Add option to invert paint colors to be used for smart invert accessibility on iOS (flutter#6176)

Author: Jonah Williams

lib/ui/painting.dart

@@ -1065,6 +1065,7 @@ class Paint {
   static const int _kMaskFilterIndex = 12;
   static const int _kMaskFilterBlurStyleIndex = 13;
   static const int _kMaskFilterSigmaIndex = 14;
+  static const int _kInvertColorIndex = 15;
 
   static const int _kIsAntiAliasOffset = _kIsAntiAliasIndex << 2;
   static const int _kColorOffset = _kColorIndex << 2;
@@ -1081,6 +1082,7 @@ class Paint {
   static const int _kMaskFilterOffset = _kMaskFilterIndex << 2;
   static const int _kMaskFilterBlurStyleOffset = _kMaskFilterBlurStyleIndex << 2;
   static const int _kMaskFilterSigmaOffset = _kMaskFilterSigmaIndex << 2;
+  static const int _kInvertColorOffset = _kInvertColorIndex << 2;
   // If you add more fields, remember to update _kDataByteCount.
   static const int _kDataByteCount = 75;
 
@@ -1363,6 +1365,18 @@ class Paint {
     }
   }
 
+  /// Whether the colors of the image are inverted when drawn.
+  ///
+  /// inverting the colors of an image applies a new color filter that will
+  /// be composed with any user provided color filters. This is primarily
+  /// used for implementing smart invert on iOS.
+  bool get invertColors {
+    return _data.getInt32(_kInvertColorOffset, _kFakeHostEndian) == 1;
+  }
+  set invertColors(bool value) {
+    _data.setInt32(_kInvertColorOffset, value ? 1 : 0, _kFakeHostEndian);
+  }
+
   @override
   String toString() {
     final StringBuffer result = new StringBuffer();
@@ -1411,8 +1425,12 @@ class Paint {
       result.write('${semicolon}filterQuality: $filterQuality');
       semicolon = '; ';
     }
-    if (shader != null)
+    if (shader != null) {
       result.write('${semicolon}shader: $shader');
+      semicolon = '; ';
+    }
+    if (invertColors)
+      result.write('${semicolon}invert: $invertColors');
     result.write(')');
     return result.toString();
   }

lib/ui/painting/paint.cc

@@ -30,6 +30,7 @@ constexpr int kColorFilterBlendModeIndex = 11;
 constexpr int kMaskFilterIndex = 12;
 constexpr int kMaskFilterBlurStyleIndex = 13;
 constexpr int kMaskFilterSigmaIndex = 14;
+constexpr int kInvertColorIndex = 15;
 constexpr size_t kDataByteCount = 75;  // 4 * (last index + 1)
 
 // Indices for objects.
@@ -47,6 +48,16 @@ constexpr uint32_t kBlendModeDefault =
 // default SkPaintDefaults_MiterLimit in Skia (which is not in a public header).
 constexpr double kStrokeMiterLimitDefault = 4.0;
 
+// A color matrix which inverts colors.
+// clang-format off
+constexpr SkScalar invert_colors[20] = {
+  -1.0,    0,    0, 1.0, 0,
+     0, -1.0,    0, 1.0, 0,
+     0,    0, -1.0, 1.0, 0,
+   1.0,  1.0,  1.0, 1.0, 0
+};
+// clang-format on
+
 // Must be kept in sync with the MaskFilter private constants in painting.dart.
 enum MaskFilterType { Null, Blur };
 
@@ -116,7 +127,19 @@ Paint::Paint(Dart_Handle paint_objects, Dart_Handle paint_data) {
   if (filter_quality)
     paint_.setFilterQuality(static_cast<SkFilterQuality>(filter_quality));
 
-  if (uint_data[kColorFilterIndex]) {
+  if (uint_data[kColorFilterIndex] && uint_data[kInvertColorIndex]) {
+    SkColor color = uint_data[kColorFilterColorIndex];
+    SkBlendMode blend_mode =
+        static_cast<SkBlendMode>(uint_data[kColorFilterBlendModeIndex]);
+    sk_sp<SkColorFilter> color_filter =
+        SkColorFilter::MakeModeFilter(color, blend_mode);
+    sk_sp<SkColorFilter> invert_filter =
+        SkColorFilter::MakeMatrixFilterRowMajor255(invert_colors);
+    paint_.setColorFilter(invert_filter->makeComposed(color_filter));
+  } else if (uint_data[kInvertColorIndex]) {
+    paint_.setColorFilter(
+        SkColorFilter::MakeMatrixFilterRowMajor255(invert_colors));
+  } else if (uint_data[kColorFilterIndex]) {
     SkColor color = uint_data[kColorFilterColorIndex];
     SkBlendMode blend_mode =
         static_cast<SkBlendMode>(uint_data[kColorFilterBlendModeIndex]);