[Impeller] Use StrokePathSourceGeometry to stroke RoundSuperellipse (flutter#169704)

This PR creates a new class, `StrokeRoundSuperellipseGeometry`, which
reuses the `StrokePathSourceGeometry` class to stroke rounded
superellipses.

Fixes flutter#168419.

Filling is unchanged. I could have used `FillPathSourceGeometry` for
non-uniform filling just like RRect, but I've decided not to after some
experimentation, mostly because the `RoundSuperellipseGeometry` class
wouldn't be simplified from uniformity as much as we hope for. This
might be worthwhile if one day we move the filling code to `Tessellator`
(just like `FilledRoundRect`), which reuses much existing code. But
that's out of the scope for now.

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responsibilities.
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we expect every widget to implement].
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- [ ] I listed at least one issue that this PR fixes in the description
above.
- [ ] I updated/added relevant documentation (doc comments with `///`).
- [ ] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [ ] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [ ] All existing and new tests are passing.

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Author: dkwingsmt

engine/src/flutter/display_list/geometry/dl_path_builder.cc

@@ -115,7 +115,7 @@ DlPathBuilder& DlPathBuilder::AddRoundSuperellipse(
   BuilderReceiver receiver(*this);
   impeller::RoundSuperellipseParam::MakeBoundsRadii(rse.GetBounds(),
                                                     rse.GetRadii())
-      .AddToPath(receiver);
+      .Dispatch(receiver);
   return *this;
 }
 

engine/src/flutter/impeller/display_list/canvas.cc

@@ -758,29 +758,26 @@ void Canvas::DrawDiffRoundRect(const RoundRect& outer,
   }
 }
 
-void Canvas::DrawRoundSuperellipse(const RoundSuperellipse& rse,
+void Canvas::DrawRoundSuperellipse(const RoundSuperellipse& round_superellipse,
                                    const Paint& paint) {
-  auto& rect = rse.GetBounds();
-  auto& radii = rse.GetRadii();
+  auto& rect = round_superellipse.GetBounds();
+  auto& radii = round_superellipse.GetRadii();
   if (radii.AreAllCornersSame() &&
       AttemptDrawBlurredRSuperellipse(rect, radii.top_left, paint)) {
     return;
   }
 
-  if (paint.style == Paint::Style::kFill) {
-    Entity entity;
-    entity.SetTransform(GetCurrentTransform());
-    entity.SetBlendMode(paint.blend_mode);
+  Entity entity;
+  entity.SetTransform(GetCurrentTransform());
+  entity.SetBlendMode(paint.blend_mode);
 
+  if (paint.style == Paint::Style::kFill) {
     RoundSuperellipseGeometry geom(rect, radii);
     AddRenderEntityWithFiltersToCurrentPass(entity, &geom, paint);
-    return;
+  } else {
+    StrokeRoundSuperellipseGeometry geom(round_superellipse, paint.stroke);
+    AddRenderEntityWithFiltersToCurrentPass(entity, &geom, paint);
   }
-
-  auto path = flutter::DlPathBuilder{}  //
-                  .AddRoundSuperellipse(rse)
-                  .TakePath();
-  DrawPath(flutter::DlPath(path), paint);
 }
 
 void Canvas::DrawCircle(const Point& center,

engine/src/flutter/impeller/entity/geometry/round_rect_geometry.h

@@ -8,6 +8,7 @@
 #include "impeller/entity/geometry/fill_path_geometry.h"
 #include "impeller/entity/geometry/geometry.h"
 #include "impeller/entity/geometry/stroke_path_geometry.h"
+#include "impeller/geometry/round_rect.h"
 
 namespace impeller {
 

engine/src/flutter/impeller/entity/geometry/round_superellipse_geometry.cc

@@ -462,4 +462,14 @@ bool RoundSuperellipseGeometry::IsAxisAlignedRect() const {
   return false;
 }
 
+StrokeRoundSuperellipseGeometry::StrokeRoundSuperellipseGeometry(
+    const RoundSuperellipse& round_superellipse,
+    const StrokeParameters& parameters)
+    : StrokePathSourceGeometry(parameters),
+      round_superellipse_source_(round_superellipse) {}
+
+const PathSource& StrokeRoundSuperellipseGeometry::GetSource() const {
+  return round_superellipse_source_;
+}
+
 }  // namespace impeller

engine/src/flutter/impeller/entity/geometry/round_superellipse_geometry.h

@@ -6,22 +6,27 @@
 #define FLUTTER_IMPELLER_ENTITY_GEOMETRY_ROUND_SUPERELLIPSE_GEOMETRY_H_
 
 #include "impeller/entity/geometry/geometry.h"
+#include "impeller/entity/geometry/stroke_path_geometry.h"
+#include "impeller/geometry/round_superellipse.h"
 #include "impeller/geometry/rounding_radii.h"
 
 namespace impeller {
-
-/// Geometry class that can generate vertices for a rounded superellipse.
+/// @brief A Geometry class that generates fillable vertices (with or without
+///        texture coordinates) directly from a round superellipse object
+///        regardless of radii uniformity.
 ///
 /// A rounded superellipse is a shape similar to a typical rounded rectangle
-/// (`RoundRect`), but with smoother transitions between the straight sides and
-/// the rounded corners. It resembles the `RoundedRectangle` shape in SwiftUI
-/// with the `.continuous` corner style. Technically, it is created by replacing
-/// the four corners of a superellipse (also known as a Lamé curve) with
-/// circular arcs.
+/// (`RoundSuperellipse`), but with smoother transitions between the straight
+/// sides and the rounded corners. It resembles the `RoundedRectangle` shape in
+/// SwiftUI with the `.continuous` corner style. Technically, it is created by
+/// replacing the four corners of a superellipse (also known as a Lamé curve)
+/// with circular arcs.
 ///
 /// The `bounds` defines the position and size of the shape. The `corner_radius`
 /// corresponds to SwiftUI's `cornerRadius` parameter, which is close to, but
 /// not exactly equals to, the radius of the corner circles.
+///
+/// @see |StrokeRoundSuperellipseGeometry|
 class RoundSuperellipseGeometry final : public Geometry {
  public:
   RoundSuperellipseGeometry(const Rect& bounds, const RoundingRadii& radii);
@@ -53,6 +58,27 @@ class RoundSuperellipseGeometry final : public Geometry {
       delete;
 };
 
+/// @brief A Geometry class that produces fillable vertices representing
+///        the stroked outline of any |RoundSuperellipse| object regardless of
+///        radii uniformity.
+///
+/// This class uses the |StrokePathSourceGeometry| base class to do the work
+/// by providing a |RoundSuperellipsePathSoure| iterator.
+///
+/// @see |RoundSuperellipseGeometry|
+class StrokeRoundSuperellipseGeometry final : public StrokePathSourceGeometry {
+ public:
+  StrokeRoundSuperellipseGeometry(const RoundSuperellipse& round_superellipse,
+                                  const StrokeParameters& parameters);
+
+ protected:
+  // |StrokePathSourceGeometry|
+  const PathSource& GetSource() const override;
+
+ private:
+  const RoundSuperellipsePathSource round_superellipse_source_;
+};
+
 }  // namespace impeller
 
 #endif  // FLUTTER_IMPELLER_ENTITY_GEOMETRY_ROUND_SUPERELLIPSE_GEOMETRY_H_

engine/src/flutter/impeller/geometry/round_superellipse.cc

@@ -38,4 +38,29 @@ RoundRect RoundSuperellipse::ToApproximateRoundRect() const {
   return RoundRect::MakeRectRadii(GetBounds(), GetRadii());
 }
 
+RoundSuperellipsePathSource::RoundSuperellipsePathSource(
+    const RoundSuperellipse& round_superellipse)
+    : round_superellipse_(round_superellipse) {}
+
+RoundSuperellipsePathSource::~RoundSuperellipsePathSource() = default;
+
+FillType RoundSuperellipsePathSource::GetFillType() const {
+  return FillType::kNonZero;
+}
+
+Rect RoundSuperellipsePathSource::GetBounds() const {
+  return round_superellipse_.GetBounds();
+}
+
+bool RoundSuperellipsePathSource::IsConvex() const {
+  return true;
+}
+
+void RoundSuperellipsePathSource::Dispatch(PathReceiver& receiver) const {
+  auto param = RoundSuperellipseParam::MakeBoundsRadii(
+      round_superellipse_.GetBounds(), round_superellipse_.GetRadii());
+  param.Dispatch(receiver);
+  receiver.PathEnd();
+}
+
 }  // namespace impeller

engine/src/flutter/impeller/geometry/round_superellipse.h

@@ -5,6 +5,7 @@
 #ifndef FLUTTER_IMPELLER_GEOMETRY_ROUND_SUPERELLIPSE_H_
 #define FLUTTER_IMPELLER_GEOMETRY_ROUND_SUPERELLIPSE_H_
 
+#include "flutter/impeller/geometry/path_source.h"
 #include "flutter/impeller/geometry/point.h"
 #include "flutter/impeller/geometry/rect.h"
 #include "flutter/impeller/geometry/rounding_radii.h"
@@ -142,6 +143,33 @@ struct RoundSuperellipse {
   RoundingRadii radii_;
 };
 
+class RoundSuperellipsePathSource : public PathSource {
+ public:
+  explicit RoundSuperellipsePathSource(
+      const RoundSuperellipse& round_superellipse);
+
+  ~RoundSuperellipsePathSource();
+
+  const RoundSuperellipse& GetRoundSuperellipse() const {
+    return round_superellipse_;
+  }
+
+  // |PathSource|
+  FillType GetFillType() const override;
+
+  // |PathSource|
+  Rect GetBounds() const override;
+
+  // |PathSource|
+  bool IsConvex() const override;
+
+  // |PathSource|
+  void Dispatch(PathReceiver& receiver) const override;
+
+ private:
+  const RoundSuperellipse round_superellipse_;
+};
+
 }  // namespace impeller
 
 namespace std {

engine/src/flutter/impeller/geometry/round_superellipse_param.cc

@@ -519,7 +519,7 @@ RoundSuperellipseParam RoundSuperellipseParam::MakeBoundsRadii(
   };
 }
 
-void RoundSuperellipseParam::AddToPath(PathReceiver& path_receiver) const {
+void RoundSuperellipseParam::Dispatch(PathReceiver& path_receiver) const {
   RoundSuperellipseBuilder builder(path_receiver);
 
   Point start = top_right.offset +

engine/src/flutter/impeller/geometry/round_superellipse_param.h

@@ -110,8 +110,8 @@ struct RoundSuperellipseParam {
   // with the bounds, which is recommended for callers.
   bool Contains(const Point& point) const;
 
-  // Add a path of this rounded superellipse to the provided path builder.
-  void AddToPath(PathReceiver& path) const;
+  // Dispatch the path operations of this rounded superellipse to the receiver.
+  void Dispatch(PathReceiver& receiver) const;
 
   // A factor used to calculate the "gap", defined as the distance from the
   // midpoint of the curved corners to the nearest sides of the bounding box.