[Linux] fix theme updates on GNOME 42

Prefer `org.gnome.desktop.interface.color-scheme` for determining the
UI color scheme. The new setting is available in GNOME 42 and later.

https://blogs.gnome.org/alexm/2021/10/04/dark-style-preference/

Fixes: flutter/flutter#101438

Author: jpnurmi

shell/platform/linux/fl_gnome_settings.cc

@@ -9,6 +9,7 @@
 static constexpr char kDesktopInterfaceSchema[] = "org.gnome.desktop.interface";
 static constexpr char kDesktopTextScalingFactorKey[] = "text-scaling-factor";
 static constexpr char kDesktopClockFormatKey[] = "clock-format";
+static constexpr char kDesktopColorSchemeKey[] = "color-scheme";
 static constexpr char kDesktopGtkThemeKey[] = "gtk-theme";
 
 static constexpr char kClockFormat12Hour[] = "12h";
@@ -53,17 +54,32 @@ static FlClockFormat fl_gnome_settings_get_clock_format(FlSettings* settings) {
   return clock_format;
 }
 
+static bool has_color_scheme(GSettings* settings) {
+  g_autoptr(GSettingsSchema) schema = nullptr;
+  g_object_get(settings, "settings-schema", &schema, nullptr);
+  return g_settings_schema_has_key(schema, kDesktopColorSchemeKey);
+}
+
 static FlColorScheme fl_gnome_settings_get_color_scheme(FlSettings* settings) {
   FlGnomeSettings* self = FL_GNOME_SETTINGS(settings);
 
   FlColorScheme color_scheme = FL_COLOR_SCHEME_LIGHT;
 
   if (self->interface_settings != nullptr) {
-    // check whether org.gnome.desktop.interface.gtk-theme ends with "-dark"
-    g_autofree gchar* value =
-        g_settings_get_string(self->interface_settings, kDesktopGtkThemeKey);
-    if (g_str_has_suffix(value, kGtkThemeDarkSuffix)) {
-      color_scheme = FL_COLOR_SCHEME_DARK;
+    if (has_color_scheme(self->interface_settings)) {
+      // org.gnome.desktop.interface.color-scheme in GNOME 42 and later
+      g_autofree gchar* value = g_settings_get_string(self->interface_settings,
+                                                      kDesktopColorSchemeKey);
+      if (g_strcmp0(value, kColorSchemePreferDark) == 0) {
+        color_scheme = FL_COLOR_SCHEME_DARK;
+      }
+    } else {
+      // check whether org.gnome.desktop.interface.gtk-theme ends with "-dark"
+      g_autofree gchar* value =
+          g_settings_get_string(self->interface_settings, kDesktopGtkThemeKey);
+      if (g_str_has_suffix(value, kGtkThemeDarkSuffix)) {
+        color_scheme = FL_COLOR_SCHEME_DARK;
+      }
     }
   }
   return color_scheme;
@@ -128,9 +144,11 @@ FlSettings* fl_gnome_settings_new(GSettings* interface_settings) {
     g_signal_connect_object(self->interface_settings, "changed::clock-format",
                             G_CALLBACK(fl_settings_emit_changed), self,
                             G_CONNECT_SWAPPED);
-    g_signal_connect_object(self->interface_settings, "changed::gtk-theme",
-                            G_CALLBACK(fl_settings_emit_changed), self,
-                            G_CONNECT_SWAPPED);
+    g_signal_connect_object(
+        self->interface_settings,
+        has_color_scheme(self->interface_settings) ? "changed::color-scheme"
+                                                   : "changed::gtk-theme",
+        G_CALLBACK(fl_settings_emit_changed), self, G_CONNECT_SWAPPED);
     g_signal_connect_object(
         self->interface_settings, "changed::text-scaling-factor",
         G_CALLBACK(fl_settings_emit_changed), self, G_CONNECT_SWAPPED);

shell/platform/linux/fl_gnome_settings_test.cc

@@ -50,6 +50,31 @@ TEST_F(FlGnomeSettingsTest, ClockFormat) {
   EXPECT_EQ(fl_settings_get_clock_format(settings), FL_CLOCK_FORMAT_12H);
 }
 
+TEST_F(FlGnomeSettingsTest, ColorScheme) {
+  g_autoptr(GSettings) interface_settings =
+      create_interface_settings("ubuntu-22.04");
+  g_settings_set_string(interface_settings, "color-scheme", "default");
+
+  g_autoptr(FlSettings) settings = fl_gnome_settings_new(interface_settings);
+  EXPECT_EQ(fl_settings_get_color_scheme(settings), FL_COLOR_SCHEME_LIGHT);
+
+  flutter::testing::MockSignalHandler settings_changed(settings, "changed");
+  EXPECT_SIGNAL(settings_changed).Times(1);
+
+  g_settings_set_string(interface_settings, "color-scheme", "prefer-light");
+  EXPECT_EQ(fl_settings_get_color_scheme(settings), FL_COLOR_SCHEME_LIGHT);
+
+  EXPECT_SIGNAL(settings_changed).Times(1);
+
+  g_settings_set_string(interface_settings, "color-scheme", "prefer-dark");
+  EXPECT_EQ(fl_settings_get_color_scheme(settings), FL_COLOR_SCHEME_DARK);
+
+  EXPECT_SIGNAL(settings_changed).Times(0);
+
+  g_settings_set_string(interface_settings, "gtk-theme", "Yaru");
+  EXPECT_EQ(fl_settings_get_color_scheme(settings), FL_COLOR_SCHEME_DARK);
+}
+
 TEST_F(FlGnomeSettingsTest, GtkTheme) {
   g_autoptr(GSettings) interface_settings =
       create_interface_settings("ubuntu-20.04");