Use Windows high contrast black/white theme with MaterialApp themes (flutter/engine#39206)

* Check high contrast themes

* Formatting

* Test message sending

* Assert channel name

* Formatting

* Calculate luminance

* Refactor brightness check

* Formatting

* Formatting

engine/src/flutter/shell/platform/windows/flutter_windows_engine.cc

@@ -668,6 +668,7 @@ void FlutterWindowsEngine::UpdateHighContrastEnabled(bool enabled) {
         ~FlutterAccessibilityFeature::kFlutterAccessibilityFeatureHighContrast;
   }
   UpdateAccessibilityFeatures(static_cast<FlutterAccessibilityFeature>(flags));
+  settings_plugin_->UpdateHighContrastMode(enabled);
 }
 
 int FlutterWindowsEngine::EnabledAccessibilityFeatures() const {

engine/src/flutter/shell/platform/windows/settings_plugin.cc

@@ -26,6 +26,32 @@ constexpr wchar_t kGetPreferredBrightnessRegValue[] = L"AppsUseLightTheme";
 constexpr wchar_t kGetTextScaleFactorRegKey[] =
     L"Software\\Microsoft\\Accessibility";
 constexpr wchar_t kGetTextScaleFactorRegValue[] = L"TextScaleFactor";
+
+// Return an approximation of the apparent luminance of a given color.
+int GetLuminance(DWORD color) {
+  int r = GetRValue(color);
+  int g = GetGValue(color);
+  int b = GetBValue(color);
+  return (r + r + r + b + (g << 2)) >> 3;
+}
+
+// Return kLight if light mode for apps is selected, otherwise return kDark.
+SettingsPlugin::PlatformBrightness GetThemeBrightness() {
+  DWORD use_light_theme;
+  DWORD use_light_theme_size = sizeof(use_light_theme);
+  LONG result = RegGetValue(HKEY_CURRENT_USER, kGetPreferredBrightnessRegKey,
+                            kGetPreferredBrightnessRegValue, RRF_RT_REG_DWORD,
+                            nullptr, &use_light_theme, &use_light_theme_size);
+
+  if (result == 0) {
+    return use_light_theme ? SettingsPlugin::PlatformBrightness::kLight
+                           : SettingsPlugin::PlatformBrightness::kDark;
+  } else {
+    // The current OS does not support dark mode. (Older Windows 10 or before
+    // Windows 10)
+    return SettingsPlugin::PlatformBrightness::kLight;
+  }
+}
 }  // namespace
 
 SettingsPlugin::SettingsPlugin(BinaryMessenger* messenger,
@@ -103,19 +129,13 @@ float SettingsPlugin::GetTextScaleFactor() {
 }
 
 SettingsPlugin::PlatformBrightness SettingsPlugin::GetPreferredBrightness() {
-  DWORD use_light_theme;
-  DWORD use_light_theme_size = sizeof(use_light_theme);
-  LONG result = RegGetValue(HKEY_CURRENT_USER, kGetPreferredBrightnessRegKey,
-                            kGetPreferredBrightnessRegValue, RRF_RT_REG_DWORD,
-                            nullptr, &use_light_theme, &use_light_theme_size);
-
-  if (result == 0) {
-    return use_light_theme ? SettingsPlugin::PlatformBrightness::kLight
-                           : SettingsPlugin::PlatformBrightness::kDark;
+  if (is_high_contrast_) {
+    DWORD window_color = GetSysColor(COLOR_WINDOW);
+    int luminance = GetLuminance(window_color);
+    return luminance >= 127 ? SettingsPlugin::PlatformBrightness::kLight
+                            : SettingsPlugin::PlatformBrightness::kDark;
   } else {
-    // The current OS does not support dark mode. (Older Windows 10 or before
-    // Windows 10)
-    return SettingsPlugin::PlatformBrightness::kLight;
+    return GetThemeBrightness();
   }
 }
 
@@ -146,4 +166,9 @@ void SettingsPlugin::WatchTextScaleFactorChanged() {
       text_scale_factor_changed_watcher_->GetHandle(), TRUE);
 }
 
+void SettingsPlugin::UpdateHighContrastMode(bool is_high_contrast) {
+  is_high_contrast_ = is_high_contrast;
+  SendSettings();
+}
+
 }  // namespace flutter

engine/src/flutter/shell/platform/windows/settings_plugin.h

@@ -23,6 +23,8 @@ namespace flutter {
 // These are typically set in the control panel.
 class SettingsPlugin {
  public:
+  enum struct PlatformBrightness { kDark, kLight };
+
   explicit SettingsPlugin(BinaryMessenger* messenger, TaskRunner* task_runner);
 
   virtual ~SettingsPlugin();
@@ -37,9 +39,10 @@ class SettingsPlugin {
   // this automatically.
   virtual void StopWatching();
 
- protected:
-  enum struct PlatformBrightness { kDark, kLight };
+  // Update the high contrast status of the system.
+  virtual void UpdateHighContrastMode(bool is_high_contrast);
 
+ protected:
   // Returns `true` if the user uses 24 hour time.
   virtual bool GetAlwaysUse24HourFormat();
 
@@ -55,6 +58,8 @@ class SettingsPlugin {
   // Starts watching text scale factor changes.
   virtual void WatchTextScaleFactorChanged();
 
+  bool is_high_contrast_ = false;
+
  private:
   std::unique_ptr<BasicMessageChannel<rapidjson::Document>> channel_;
 

engine/src/flutter/shell/platform/windows/settings_plugin_unittests.cc

@@ -21,6 +21,8 @@ class MockSettingsPlugin : public SettingsPlugin {
 
   virtual ~MockSettingsPlugin() = default;
 
+  bool is_high_contrast() { return is_high_contrast_; }
+
   // |SettingsPlugin|
   MOCK_METHOD0(GetAlwaysUse24HourFormat, bool());
   MOCK_METHOD0(GetTextScaleFactor, float());
@@ -70,5 +72,24 @@ TEST(SettingsPluginTest, StartWatchingStartsWatchingChanges) {
   settings_plugin.StartWatching();
 }
 
+TEST(SettingsPluginTest, HighContrastModeHonored) {
+  int times = 0;
+  TestBinaryMessenger messenger(
+      [&times](const std::string& channel, const uint8_t* message,
+               size_t message_size, BinaryReply reply) {
+        ASSERT_EQ(channel, "flutter/settings");
+        times++;
+      });
+  ::testing::NiceMock<MockSettingsPlugin> settings_plugin(&messenger, nullptr);
+
+  settings_plugin.UpdateHighContrastMode(true);
+  EXPECT_TRUE(settings_plugin.is_high_contrast());
+
+  settings_plugin.UpdateHighContrastMode(false);
+  EXPECT_FALSE(settings_plugin.is_high_contrast());
+
+  EXPECT_EQ(times, 2);
+}
+
 }  // namespace testing
 }  // namespace flutter