Add tests for light vs. dark theme detection

Author: jpnurmi

shell/platform/linux/BUILD.gn

@@ -169,6 +169,7 @@ executable("flutter_linux_unittests") {
     "fl_method_channel_test.cc",
     "fl_method_codec_test.cc",
     "fl_method_response_test.cc",
+    "fl_settings_plugin_test.cc",
     "fl_standard_message_codec_test.cc",
     "fl_standard_method_codec_test.cc",
     "fl_string_codec_test.cc",

shell/platform/linux/fl_settings_plugin_test.cc

@@ -0,0 +1,69 @@
+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Included first as it collides with the X11 headers.
+#include "gtest/gtest.h"
+
+#include "flutter/shell/platform/embedder/test_utils/proc_table_replacement.h"
+#include "flutter/shell/platform/linux/fl_engine_private.h"
+#include "flutter/shell/platform/linux/public/flutter_linux/fl_engine.h"
+#include "flutter/shell/platform/linux/public/flutter_linux/fl_json_message_codec.h"
+#include "flutter/shell/platform/linux/public/flutter_linux/fl_value.h"
+#include "flutter/shell/platform/linux/testing/fl_test.h"
+
+static void test_system_theme(const gchar* color, const gchar* expected_theme) {
+  gtk_init(nullptr, nullptr);
+
+  GdkScreen* screen = gdk_screen_get_default();
+  GtkWidget* window = gtk_window_new(GTK_WINDOW_TOPLEVEL);
+
+  g_autofree gchar* css = g_strdup_printf("* { color: %s; }", color);
+  g_autoptr(GtkCssProvider) css_provider = gtk_css_provider_new();
+  gtk_css_provider_load_from_data(css_provider, css, -1, NULL);
+
+  gtk_style_context_add_provider_for_screen(screen,
+                                            GTK_STYLE_PROVIDER(css_provider),
+                                            GTK_STYLE_PROVIDER_PRIORITY_USER);
+
+  g_autoptr(FlEngine) engine = make_mock_engine();
+  FlutterEngineProcTable* embedder_api = fl_engine_get_embedder_api(engine);
+
+  g_autofree gchar* system_theme = nullptr;
+  embedder_api->SendPlatformMessage = MOCK_ENGINE_PROC(
+      SendPlatformMessage,
+      ([&system_theme](auto engine, const FlutterPlatformMessage* message) {
+        EXPECT_STREQ(message->channel, "flutter/settings");
+
+        g_autoptr(FlJsonMessageCodec) codec = fl_json_message_codec_new();
+        g_autoptr(GBytes) data =
+            g_bytes_new(message->message, message->message_size);
+        g_autoptr(GError) error = nullptr;
+        g_autoptr(FlValue) settings = fl_message_codec_decode_message(
+            FL_MESSAGE_CODEC(codec), data, &error);
+        EXPECT_NE(settings, nullptr);
+        EXPECT_EQ(error, nullptr);
+
+        FlValue* value = fl_value_lookup_string(settings, "platformBrightness");
+        EXPECT_NE(value, nullptr);
+
+        system_theme = g_strdup(fl_value_get_string(value));
+
+        return kSuccess;
+      }));
+
+  EXPECT_TRUE(fl_engine_start(engine, nullptr));
+  EXPECT_STREQ(expected_theme, system_theme);
+
+  gtk_style_context_remove_provider_for_screen(
+      screen, GTK_STYLE_PROVIDER(css_provider));
+  gtk_widget_destroy(window);
+}
+
+TEST(FlSettingsPluginTest, TestLightSystemTheme) {
+  test_system_theme("#111", "light");
+}
+
+TEST(FlSettingsPluginTest, TestDarkSystemTheme) {
+  test_system_theme("#eee", "dark");
+}