Enable sweep gradients (flutter/engine#4950)

* Enable sweep gradients

* Use _matrix4IsValid

* Add assert for startAngle < endAngle

* Use radians for startAngle and endAngle

* Update AUTHORS

* Better assert on matrix4IsValid

engine/src/flutter/AUTHORS

@@ -8,3 +8,4 @@ Jim Simon <jim.j.simon@gmail.com>
 Ali Bitek <alibitek@protonmail.ch>
 Jacob Greenfield <jacob.greenfield.256@gmail.com>
 Dan Field <dfield@gmail.com>
+Victor Choueiri <victor@ctrlanddev.com>

engine/src/flutter/lib/ui/painting.dart

@@ -2231,8 +2231,8 @@ Float32List _encodeTwoPoints(Offset pointA, Offset pointB) {
 
 /// A shader (as used by [Paint.shader]) that renders a color gradient.
 ///
-/// There are two useful types of gradients, created by [new Gradient.linear]
-/// and [new Gradient.radial].
+/// There are several types of gradients, represented by the various constructors
+/// on this class.
 class Gradient extends Shader {
 
   void _constructor() native 'Gradient_constructor';
@@ -2306,9 +2306,8 @@ class Gradient extends Shader {
   ]) : assert(_offsetIsValid(center)),
        assert(colors != null),
        assert(tileMode != null),
+       assert(matrix4 == null || _matrix4IsValid(matrix4)),
        super._() {
-    if (matrix4 != null && matrix4.length != 16)
-      throw new ArgumentError('"matrix4" must have 16 entries.');
     _validateColorStops(colors, colorStops);
     final Int32List colorsBuffer = _encodeColorList(colors);
     final Float32List colorStopsBuffer = colorStops == null ? null : new Float32List.fromList(colorStops);
@@ -2317,6 +2316,56 @@ class Gradient extends Shader {
   }
   void _initRadial(double centerX, double centerY, double radius, Int32List colors, Float32List colorStops, int tileMode, Float64List matrix4) native 'Gradient_initRadial';
 
+  /// Creates a sweep gradient centered at `center` that starts at `startAngle`
+  /// and ends at `endAngle`.
+  ///
+  /// `startAngle` and `endAngle` should be provided in radians, with zero
+  /// radians being the horizontal line to the right of the `center` and with
+  /// positive angles going clockwise around the `center`.
+  ///
+  /// If `colorStops` is provided, `colorStops[i]` is a number from 0.0 to 1.0
+  /// that specifies where `color[i]` begins in the gradient. If `colorStops` is
+  /// not provided, then only two stops, at 0.0 and 1.0, are implied (and
+  /// `color` must therefore only have two entries).
+  ///
+  /// The behavior before `startAngle` and after `endAngle` is described by the
+  /// `tileMode` argument. For details, see the [TileMode] enum.
+  ///
+  /// ![](https://flutter.github.io/assets-for-api-docs/dart-ui/tile_mode_clamp_sweep.png)
+  /// ![](https://flutter.github.io/assets-for-api-docs/dart-ui/tile_mode_mirror_sweep.png)
+  /// ![](https://flutter.github.io/assets-for-api-docs/dart-ui/tile_mode_repeated_sweep.png)
+  ///
+  /// If `center`, `colors`, `tileMode`, `startAngle`, or `endAngle` are null,
+  /// or if `colors` or `colorStops` contain null values, this constructor will
+  /// throw a [NoSuchMethodError].
+  ///
+  /// If `matrix4` is provided, the gradient fill will be transformed by the 
+  /// specified 4x4 matrix relative to the local coordinate system. `matrix4` must
+  /// be a column-major matrix packed into a list of 16 values.
+  Gradient.sweep(
+    Offset center,
+    List<Color> colors, [
+    List<double> colorStops,
+    TileMode tileMode = TileMode.clamp,
+    double startAngle = 0.0,
+    double endAngle = math.pi * 2,
+    Float64List matrix4,
+  ]) : assert(_offsetIsValid(center)),
+       assert(colors != null),
+       assert(tileMode != null),
+       assert(startAngle != null),
+       assert(endAngle != null),
+       assert(startAngle < endAngle),
+       assert(matrix4 == null || _matrix4IsValid(matrix4)),
+       super._() {
+    _validateColorStops(colors, colorStops);
+    final Int32List colorsBuffer = _encodeColorList(colors);
+    final Float32List colorStopsBuffer = colorStops == null ? null : new Float32List.fromList(colorStops);
+    _constructor();
+    _initSweep(center.dx, center.dy, colorsBuffer, colorStopsBuffer, tileMode.index, startAngle, endAngle, matrix4);
+  }
+  void _initSweep(double centerX, double centerY, Int32List colors, Float32List colorStops, int tileMode, double startAngle, double endAngle, Float64List matrix) native 'Gradient_initSweep';
+
   static void _validateColorStops(List<Color> colors, List<double> colorStops) {
     if (colorStops == null) {
       if (colors.length != 2)

engine/src/flutter/lib/ui/painting/gradient.cc

@@ -22,7 +22,8 @@ IMPLEMENT_WRAPPERTYPEINFO(ui, Gradient);
 
 #define FOR_EACH_BINDING(V) \
   V(Gradient, initLinear)   \
-  V(Gradient, initRadial)
+  V(Gradient, initRadial)   \
+  V(Gradient, initSweep)
 
 FOR_EACH_BINDING(DART_NATIVE_CALLBACK)
 
@@ -79,6 +80,33 @@ void CanvasGradient::initRadial(double center_x,
       colors.num_elements(), tile_mode, 0, has_matrix ? &sk_matrix : nullptr)));
 }
 
+void CanvasGradient::initSweep(double center_x,
+                               double center_y,
+                               const tonic::Int32List& colors,
+                               const tonic::Float32List& color_stops,
+                               SkShader::TileMode tile_mode,
+                               double start_angle,
+                               double end_angle,
+                               const tonic::Float64List& matrix4) {
+  FXL_DCHECK(colors.num_elements() == color_stops.num_elements() ||
+             color_stops.data() == nullptr);
+
+  static_assert(sizeof(SkColor) == sizeof(int32_t),
+                "SkColor doesn't use int32_t.");
+
+  SkMatrix sk_matrix;
+  bool has_matrix = matrix4.data() != nullptr;
+  if (has_matrix) {
+    sk_matrix = ToSkMatrix(matrix4);
+  }
+
+  set_shader(SkGradientShader::MakeSweep(
+      center_x, center_y, reinterpret_cast<const SkColor*>(colors.data()),
+      color_stops.data(), colors.num_elements(), tile_mode,
+      start_angle * 180.0 / M_PI, end_angle * 180.0 / M_PI, 0,
+      has_matrix ? &sk_matrix : nullptr));
+}
+
 CanvasGradient::CanvasGradient() = default;
 
 CanvasGradient::~CanvasGradient() = default;

engine/src/flutter/lib/ui/painting/gradient.h

@@ -43,6 +43,15 @@ class CanvasGradient : public Shader {
                   SkShader::TileMode tile_mode,
                   const tonic::Float64List& matrix4);
 
+  void initSweep(double center_x,
+                 double center_y,
+                 const tonic::Int32List& colors,
+                 const tonic::Float32List& color_stops,
+                 SkShader::TileMode tile_mode,
+                 double start_angle,
+                 double end_angle,
+                 const tonic::Float64List& matrix4);
+
   static void RegisterNatives(tonic::DartLibraryNatives* natives);
 
  private: