GlobalCanvasDragInfo.java

/*
 * Copyright (C) 2010 The Android Open Source Project
 *
 * Licensed under the Eclipse Public License, Version 1.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.eclipse.org/org/documents/epl-v10.php
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.android.ide.eclipse.adt.internal.editors.layout.gle2;

import com.android.ide.common.api.Rect;


/**
 * This singleton is used to keep track of drag'n'drops initiated within this
 * session of Eclipse. A drag can be initiated from a palette or from a canvas
 * and its content is an Android View fully-qualified class name.
 * <p/>
 * Overall this is a workaround: the issue is that the drag'n'drop SWT API does not
 * allow us to know the transfered data during the initial drag -- only when the
 * data is dropped do we know what it is about (and to be more exact there is a workaround
 * to do just that which works on Windows but not on Linux/Mac SWT).
 * <p/>
 * In the GLE we'd like to adjust drag feedback to the data being actually dropped.
 * The singleton instance of this class will be used to track the data currently dragged
 * off a canvas or its palette and then set back to null when the drag'n'drop is finished.
 * <p/>
 * Note that when a drag starts in one instance of Eclipse and the dragOver/drop is done
 * in a <em>separate</em> instance of Eclipse, the dragged FQCN won't be registered here
 * and will be null.
 */
final class GlobalCanvasDragInfo {

    private static final GlobalCanvasDragInfo sInstance = new GlobalCanvasDragInfo();

    private SimpleElement[] mCurrentElements = null;
    private SelectionItem[] mCurrentSelection;
    private Object mSourceCanvas = null;
    private Runnable mRemoveSourceHandler;
    private Rect mDragBounds;

    /** Private constructor. Use {@link #getInstance()} to retrieve the singleton. */
    private GlobalCanvasDragInfo() {
        // pass
    }

    /** Returns the singleton instance. */
    public static GlobalCanvasDragInfo getInstance() {
        return sInstance;
    }

    /**
     * Registers the XML elements being dragged.
     *
     * @param elements The elements being dragged
     * @param selection The selection (which can be null, for example when the
     *            user drags from the palette)
     * @param sourceCanvas An object representing the source we are dragging
     *            from (used for identity comparisons only)
     * @param removeSourceHandler A runnable (or null) which can clean up the
     *            source. It should only be invoked if the drag operation is a
     *            move, not a copy.
     */
    public void startDrag(SimpleElement[] elements, SelectionItem[] selection,
            Object sourceCanvas, Runnable removeSourceHandler) {
        mCurrentElements = elements;
        mCurrentSelection = selection;
        mSourceCanvas = sourceCanvas;
        mRemoveSourceHandler = removeSourceHandler;
    }

    /** Unregisters elements being dragged. */
    public void stopDrag() {
        mCurrentElements = null;
        mCurrentSelection = null;
        mSourceCanvas = null;
        mRemoveSourceHandler = null;
        mDragBounds = null;
    }

    public boolean isDragging() {
        return mCurrentElements != null;
    }

    /** Returns the elements being dragged. */
    public SimpleElement[] getCurrentElements() {
        return mCurrentElements;
    }

    /** Returns the selection originally dragged.
     * Can be null if the drag did not start in a canvas.
     */
    public SelectionItem[] getCurrentSelection() {
        return mCurrentSelection;
    }

    /**
     * Returns the object that call {@link #startDrag(SimpleElement[], SelectionItem[], Object)}.
     * Can be null.
     * This is not meant to access the object indirectly, it is just meant to compare if the
     * source and the destination of the drag'n'drop are the same, so object identity
     * is all what matters.
     */
    public Object getSourceCanvas() {
        return mSourceCanvas;
    }

    /**
     * Removes source of the drag. This should only be called when the drag and
     * drop operation is a move (not a copy).
     */
    public void removeSource() {
        if (mRemoveSourceHandler != null) {
            mRemoveSourceHandler.run();
            mRemoveSourceHandler = null;
        }
    }

    /**
     * Get the bounds of the drag, relative to the starting mouse position. For example,
     * if you have a rectangular view of size 100x80, and you start dragging at position
     * (15,20) from the top left corner of this rectangle, then the drag bounds would be
     * (-15,-20, 100x80).
     * <p>
     * NOTE: The coordinate units will be in SWT/control pixels, not Android view pixels.
     * In other words, they are affected by the canvas zoom: If you zoom the view and the
     * bounds of a view grow, the drag bounds will be larger.
     *
     * @return the drag bounds, or null if there are no bounds for the current drag
     */
    public Rect getDragBounds() {
        return mDragBounds;
    }

    /**
     * Set the bounds of the drag, relative to the starting mouse position. See
     * {@link #getDragBounds()} for details on the semantics of the drag bounds.
     *
     * @param dragBounds the new drag bounds, or null if there are no drag bounds
     */
    public void setDragBounds(Rect dragBounds) {
        mDragBounds = dragBounds;
    }
}