Implement cross overview drag & drop

The gnome-panel allows the user to hover over a tasklist entry
while draging to activate a minimized or obscured window and drop onto it.

Implement a similar behaviour by allowing draging to the activities button or
the hotcorner (and thus opening the overview), which allows the user to
activate any window (even on different workspaces) as a drop target.

https://bugzilla.gnome.org/show_bug.cgi?id=601731

src/gnome-shell-plugin.c

@@ -113,6 +113,8 @@ struct _GnomeShellPlugin
   int glx_error_base;
   int glx_event_base;
   guint have_swap_event : 1;
+
+  ShellGlobal *global;
 };
 
 struct _GnomeShellPluginClass
@@ -320,7 +322,6 @@ gnome_shell_plugin_start (MetaPlugin *plugin)
   int status;
   const char *shell_js;
   char **search_path;
-  ShellGlobal *global;
   const char *glx_extensions;
 
   bindtextdomain (GETTEXT_PACKAGE, LOCALEDIR);
@@ -379,10 +380,10 @@ gnome_shell_plugin_start (MetaPlugin *plugin)
                      gvc_muted_debug_log_handler, NULL);
 
   /* Initialize the global object here. */
-  global = shell_global_get ();
+  shell_plugin->global = shell_global_get ();
 
-  _shell_global_set_plugin (global, META_PLUGIN(shell_plugin));
-  _shell_global_set_gjs_context (global, shell_plugin->gjs_context);
+  _shell_global_set_plugin (shell_plugin->global, META_PLUGIN(shell_plugin));
+  _shell_global_set_gjs_context (shell_plugin->global, shell_plugin->gjs_context);
 
   add_statistics (shell_plugin);
 
@@ -511,9 +512,9 @@ static gboolean
 gnome_shell_plugin_xevent_filter (MetaPlugin *plugin,
                                   XEvent     *xev)
 {
-#ifdef GLX_INTEL_swap_event
-  GnomeShellPlugin *shell_plugin = GNOME_SHELL_PLUGIN (plugin);
 
+  GnomeShellPlugin *shell_plugin = GNOME_SHELL_PLUGIN (plugin);
+#ifdef GLX_INTEL_swap_event
   if (shell_plugin->have_swap_event &&
       xev->type == (shell_plugin->glx_event_base + GLX_BufferSwapComplete))
     {
@@ -545,6 +546,12 @@ gnome_shell_plugin_xevent_filter (MetaPlugin *plugin,
       && xev->xcrossing.window == clutter_x11_get_stage_window (CLUTTER_STAGE (clutter_stage_get_default ())))
     return TRUE;
 
+  /*
+   * Pass the event to shell-global
+   */
+  if (_shell_global_check_xdnd_event (shell_plugin->global, xev))
+    return TRUE;
+
   return clutter_x11_handle_event (xev) != CLUTTER_X11_FILTER_CONTINUE;
 }
 

src/shell-global-private.h

@@ -11,4 +11,6 @@ void _shell_global_set_plugin      (ShellGlobal  *global,
 void _shell_global_set_gjs_context (ShellGlobal  *global,
                                     GjsContext   *context);
 
+gboolean _shell_global_check_xdnd_event (ShellGlobal  *global,
+                                         XEvent       *xev);
 #endif /* __SHELL_GLOBAL_PRIVATE_H__ */

src/shell-global.c

@@ -6,6 +6,7 @@
 #include "shell-enum-types.h"
 #include "shell-perf-log.h"
 #include "shell-window-tracker.h"
+#include "shell-marshal.h"
 #include "shell-wm.h"
 #include "st.h"
 
@@ -70,6 +71,8 @@ struct _ShellGlobal {
 
   /* For sound notifications */
   ca_context *sound_context;
+
+  guint32 xdnd_timestamp;
 };
 
 enum {
@@ -92,8 +95,19 @@ enum {
   PROP_FOCUS_MANAGER,
 };
 
+/* Signals */
+enum
+{
+ XDND_POSITION_CHANGED,
+ XDND_LEAVE,
+ XDND_ENTER,
+ LAST_SIGNAL
+};
+
 G_DEFINE_TYPE(ShellGlobal, shell_global, G_TYPE_OBJECT);
 
+static guint shell_global_signals [LAST_SIGNAL] = { 0 };
+
 static void
 shell_global_set_property(GObject         *object,
                           guint            prop_id,
@@ -238,6 +252,36 @@ shell_global_class_init (ShellGlobalClass *klass)
   gobject_class->get_property = shell_global_get_property;
   gobject_class->set_property = shell_global_set_property;
 
+  /* Emitted from gnome-shell-plugin.c during event handling */
+  shell_global_signals[XDND_POSITION_CHANGED] =
+      g_signal_new ("xdnd-position-changed",
+                    G_TYPE_FROM_CLASS (klass),
+                    G_SIGNAL_RUN_LAST,
+                    0,
+                    NULL, NULL,
+                    _shell_marshal_VOID__INT_INT,
+                    G_TYPE_NONE, 2, G_TYPE_INT, G_TYPE_INT);
+
+  /* Emitted from gnome-shell-plugin.c during event handling */
+  shell_global_signals[XDND_LEAVE] =
+      g_signal_new ("xdnd-leave",
+                    G_TYPE_FROM_CLASS (klass),
+                    G_SIGNAL_RUN_LAST,
+                    0,
+                    NULL, NULL,
+                    g_cclosure_marshal_VOID__VOID,
+                    G_TYPE_NONE, 0);
+
+  /* Emitted from gnome-shell-plugin.c during event handling */
+  shell_global_signals[XDND_ENTER] =
+      g_signal_new ("xdnd-enter",
+                    G_TYPE_FROM_CLASS (klass),
+                    G_SIGNAL_RUN_LAST,
+                    0,
+                    NULL, NULL,
+                    g_cclosure_marshal_VOID__VOID,
+                    G_TYPE_NONE, 0);
+
   g_object_class_install_property (gobject_class,
                                    PROP_OVERLAY_GROUP,
                                    g_param_spec_object ("overlay-group",
@@ -1130,6 +1174,39 @@ grab_notify (GtkWidget *widget, gboolean was_grabbed, gpointer user_data)
   shell_global_set_stage_input_mode (global, global->input_mode);
 }
 
+/**
+ * shell_global_init_xdnd:
+ * @global: the #ShellGlobal
+ *
+ * Enables tracking of Xdnd events
+ */
+void shell_global_init_xdnd (ShellGlobal *global)
+{
+  long xdnd_version = 5;
+
+  MetaScreen *screen = shell_global_get_screen (global);
+  Window output_window = meta_get_overlay_window (screen);
+
+  MetaDisplay *display = meta_screen_get_display (screen);
+  Display *xdisplay = meta_display_get_xdisplay (display);
+
+  ClutterStage *stage = CLUTTER_STAGE(meta_plugin_get_stage (global->plugin));
+  Window stage_win = clutter_x11_get_stage_window (stage);
+
+  XChangeProperty (xdisplay, stage_win, gdk_x11_get_xatom_by_name ("XdndAware"), XA_ATOM,
+                  32, PropModeReplace, (const unsigned char *)&xdnd_version, 1);
+
+  XChangeProperty (xdisplay, output_window, gdk_x11_get_xatom_by_name ("XdndProxy"), XA_WINDOW,
+                  32, PropModeReplace, (const unsigned char *)&stage_win, 1);
+
+  /*
+   * XdndProxy is additionally set on the proxy window as verification that the
+   * XdndProxy property on the target window isn't a left-over
+   */
+  XChangeProperty (xdisplay, stage_win, gdk_x11_get_xatom_by_name ("XdndProxy"), XA_WINDOW,
+                  32, PropModeReplace, (const unsigned char *)&stage_win, 1);
+}
+
 /**
  * shell_global_format_time_relative_pretty:
  * @global:
@@ -1422,6 +1499,10 @@ shell_global_get_current_time (ShellGlobal *global)
   guint32 time;
   MetaDisplay *display;
 
+  /* In case we have a xdnd timestamp use it */
+  if (global->xdnd_timestamp != 0)
+    return global->xdnd_timestamp;
+
   /* meta_display_get_current_time() will return the correct time
      when handling an X or Gdk event, but will return CurrentTime
      from some Clutter event callbacks.
@@ -1689,3 +1770,67 @@ shell_global_play_theme_sound (ShellGlobal *global,
 {
   ca_context_play (global->sound_context, 0, CA_PROP_EVENT_ID, name, NULL);
 }
+
+/*
+ * Process Xdnd events
+ *
+ * We pass the position and leave events to JS via a signal
+ * where the actual drag & drop handling happens.
+ *
+ * http://www.freedesktop.org/wiki/Specifications/XDND
+ */
+gboolean _shell_global_check_xdnd_event (ShellGlobal  *global,
+                                         XEvent       *xev)
+{
+  MetaScreen *screen = meta_plugin_get_screen (global->plugin);
+  Window output_window = meta_get_overlay_window (screen);
+  MetaDisplay *display = meta_screen_get_display (screen);
+  Display *xdisplay = meta_display_get_xdisplay (display);
+
+  ClutterStage *stage = CLUTTER_STAGE (meta_plugin_get_stage (global->plugin));
+  Window stage_win = clutter_x11_get_stage_window (stage);
+
+  if (xev->xany.window != output_window && xev->xany.window != stage_win)
+    return FALSE;
+
+  if (xev->xany.type == ClientMessage && xev->xclient.message_type == gdk_x11_get_xatom_by_name ("XdndPosition"))
+    {
+      XEvent xevent;
+      Window src = xev->xclient.data.l[0];
+
+      memset (&xevent, 0, sizeof(xevent));
+      xevent.xany.type = ClientMessage;
+      xevent.xany.display = xdisplay;
+      xevent.xclient.window = src;
+      xevent.xclient.message_type = gdk_x11_get_xatom_by_name ("XdndStatus");
+      xevent.xclient.format = 32;
+      xevent.xclient.data.l[0] = output_window;
+      /* flags: bit 0: will we accept the drop? bit 1: do we want more position messages */
+      xevent.xclient.data.l[1] = 2;
+      xevent.xclient.data.l[4] = None;
+
+      XSendEvent (xdisplay, src, False, 0, &xevent);
+
+      /* Store the timestamp of the xdnd position event */
+      global->xdnd_timestamp = xev->xclient.data.l[3];
+      g_signal_emit_by_name (G_OBJECT (global), "xdnd-position-changed",
+                            (int)(xev->xclient.data.l[2] >> 16), (int)(xev->xclient.data.l[2] & 0xFFFF));
+      global->xdnd_timestamp = 0;
+
+      return TRUE;
+    }
+   else if (xev->xany.type == ClientMessage && xev->xclient.message_type == gdk_x11_get_xatom_by_name ("XdndLeave"))
+    {
+      g_signal_emit_by_name (G_OBJECT (global), "xdnd-leave");
+
+      return TRUE;
+    }
+   else if (xev->xany.type == ClientMessage && xev->xclient.message_type == gdk_x11_get_xatom_by_name ("XdndEnter"))
+    {
+      g_signal_emit_by_name (G_OBJECT (global), "xdnd-enter");
+
+      return TRUE;
+    }
+
+    return FALSE;
+}

src/shell-global.h

@@ -132,6 +132,8 @@ void shell_global_run_at_leisure (ShellGlobal         *global,
 void shell_global_play_theme_sound (ShellGlobal       *global,
                                     const char        *name);
 
+void shell_global_init_xdnd (ShellGlobal *global);
+
 G_END_DECLS
 
 #endif /* __SHELL_GLOBAL_H__ */

src/shell-marshal.list

@@ -4,3 +4,4 @@ VOID:BOXED
 VOID:BOXED,OBJECT
 VOID:OBJECT,OBJECT
 VOID:STRING,OBJECT,BOOLEAN
+VOID:INT,INT