month-view: save the event when dragged and dropped

This magical commit finally implements the Drag n' Drop
feature for Month view by changing the start and end dates
of the events according to the cell where the user drops
the event.

src/gcal-month-view.c

@@ -847,6 +847,74 @@ gcal_month_view_drag_motion (GtkWidget      *widget,
   return priv->dnd_cell != -1;
 }
 
+static gboolean
+gcal_month_view_drag_drop (GtkWidget      *widget,
+                           GdkDragContext *context,
+                           gint            x,
+                           gint            y,
+                           guint           time)
+{
+  GcalMonthViewPrivate *priv = gcal_month_view_get_instance_private (GCAL_MONTH_VIEW (widget));
+  GtkWidget *event_widget;
+  GDateTime *start_dt, *end_dt;
+  GcalEvent *event;
+  gint cell, diff;
+
+  cell = get_dnd_cell (widget, x, y);
+  event_widget = gtk_drag_get_source_widget (context);
+  event = gcal_event_widget_get_event (GCAL_EVENT_WIDGET (event_widget));
+
+  /* Move the event's date */
+  start_dt = gcal_event_get_date_start (event);
+  end_dt = gcal_event_get_date_end (event);
+
+  diff = cell - g_date_time_get_day_of_month (start_dt) + 1;
+
+  if (diff != 0)
+    {
+      GDateTime *new_start = g_date_time_add_days (start_dt, diff);
+
+      gcal_event_set_date_start (event, new_start);
+
+      /* The event may have a NULL end date, so we have to check it here */
+      if (end_dt)
+        {
+          GDateTime *new_end = g_date_time_add_days (end_dt, diff);
+
+          gcal_event_set_date_end (event, new_end);
+          g_clear_pointer (&new_end, g_date_time_unref);
+        }
+
+      gcal_manager_update_event (priv->manager, event);
+
+      g_clear_pointer (&new_start, g_date_time_unref);
+    }
+
+  /* Cancel the DnD */
+  priv->dnd_cell = -1;
+  gtk_drag_unhighlight (widget);
+
+  gtk_drag_finish (context, TRUE, FALSE, time);
+
+  gtk_widget_queue_draw (widget);
+
+  return TRUE;
+}
+
+static void
+gcal_month_view_drag_leave (GtkWidget      *widget,
+                            GdkDragContext *context,
+                            guint           time)
+{
+  GcalMonthViewPrivate *priv = gcal_month_view_get_instance_private (GCAL_MONTH_VIEW (widget));
+
+  /* Cancel the drag */
+  priv->dnd_cell = -1;
+  gtk_drag_unhighlight (widget);
+
+  gtk_widget_queue_draw (widget);
+}
+
 static void
 add_new_event_button_cb (GtkWidget *button,
                          gpointer   user_data)
@@ -892,6 +960,8 @@ gcal_month_view_class_init (GcalMonthViewClass *klass)
   widget_class->direction_changed = gcal_month_view_direction_changed;
   widget_class->key_press_event = gcal_month_view_key_press;
   widget_class->drag_motion = gcal_month_view_drag_motion;
+  widget_class->drag_drop = gcal_month_view_drag_drop;
+  widget_class->drag_leave = gcal_month_view_drag_leave;
 
   subscriber_view_class = GCAL_SUBSCRIBER_VIEW_CLASS (klass);
   subscriber_view_class->get_child_cell = gcal_month_view_get_child_cell;