TwoPointConicGradients again (flutter#5299)

* update docs for getBounds

* Add computeMetrics suggestion

* better explanation

* Support for TwoPointConical gradients

lib/ui/painting.dart

@@ -2310,25 +2310,45 @@ class Gradient extends Shader {
   /// 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.
+  /// 
+  /// If `focal` is provided and not equal to `center` and `focalRadius` is 
+  /// provided and not equal to 0.0, the generated shader will be a two point
+  /// conical radial gradient, with `focal` being the center of the focal 
+  /// circle and `focalRadius` being the radius of that circle. If `focal` is
+  /// provided and not equal to `center`, at least one of the two offsets must
+  /// not be equal to [Offset.zero].
   Gradient.radial(
     Offset center,
     double radius,
     List<Color> colors, [
     List<double> colorStops,
     TileMode tileMode = TileMode.clamp,
-    Float64List matrix4
+    Float64List matrix4,
+    Offset focal,
+    double focalRadius = 0.0
   ]) : assert(_offsetIsValid(center)),
        assert(colors != null),
        assert(tileMode != null),
        assert(matrix4 == null || _matrix4IsValid(matrix4)),
        super._() {
+    focalRadius ??= 0.0;
     _validateColorStops(colors, colorStops);
     final Int32List colorsBuffer = _encodeColorList(colors);
     final Float32List colorStopsBuffer = colorStops == null ? null : new Float32List.fromList(colorStops);
-    _constructor();
-    _initRadial(center.dx, center.dy, radius, colorsBuffer, colorStopsBuffer, tileMode.index, matrix4);
+
+    // If focal is null or focal radius is null, this should be treated as a regular radial gradient
+    // If focal == center and the focal radius is 0.0, it's still a regular radial gradient
+    if (focal == null || (focal == center && focalRadius == 0.0)) {
+      _constructor();
+      _initRadial(center.dx, center.dy, radius, colorsBuffer, colorStopsBuffer, tileMode.index, matrix4);
+    } else {
+      assert(center != Offset.zero || focal != Offset.zero); // will result in exception(s) in Skia side
+      _constructor();
+      _initConical(focal.dx, focal.dy, focalRadius, center.dx, center.dy, radius, colorsBuffer, colorStopsBuffer, tileMode.index, matrix4);
+    }
   }
   void _initRadial(double centerX, double centerY, double radius, Int32List colors, Float32List colorStops, int tileMode, Float64List matrix4) native 'Gradient_initRadial';
+  void _initConical(double startX, double startY, double startRadius, double endX, double endY, double endRadius, Int32List colors, Float32List colorStops, int tileMode, Float64List matrix4) native 'Gradient_initTwoPointConical';
 
   /// Creates a sweep gradient centered at `center` that starts at `startAngle`
   /// and ends at `endAngle`.

lib/ui/painting/gradient.cc

@@ -25,7 +25,8 @@ IMPLEMENT_WRAPPERTYPEINFO(ui, Gradient);
 #define FOR_EACH_BINDING(V) \
   V(Gradient, initLinear)   \
   V(Gradient, initRadial)   \
-  V(Gradient, initSweep)
+  V(Gradient, initSweep)    \
+  V(Gradient, initTwoPointConical)
 
 FOR_EACH_BINDING(DART_NATIVE_CALLBACK)
 
@@ -109,6 +110,35 @@ void CanvasGradient::initSweep(double center_x,
       has_matrix ? &sk_matrix : nullptr)));
 }
 
+void CanvasGradient::initTwoPointConical(double start_x,
+                                         double start_y,
+                                         double start_radius,
+                                         double end_x,
+                                         double end_y,
+                                         double end_radius,
+                                         const tonic::Int32List& colors,
+                                         const tonic::Float32List& color_stops,
+                                         SkShader::TileMode tile_mode,
+                                         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(UIDartState::CreateGPUObject(SkGradientShader::MakeTwoPointConical(
+      SkPoint::Make(start_x, start_y), start_radius,
+      SkPoint::Make(end_x, end_y), end_radius,
+      reinterpret_cast<const SkColor*>(colors.data()), color_stops.data(),
+      colors.num_elements(), tile_mode, 0, has_matrix ? &sk_matrix : nullptr)));
+}
+
 CanvasGradient::CanvasGradient() = default;
 
 CanvasGradient::~CanvasGradient() = default;

lib/ui/painting/gradient.h

@@ -52,6 +52,17 @@ class CanvasGradient : public Shader {
                  double end_angle,
                  const tonic::Float64List& matrix4);
 
+  void initTwoPointConical(double start_x,
+                           double start_y,
+                           double start_radius,
+                           double end_x,
+                           double end_y,
+                           double end_radius,
+                           const tonic::Int32List& colors,
+                           const tonic::Float32List& color_stops,
+                           SkShader::TileMode tile_mode,
+                           const tonic::Float64List& matrix4);
+
   static void RegisterNatives(tonic::DartLibraryNatives* natives);
 
  private:

testing/dart/gradient_test.dart

@@ -0,0 +1,72 @@
+// Copyright 2018 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'dart:typed_data' show Float64List;
+import 'dart:ui';
+
+import 'package:test/test.dart';
+
+void main() {
+  test('Gradient.radial with no focal point', () {
+    expect(
+      new Gradient.radial(
+          Offset.zero,
+          null,
+          <Color>[const Color(0xFFFFFFFF), const Color(0xFFFFFFFF)],
+          <double>[0.0, 1.0],
+          TileMode.mirror),
+      isNotNull,
+    );
+  });
+
+  // this is just a radial gradient, focal point is discarded.
+  test('radial center and focal == Offset.zero and focalRadius == 0.0 is ok',
+      () {
+    expect(
+        () => new Gradient.radial(
+              Offset.zero,
+              0.0,
+              <Color>[const Color(0xFFFFFFFF), const Color(0xFFFFFFFF)],
+              <double>[0.0, 1.0],
+              TileMode.mirror,
+              null,
+              Offset.zero,
+              0.0,
+            ),
+        isNotNull);
+  });
+
+  test('radial center != focal and focalRadius == 0.0 is ok', () {
+    expect(
+        () => new Gradient.radial(
+              Offset.zero,
+              0.0,
+              <Color>[const Color(0xFFFFFFFF), const Color(0xFFFFFFFF)],
+              <double>[0.0, 1.0],
+              TileMode.mirror,
+              null,
+              const Offset(2.0, 2.0),
+              0.0,
+            ),
+        isNotNull);
+  });
+
+  // this would result in div/0 on skia side.
+  test('radial center and focal == Offset.zero and focalRadius != 0.0 assert',
+      () {
+    expect(
+      () => new Gradient.radial(
+            Offset.zero,
+            0.0,
+            <Color>[const Color(0xFFFFFFFF), const Color(0xFFFFFFFF)],
+            <double>[0.0, 1.0],
+            TileMode.mirror,
+            null,
+            Offset.zero,
+            1.0,
+          ),
+      throwsA(const isInstanceOf<AssertionError>()),
+    );
+  });
+}