lib: add util to check for inverted scroll

This has limited functionality because what we really need is support for
wayland!183 as it would only work unsandboxed.

But it provides a convenient abstraction point for implementing that as
well once we have it.

lib/meson.build

@@ -29,6 +29,7 @@ libmks_private_sources = [
   'mks-css.c',
   'mks-gl-context.c',
   'mks-read-only-list-model.c',
+  'mks-util.c',
 
   gnome.gdbus_codegen('mks-qemu',
          autocleanup: 'all',

lib/mks-util-private.h

@@ -0,0 +1,29 @@
+/* mks-util-private.h
+ *
+ * Copyright 2023 Christian Hergert <chergert@redhat.com>
+ *
+ * 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 3 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
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ */
+
+#pragma once
+
+#include <gdk/gdk.h>
+
+G_BEGIN_DECLS
+
+gboolean mks_scroll_event_is_inverted (GdkEvent *event);
+
+G_END_DECLS

lib/mks-util.c

@@ -0,0 +1,88 @@
+/* mks-util.c
+ *
+ * Copyright 2023 Christian Hergert <chergert@redhat.com>
+ *
+ * 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 3 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
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ */
+
+#include "config.h"
+
+#include "mks-util-private.h"
+
+static GSettings *mouse_settings;
+static GSettings *touchpad_settings;
+static gsize initialized;
+
+static GSettings *
+load_gsettings (const char *schema_id)
+{
+  GSettingsSchemaSource *source = g_settings_schema_source_get_default ();
+  g_autoptr(GSettingsSchema) schema = g_settings_schema_source_lookup (source, schema_id, TRUE);
+
+  if (schema != NULL)
+    return g_settings_new (schema_id);
+
+  return NULL;
+}
+
+static void
+_mks_util_init (void)
+{
+  if (g_once_init_enter (&initialized))
+    {
+      mouse_settings = load_gsettings ("org.gnome.desktop.peripherals.mouse");
+      touchpad_settings = load_gsettings ("org.gnome.desktop.peripherals.touchpad");
+      g_once_init_leave (&initialized, TRUE);
+    }
+}
+
+/* This is abstracted in a way that as soon as GdkEvent contains enough
+ * information to know if the GdkScrollEvent contains inverted axis
+ * directoin we can use that instead of checking the GSetting.
+ *
+ * TODO: This won't handle Flatpak because we won't have access to the
+ * host setting for the GSetting. Additionally, it wont work with jhbuild
+ * for the same reasons (likely using alternate GSettings/dconf).
+ *
+ * But this is better than nothing for the time being and provides an
+ * abstraction point once support for wayland!183 lands.
+ */
+gboolean
+mks_scroll_event_is_inverted (GdkEvent *event)
+{
+  GdkScrollUnit unit;
+
+  g_return_val_if_fail (gdk_event_get_event_type (event) == GDK_SCROLL, FALSE);
+
+  _mks_util_init ();
+
+  if (mouse_settings == NULL || touchpad_settings == NULL)
+    return FALSE;
+
+  unit = gdk_scroll_event_get_unit (event);
+
+  switch (unit)
+    {
+    case GDK_SCROLL_UNIT_WHEEL:
+      return g_settings_get_boolean (mouse_settings, "natural-scroll");
+
+    case GDK_SCROLL_UNIT_SURFACE:
+      return g_settings_get_boolean (touchpad_settings, "natural-scroll");
+
+    default:
+      return FALSE;
+    }
+}