bundles/org.eclipse.jface/src/org/eclipse/jface/viewers/ViewerDropAdapter.java

/*******************************************************************************
 * Copyright (c) 2000, 2004 IBM Corporation and others.
 * All rights reserved. This program and the accompanying materials
 * are made available under the terms of the Eclipse Public License v1.0
 * which accompanies this distribution, and is available at
 * http://www.eclipse.org/legal/epl-v10.html
 *
 * Contributors:
 *     IBM Corporation - initial API and implementation
 *******************************************************************************/
package org.eclipse.jface.viewers;

import org.eclipse.swt.dnd.DND;
import org.eclipse.swt.dnd.DropTargetAdapter;
import org.eclipse.swt.dnd.DropTargetEvent;
import org.eclipse.swt.dnd.TransferData;
import org.eclipse.swt.graphics.Point;
import org.eclipse.swt.graphics.Rectangle;
import org.eclipse.swt.widgets.Item;
import org.eclipse.swt.widgets.TableItem;
import org.eclipse.swt.widgets.TreeItem;

/**
 * This adapter class provides generic drag-and-drop support for a viewer.
 * <p>
 * Subclasses must implement the following methods:
 * <ul>
 *   <li><code>validateDrop</code> - identifies valid drop targets in viewer</li>
 *   <li><code>performDrop</code> - carries out a drop into a viewer</li>
 * </ul>
 * The <code>setFeedbackEnabled</code> method can be called to turn on and off 
 * visual insertion feedback (on by default).
 * </p>
 */
public abstract class ViewerDropAdapter extends DropTargetAdapter {

    /**
     * Constant describing the position of the cursor relative 
     * to the target object.  This means the mouse is positioned
     * slightly before the target.
     * @see #getCurrentLocation()
     */
    public static final int LOCATION_BEFORE = 1;

    /**
     * Constant describing the position of the cursor relative 
     * to the target object.  This means the mouse is positioned
     * slightly after the target.
     * @see #getCurrentLocation()
     */
    public static final int LOCATION_AFTER = 2;

    /**
     * Constant describing the position of the cursor relative 
     * to the target object.  This means the mouse is positioned
     * directly on the target.
     * @see #getCurrentLocation()
     */
    public static final int LOCATION_ON = 3;

    /**
     * Constant describing the position of the cursor relative 
     * to the target object.  This means the mouse is not positioned
     * over or near any valid target.
     * @see #getCurrentLocation()
     */
    public static final int LOCATION_NONE = 4;

    /**
     * The viewer to which this drop support has been added.
     */
    private Viewer viewer;

    /**
     * The current operation.
     */
    private int currentOperation = DND.DROP_NONE;

    /**
     * The last valid operation.
     */
    private int lastValidOperation = DND.DROP_NONE;

    /**
     * The data item currently under the mouse.
     */
    private Object currentTarget;

    /**
     * Information about the position of the mouse relative to the
     * target (before, on, or after the target.  Location is one of
     * the <code>LOCATION_* </code> constants defined in this type.
     */
    private int currentLocation;

    /**
     * A flag that allows adapter users to turn the insertion
     * feedback on or off. Default is <code>true</code>.
     */
    private boolean feedbackEnabled = true;

    /**
     * A flag that allows adapter users to turn auto scrolling
     * and expanding on or off. Default is <code>true</code>.
     */
    private boolean scrollExpandEnabled = true;

    /**
     * A flag that allows adapter users to turn selection feedback
     *  on or off. Default is <code>true</code>.
     */
    private boolean selectFeedbackEnabled = true;

    /**
     * Creates a new drop adapter for the given viewer.
     *
     * @param viewer the viewer
     */
    protected ViewerDropAdapter(Viewer viewer) {
        this.viewer = viewer;
    }

    /**
     * Returns the position of the given event's coordinates relative to its target.
     * The position is determined to be before, after, or on the item, based on
     * some threshold value.
     *
     * @param event the event
     * @return one of the <code>LOCATION_* </code>constants defined in this class
     */
    protected int determineLocation(DropTargetEvent event) {
        if (!(event.item instanceof Item)) {
            return LOCATION_NONE;
        }
        Item item = (Item) event.item;
        Point coordinates = new Point(event.x, event.y);
        coordinates = viewer.getControl().toControl(coordinates);
        if (item != null) {
            Rectangle bounds = getBounds(item);
            if (bounds == null) {
                return LOCATION_NONE;
            }
            if ((coordinates.y - bounds.y) < 5) {
                return LOCATION_BEFORE;
            }
            if ((bounds.y + bounds.height - coordinates.y) < 5) {
                return LOCATION_AFTER;
            }
        }
        return LOCATION_ON;
    }

    /**
     * Returns the target item of the given drop event.
     *
     * @param event the event
     * @return The target of the drop, may be <code>null</code>.
     */
    protected Object determineTarget(DropTargetEvent event) {
        return event.item == null ? null : event.item.getData();
    }

    /* (non-Javadoc)
     * Method declared on DropTargetAdapter.
     * The mouse has moved over the drop target.  If the
     * target item has changed, notify the action and check
     * that it is still enabled.
     */
    private void doDropValidation(DropTargetEvent event) {
        //update last valid operation
        if (event.detail != DND.DROP_NONE) {
            lastValidOperation = event.detail;
        }
        //valid drop and set event detail accordingly
        if (validateDrop(currentTarget, event.detail, event.currentDataType)) {
            currentOperation = lastValidOperation;
        } else {
            currentOperation = DND.DROP_NONE;
        }
        event.detail = currentOperation;
    }

    /* (non-Javadoc)
     * Method declared on DropTargetAdapter.
     * The drag has entered this widget's region.  See
     * if the drop should be allowed.
     */
    public void dragEnter(DropTargetEvent event) {
        currentTarget = determineTarget(event);
        doDropValidation(event);
    }

    /* (non-Javadoc)
     * Method declared on DropTargetAdapter.
     * The drop operation has changed, see if the action
     * should still be enabled.
     */
    public void dragOperationChanged(DropTargetEvent event) {
        currentTarget = determineTarget(event);
        doDropValidation(event);
    }

    /* (non-Javadoc)
     * Method declared on DropTargetAdapter.
     * The mouse has moved over the drop target.  If the
     * target item has changed, notify the action and check
     * that it is still enabled.
     */
    public void dragOver(DropTargetEvent event) {
        //use newly revealed item as target if scrolling occurs
        Object target = determineTarget(event);

        //set the location feedback
        int oldLocation = currentLocation;
        currentLocation = determineLocation(event);
        setFeedback(event, currentLocation);

        //see if anything has really changed before doing validation.
        if (target != currentTarget || currentLocation != oldLocation) {
            currentTarget = target;
            doDropValidation(event);
        }
    }

    /* (non-Javadoc)
     * Method declared on DropTargetAdapter.
     * The user has dropped something on the desktop viewer.
     */
    public void drop(DropTargetEvent event) {
        currentLocation = determineLocation(event);

        //perform the drop behaviour
        if (!performDrop(event.data)) {
            event.detail = DND.DROP_NONE;
        }
        currentOperation = event.detail;
    }

    /* (non-Javadoc)
     * Method declared on DropTargetAdapter.
     * Last chance for the action to disable itself
     */
    public void dropAccept(DropTargetEvent event) {
        if (!validateDrop(currentTarget, event.detail, event.currentDataType)) {
            event.detail = DND.DROP_NONE;
        }
    }

    /**
     * Returns the bounds of the given SWT tree or table item.
     *
     * @param item the SWT Item
     * @return the bounds, or <code>null</code> if it is not a known type of item
     */
    protected Rectangle getBounds(Item item) {
        if (item instanceof TreeItem) {
            return ((TreeItem) item).getBounds();
        }
        if (item instanceof TableItem) {
            return ((TableItem) item).getBounds(0);
        }
        return null;
    }

    /**
     * Returns a constant describing the position of the mouse relative to the
     * target (before, on, or after the target.  
     *
     * @return one of the <code>LOCATION_* </code> constants defined in this type
     */
    protected int getCurrentLocation() {
        return currentLocation;
    }

    /**
     * Returns the current operation.
     *
     * @return a <code>DROP_*</code> constant from class <code>DND</code>
     *
     * @see DND#DROP_COPY
     * @see DND#DROP_MOVE
     * @see DND#DROP_LINK
     * @see DND#DROP_NONE
     */
    protected int getCurrentOperation() {
        return currentOperation;
    }

    /**
     * Returns the target object currently under the mouse.
     *
     * @return the current target object
     */
    protected Object getCurrentTarget() {
        return currentTarget;
    }

    /**
     * Returns whether visible insertion feedback should be presented to the user.
     * <p>
     * Typical insertion feedback is the horizontal insertion bars that appear 
     * between adjacent items while dragging.
     * </p>
     *
     * @return <code>true</code> if visual feedback is desired, and <code>false</code> if not
     */
    public boolean getFeedbackEnabled() {
        return feedbackEnabled;
    }

    /**
     * Returns the object currently selected by the viewer.
     *
     * @return the selected object, or <code>null</code> if either no object or 
     *   multiple objects are selected
     */
    protected Object getSelectedObject() {
        ISelection selection = viewer.getSelection();
        if (selection instanceof IStructuredSelection && !selection.isEmpty()) {
            IStructuredSelection structured = (IStructuredSelection) selection;
            return structured.getFirstElement();
        }
        return null;
    }

    /**
     * Returns the viewer to which this drop support has been added.
     */
    protected Viewer getViewer() {
        return viewer;
    }

    /**
     * @deprecated this method should not be used. Exception handling has been 
     * 	removed from DropTargetAdapter methods overridden by this class.
     * Handles any exception that occurs during callback, including 
     * rethrowing behavior.
     * <p>
     * [Issue: Implementation prints stack trace and eats exception to avoid
     *  crashing VA/J.
     *  Consider conditionalizing the implementation to do one thing in VAJ
     *  and something more reasonable in other operating environments.
     * ]
     * </p>
     *
     * @param exception the exception
     * @param event the event
     */
    protected void handleException(Throwable exception, DropTargetEvent event) {
        // Currently we never rethrow because VA/Java crashes if an SWT
        // callback throws anything. Generally catching Throwable is bad, but in
        // this cases it's better than hanging the image.
        exception.printStackTrace();
        event.detail = DND.DROP_NONE;
    }

    /**
     * Performs any work associated with the drop.
     * <p>
     * Subclasses must implement this method to provide drop behavior.
     * </p>
     *
     * @param data the drop data
     * @return <code>true</code> if the drop was successful, and 
     *   <code>false</code> otherwise
     */
    public abstract boolean performDrop(Object data);

    /* (non-Javadoc)
     * Method declared on DropTargetAdapter.
     * The mouse has moved over the drop target.  If the
     * target item has changed, notify the action and check
     * that it is still enabled.
     */
    private void setFeedback(DropTargetEvent event, int location) {
        if (feedbackEnabled) {
            switch (location) {
            case LOCATION_BEFORE:
                event.feedback = DND.FEEDBACK_INSERT_BEFORE;
                break;
            case LOCATION_AFTER:
                event.feedback = DND.FEEDBACK_INSERT_AFTER;
                break;
            case LOCATION_ON:
            default:
                event.feedback = DND.FEEDBACK_SELECT;
                break;
            }
        }
        
         // Explicitly inhibit SELECT feedback if desired
		if (!selectFeedbackEnabled)
			event.feedback &= ~DND.FEEDBACK_SELECT;

		if (scrollExpandEnabled)
			event.feedback |= DND.FEEDBACK_EXPAND | DND.FEEDBACK_SCROLL;
    }

    /**
	 * Sets whether visible insertion feedback should be presented to the user.
	 * <p>
	 * Typical insertion feedback is the horizontal insertion bars that appear
	 * between adjacent items while dragging.
	 * </p>
	 * 
	 * @param value
	 *            <code>true</code> if visual feedback is desired, and
	 *            <code>false</code> if not
	 */
    public void setFeedbackEnabled(boolean value) {
        feedbackEnabled = value;
    }

    /**
     * Sets whether selection feedback should be provided during dragging.
     *
     * @param value <code>true</code> if selection feedback is desired, and
     *   <code>false</code> if not
     *   
     * @since 3.2
     */
    public void setSelectionFeedbackEnabled(boolean value) {
        selectFeedbackEnabled = value;
    }

    /**
     * Sets whether auto scrolling and expanding should be provided during dragging.
     *
     * @param value <code>true</code> if scrolling and expanding is desired, and
     *   <code>false</code> if not
     * @since 2.0
     */
    public void setScrollExpandEnabled(boolean value) {
        scrollExpandEnabled = value;
    }

    /**
     * Validates dropping on the given object. This method is called whenever some 
     * aspect of the drop operation changes.
     * <p>
     * Subclasses must implement this method to define which drops make sense.
     * </p>
     * 
     * @param target the object that the mouse is currently hovering over, or
     *   <code>null</code> if the mouse is hovering over empty space
     * @param operation the current drag operation (copy, move, etc.)
     * @param transferType the current transfer type
     * @return <code>true</code> if the drop is valid, and <code>false</code>
     *   otherwise
     */
    public abstract boolean validateDrop(Object target, int operation,
            TransferData transferType);
}