Add support for Increase Contrast on iOS (#15343)

Author: miquelbeltran

lib/ui/window.dart

@@ -1212,6 +1212,7 @@ class AccessibilityFeatures {
   static const int _kDisableAnimationsIndex = 1 << 2;
   static const int _kBoldTextIndex = 1 << 3;
   static const int _kReduceMotionIndex = 1 << 4;
+  static const int _kHighContrastIndex = 1 << 5;
 
   // A bitfield which represents each enabled feature.
   final int _index;
@@ -1239,6 +1240,11 @@ class AccessibilityFeatures {
   /// Only supported on iOS.
   bool get reduceMotion => _kReduceMotionIndex & _index != 0;
 
+  /// The platform is requesting that UI be rendered with darker colors.
+  ///
+  /// Only supported on iOS.
+  bool get highContrast => _kHighContrastIndex & _index != 0;
+
   @override
   String toString() {
     final List<String> features = <String>[];
@@ -1252,6 +1258,8 @@ class AccessibilityFeatures {
       features.add('boldText');
     if (reduceMotion)
       features.add('reduceMotion');
+    if (highContrast)
+      features.add('highContrast');
     return 'AccessibilityFeatures$features';
   }
 

lib/ui/window/window.h

@@ -44,6 +44,7 @@ enum class AccessibilityFeatureFlag : int32_t {
   kDisableAnimations = 1 << 2,
   kBoldText = 1 << 3,
   kReduceMotion = 1 << 4,
+  kHighContrast = 1 << 5,
 };
 
 class WindowClient {

lib/web_ui/lib/src/ui/window.dart

@@ -996,6 +996,7 @@ class AccessibilityFeatures {
   static const int _kDisableAnimationsIndex = 1 << 2;
   static const int _kBoldTextIndex = 1 << 3;
   static const int _kReduceMotionIndex = 1 << 4;
+  static const int _kHighContrastIndex = 1 << 5;
 
   // A bitfield which represents each enabled feature.
   final int _index;
@@ -1023,6 +1024,11 @@ class AccessibilityFeatures {
   /// Only supported on iOS.
   bool get reduceMotion => _kReduceMotionIndex & _index != 0;
 
+  /// The platform is requesting that UI be rendered with darker colors.
+  ///
+  /// Only supported on iOS.
+  bool get highContrast => _kHighContrastIndex & _index != 0;
+
   @override
   String toString() {
     final List<String> features = <String>[];
@@ -1041,6 +1047,9 @@ class AccessibilityFeatures {
     if (reduceMotion) {
       features.add('reduceMotion');
     }
+    if (highContrast) {
+      features.add('highContrast');
+    }
     return 'AccessibilityFeatures$features';
   }
 

shell/platform/darwin/ios/framework/Source/FlutterViewController.mm

@@ -298,6 +298,11 @@ - (void)setupNotificationCenterObservers {
                  name:UIAccessibilityBoldTextStatusDidChangeNotification
                object:nil];
 
+  [center addObserver:self
+             selector:@selector(onAccessibilityStatusChanged:)
+                 name:UIAccessibilityDarkerSystemColorsStatusDidChangeNotification
+               object:nil];
+
   [center addObserver:self
              selector:@selector(onUserSettingsChanged:)
                  name:UIContentSizeCategoryDidChangeNotification
@@ -962,6 +967,8 @@ - (void)onAccessibilityStatusChanged:(NSNotification*)notification {
     flags |= static_cast<int32_t>(flutter::AccessibilityFeatureFlag::kReduceMotion);
   if (UIAccessibilityIsBoldTextEnabled())
     flags |= static_cast<int32_t>(flutter::AccessibilityFeatureFlag::kBoldText);
+  if (UIAccessibilityDarkerSystemColorsEnabled())
+    flags |= static_cast<int32_t>(flutter::AccessibilityFeatureFlag::kHighContrast);
 #if TARGET_OS_SIMULATOR
   // There doesn't appear to be any way to determine whether the accessibility
   // inspector is enabled on the simulator. We conservatively always turn on the