitionSupport)AppContext.getAppContext().
//                get(DragRecognitionSupport.class);
//
//        if (support == null) {
//            support = new DragRecognitionSupport();
//            AppContext.getAppContext().put(DragRecognitionSupport.class, support);
//        }

        DragRecognitionSupport support = (DragRecognitionSupport)
                UIManager.get("sharedInstance.dragRecognitionSupport");
        if (support == null) {
            support = new DragRecognitionSupport();
            UIManager.put("sharedInstance.dragRecognitionSupport", support);
        }
        return support;
    }

    private int mapDragOperationFromModifiers(final MouseEvent me,
                                              final TransferHandler th) {

        if (th == null || !SwingUtilities.isLeftMouseButton(me)) {
            return TransferHandler.NONE;
        }
        // PENDING JW: c'p from SunDragSourceContextPeer
        return SwingXUtilities.
                                      convertModifiersToDropAction(me.getModifiersEx(),
                                                                   th.getSourceActions(component));
    }

    private void clearState() {
        dndArmedEvent = null;
        component = null;
    }

    /**
     * If a dnd recognition has been going on, return the MouseEvent that started the recognition. Otherwise, return
     * null.
     */
    public static MouseEvent mouseReleased(final MouseEvent me) {
        return getDragRecognitionSupport().
                                                  mouseReleasedImpl(me);
    }

    /**
     * If a dnd recognition has been going on, return the MouseEvent that started the recognition. Otherwise, return
     * null.
     */
    private MouseEvent mouseReleasedImpl(final MouseEvent me) {
        /* no recognition has been going on */
        if (dndArmedEvent == null) {
            return null;
        }

        MouseEvent retEvent = null;

        if (me.getSource() == component) {
            retEvent = dndArmedEvent;
        } // else component has changed unexpectedly, so return null

        clearState();
        return retEvent;
    }

    /**
     * Returns whether or not a drag gesture recognition is ongoing.
     */
    public static boolean mouseDragged(final MouseEvent me, final BeforeDrag bd) {
        return getDragRecognitionSupport().
                                                  mouseDraggedImpl(me, bd);
    }

    /**
     * Returns whether or not a drag gesture recognition is ongoing.
     */
    private boolean mouseDraggedImpl(final MouseEvent me, final BeforeDrag bd) {
        /* no recognition is in progress */
        if (dndArmedEvent == null) {
            return false;
        }

        /* component has changed unexpectedly, so bail */
        if (me.getSource() != component) {
            clearState();
            return false;
        }

        int dx = Math.abs(me.getX() - dndArmedEvent.getX());
        int dy = Math.abs(me.getY() - dndArmedEvent.getY());
        if ((dx > motionThreshold) || (dy > motionThreshold)) {
            TransferHandler th = component.getTransferHandler();
            int action = mapDragOperationFromModifiers(me, th);
            if (action != TransferHandler.NONE) {
                /* notify the BeforeDrag instance */
                if (bd != null) {
                    bd.dragStarting(dndArmedEvent);
                }
                th.exportAsDrag(component, dndArmedEvent, action);
                clearState();
            }
        }

        return true;
    }

    /**
     * This interface allows us to pass in a handler to mouseDragged, so that we can be notified immediately before a
     * drag begins.
     */
    public interface BeforeDrag {
        void dragStarting(MouseEvent me);
    }
}