Here's a simple example of drag-and-drop within a single GTK3 application. The code compiles cleanly as of Vala 0.20.

Credit: the code was taken from and cleaned up for modern GTK and Vala.

/* TestDnD - dnd.vala : Simple tutorial for GTK+ Drag-N-Drop
 * Copyright (C) 2005 Ryan McDougall.
 * Vala port 2008 by Frederik
 * Cleanup 2013 by Yaron Sheffer
 *  This program is free software; you can redistribute it and/or modify
 *  it under the terms of the GNU General Public License as published by
 *  the Free Software Foundation; either version 2 of the License, or
 *  (at your option) any later version.
 *  This program is distributed in the hope that it will be useful,
 *  but WITHOUT ANY WARRANTY; without even the implied warranty of
 *  GNU Library General Public License for more details.
 *  You should have received a copy of the GNU General Public License
 *  along with this program; if not, write to the Free Software
 *  Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.

// Compile with: valac dnd.vala --pkg gtk+-3.0

using Gtk;
using Gdk;

const int BYTE_BITS = 8;
const int WORD_BITS = 16;
const int DWORD_BITS = 32;

 * Define a list of data types called "targets" that a destination widget will
 * accept. The string type is arbitrary, and negotiated between DnD widgets by
 * the developer. An enum or Quark can serve as the integer target id.
enum Target {

/* datatype (string), restrictions on DnD (Gtk.TargetFlags), datatype (int) */
const TargetEntry[] target_list = {
    { "INTEGER",    0, Target.INT32 },
    { "STRING",     0, Target.STRING },
    { "text/plain", 0, Target.STRING },
    { "application/x-rootwindow-drop", 0, Target.ROOTWIN }

public class Well : Label {

    public Well () {
        set_text ("[a well]");

        // Make this widget a DnD destination.
        Gtk.drag_dest_set (
                this,                     // widget that will accept a drop
                DestDefaults.MOTION       // default actions for dest on DnD
                | DestDefaults.HIGHLIGHT,
                target_list,              // lists of target to support
                DragAction.COPY           // what to do with data after dropped

        // All possible destination signals

    /** Emitted when a drag is over the destination */
    private bool on_drag_motion (Widget widget, DragContext context,
                                 int x, int y, uint time)
        // Fancy stuff here. This signal spams the console something horrible.
        // print ("%s: on_drag_motion\n",;
        return false;

    /** Emitted when a drag leaves the destination */
    private void on_drag_leave (Widget widget, DragContext context, uint time) {
        print ("%s: on_drag_leave\n",;

     * Emitted when the user releases (drops) the selection. It should check
     * that the drop is over a valid part of the widget (if its a complex
     * widget), and itself to return true if the operation should continue. Next
     * choose the target type it wishes to ask the source for. Finally call
     * Gtk.drag_get_data which will emit "drag_data_get" on the source.
    private bool on_drag_drop (Widget widget, DragContext context,
                               int x, int y, uint time)
        print ("%s: on_drag_drop\n",;

        // Check to see if (x, y) is a valid drop site within widget
        bool is_valid_drop_site = true;

        // If the source offers a target
        if (context.list_targets() != null) {
            // Choose the best target type
            var target_type = (Atom) context.list_targets().nth_data (Target.INT32);

            // Request the data from the source.
            Gtk.drag_get_data (
                    widget,         // will receive 'drag_data_received' signal
                    context,        // represents the current state of the DnD
                    target_type,    // the target type we want
                    time            // time stamp
        } else {
            // No target offered by source => error
            is_valid_drop_site = false;

        return is_valid_drop_site;

     * Emitted when the data has been received from the source. It should check
     * the SelectionData sent by the source, and do something with it. Finally
     * it needs to finish the operation by calling Gtk.drag_finish, which will
     * emit the "data_delete" signal if told to.
    private void on_drag_data_received (Widget widget, DragContext context,
                                        int x, int y,
                                        SelectionData selection_data,
                                        uint target_type, uint time)
        bool dnd_success = false;
        bool delete_selection_data = false;

        print ("%s: on_drag_data_received\n",;

        // Deal with what we are given from source
        if ((selection_data != null) && (selection_data.get_length() >= 0)) {
            if (context.get_suggested_action() == DragAction.ASK) {
                // Ask the user to move or copy, then set the context action.

            if (context.get_suggested_action() == DragAction.MOVE) {
                delete_selection_data = true;

            // Check that we got the format we can use
            print (" Receiving ");
            switch (target_type) {
            case Target.INT32:
                long* data = (long*) selection_data.get_data();
                print ("integer: %ld", (*data));
                dnd_success = true;
            case Target.STRING:
                print ("string: %s", (string) selection_data.get_data());
                dnd_success = true;
                print ("nothing good");

            print (".\n");

        if (dnd_success == false) {
            print ("DnD data transfer failed!\n");

        Gtk.drag_finish (context, dnd_success, delete_selection_data, time);

public class Coins : Button {

    public Coins () {
        set_label ("[coins]");

        // Make the this widget a DnD source.
        // Why doesn't Gtk.Label work here?
        Gtk.drag_source_set (
                this,                      // widget will be drag-able
                ModifierType.BUTTON1_MASK, // modifier that will start a drag
                target_list,               // lists of target to support
                DragAction.COPY            // what to do with data after dropped

        // All possible source signals

     * Emitted when DnD begins. This is often used to present custom graphics.
    private void on_drag_begin (Widget widget, DragContext context) {
        print ("%s: on_drag_begin\n",;

     * Emitted when the destination requests data from the source via
     * Gtk.drag_get_data. It should attempt to provide its data in the form
     * requested in the target_type passed to it from the destination. If it
     * cannot, it should default to a "safe" type such as a string or text, even
     * if only to print an error. Then use Gtk.SelectionData.set to put the
     * source data into the allocated selection_data object, which will then be
     * passed to the destination. This will cause "drag_data_received" to be
     * emitted on the destination. Gtk.SelectionData is based on X's selection
     * mechanism which, via X properties, is only capable of storing data in
     * blocks of 8, 16, or 32 bit units.
    private void on_drag_data_get (Widget widget, DragContext context,
                                   SelectionData selection_data,
                                   uint target_type, uint time)
        string string_data = "This is data from the source.";
        long integer_data = 42;

        print ("%s: on_drag_data_get\n",;

        print (" Sending ");
        switch (target_type) {
            // case Target.SOME_OBJECT:
            // Serialize the object and send as a string of bytes.
            // Pixbufs, (UTF-8) text, and URIs have their own convenience
            // setter functions
        case Target.INT32:
            print ("integer: %ld", integer_data);
            uchar [] buf;
            convert_long_to_bytes(integer_data, out buf);
            selection_data.set (
                    selection_data.get_target(),      // target type
                    BYTE_BITS,                 // number of bits per 'unit'
                    buf // pointer to data to be sent
        case Target.STRING:
            print ("string: %s", string_data);
            selection_data.set (
                    (uchar [])string_data.to_utf8()
        case Target.ROOTWIN:
            print ("Dropped on the root window!\n");
            // Default to some a safe target instead of fail.
            assert_not_reached ();

        print (".\n");

     * Emitted after "drag_data_received" is handled, and Gtk.drag_finish is
     * called with the "delete" parameter set to true (when DnD is
     * DragAction.MOVE).
    private void on_drag_data_delete (Widget widget, DragContext context) {
        // We aren't moving or deleting anything here
        print ("%s: on_drag_data_delete\n",;

    /** Emitted when DnD ends. This is used to clean up any leftover data. */
    private void on_drag_end (Widget widget, DragContext context) {
        print ("%s: on_drag_end\n",;

     * Convert a "long" into a buffer of bytes
     * Note: we assume a little-endian machine
    private void convert_long_to_bytes(long number, out uchar [] buffer) {
        buffer = new uchar[sizeof(long)];
        for (int i=0; i<sizeof(long); i++) {
            buffer[i] = (uchar) (number & 0xFF);
            number = number >> 8;

static int main (string[] args) {

    // Always start GTK+ first!
    Gtk.init (ref args);

    // Create the widgets
    var window = new Gtk.Window (Gtk.WindowType.TOPLEVEL);
    var grid = new Grid();
    grid.orientation = Orientation.HORIZONTAL;
    grid.row_homogeneous = false;
    grid.row_spacing = 5;
    var coin_source = new Coins ();
    var well_dest = new Well ();
    var directions_label = new Label ("drag a coin and drop it in the well");

    // Pack the widgets
    window.add (grid);
    grid.add (coin_source);
    coin_source.vexpand = true;
    grid.add (directions_label);
    directions_label.hexpand = true;
    grid.add (well_dest);

    // Make the window big enough for some DnD action
    window.set_default_size (450, 50);

    // Connect the signals

    // Show the widgets
    window.show_all ();

    // Start the event loop
    Gtk.main ();

    return 0;


Projects/Vala/DragAndDropSample (last edited 2013-11-22 16:48:24 by WilliamJonMcCann)