Canvas.drawShadow API based on SkShadowUtils::drawShadow (flutter/engine#3486)

engine/src/flutter/flow/layers/physical_model_layer.cc

@@ -50,16 +50,8 @@ void PhysicalModelLayer::Paint(PaintContext& context) {
   path.addRRect(rrect_);
 
   if (elevation_ != 0) {
-    SkShadowFlags flags = SkColorGetA(color_) == 0xff ?
-        SkShadowFlags::kNone_ShadowFlag :
-        SkShadowFlags::kTransparentOccluder_ShadowFlag;
-    SkShadowUtils::DrawShadow(&context.canvas, path,
-                              elevation_ * 4,
-                              SkPoint3::Make(0.0f, -700.0f, 2800.0f),
-                              2800.0f,
-                              0.25f, 0.25f,
-                              SK_ColorBLACK,
-                              flags);
+    DrawShadow(&context.canvas, path, SK_ColorBLACK, elevation_,
+               SkColorGetA(color_) != 0xff);
   }
 
   if (needs_system_composite())
@@ -79,4 +71,19 @@ void PhysicalModelLayer::Paint(PaintContext& context) {
   PaintChildren(context);
 }
 
+void PhysicalModelLayer::DrawShadow(SkCanvas* canvas, const SkPath& path,
+                                    SkColor color, int elevation,
+                                    bool transparentOccluder) {
+    SkShadowFlags flags = transparentOccluder ?
+        SkShadowFlags::kTransparentOccluder_ShadowFlag :
+        SkShadowFlags::kNone_ShadowFlag;
+    SkShadowUtils::DrawShadow(canvas, path,
+                              elevation * 4,
+                              SkPoint3::Make(0.0f, -700.0f, 2800.0f),
+                              2800.0f,
+                              0.25f, 0.25f,
+                              color,
+                              flags);
+}
+
 }  // namespace flow

engine/src/flutter/flow/layers/physical_model_layer.h

@@ -18,6 +18,9 @@ class PhysicalModelLayer : public ContainerLayer {
   void set_elevation(int elevation) { elevation_ = elevation; }
   void set_color(SkColor color) { color_ = color; }
 
+  static void DrawShadow(SkCanvas* canvas, const SkPath& path,
+                         SkColor color, int elevation, bool transparentOccluder);
+
  protected:
   void Preroll(PrerollContext* context, const SkMatrix& matrix) override;
   void Paint(PaintContext& context) override;

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

@@ -1548,6 +1548,18 @@ class Canvas extends NativeFieldWrapperClass2 {
                   Int32List colors,
                   int blendMode,
                   Float32List cullRect) native "Canvas_drawAtlas";
+
+  /// Draws a shadow for a [Path] representing the given material elevation.
+  ///
+  /// transparentOccluder should be true if the occluding object is not opaque.
+  void drawShadow(Path path, Color color, int elevation, bool transparentOccluder) {
+    _drawShadow(path, color.value, elevation, transparentOccluder);
+  }
+
+  void _drawShadow(Path path,
+                   int color,
+                   int elevation,
+                   bool transparentOccluder) native "Canvas_drawShadow";
 }
 
 /// An object representing a sequence of recorded graphical operations.

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

@@ -6,6 +6,7 @@
 
 #include <math.h>
 
+#include "flutter/flow/layers/physical_model_layer.h"
 #include "flutter/lib/ui/painting/image.h"
 #include "flutter/lib/ui/painting/matrix.h"
 #include "lib/tonic/converter/dart_converter.h"
@@ -53,7 +54,8 @@ IMPLEMENT_WRAPPERTYPEINFO(ui, Canvas);
   V(Canvas, drawPicture)            \
   V(Canvas, drawPoints)             \
   V(Canvas, drawVertices)           \
-  V(Canvas, drawAtlas)
+  V(Canvas, drawAtlas)              \
+  V(Canvas, drawShadow)
 
 FOR_EACH_BINDING(DART_NATIVE_CALLBACK)
 
@@ -395,6 +397,17 @@ void Canvas::drawAtlas(const Paint& paint,
       paint.paint());
 }
 
+void Canvas::drawShadow(const CanvasPath* path,
+                        SkColor color,
+                        int elevation,
+                        bool transparentOccluder) {
+  flow::PhysicalModelLayer::DrawShadow(canvas_,
+                                       path->path(),
+                                       color,
+                                       elevation,
+                                       transparentOccluder);
+}
+
 void Canvas::Clear() {
   canvas_ = nullptr;
 }

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

@@ -15,6 +15,7 @@
 #include "lib/tonic/typed_data/float64_list.h"
 #include "lib/tonic/typed_data/int32_list.h"
 #include "third_party/skia/include/core/SkCanvas.h"
+#include "third_party/skia/include/utils/SkShadowUtils.h"
 
 namespace tonic {
 class DartLibraryNatives;
@@ -159,6 +160,11 @@ class Canvas : public ftl::RefCountedThreadSafe<Canvas>,
                  SkBlendMode blend_mode,
                  const tonic::Float32List& cull_rect);
 
+  void drawShadow(const CanvasPath* path,
+                  SkColor color,
+                  int elevation,
+                  bool transparentOccluder);
+
   SkCanvas* canvas() const { return canvas_; }
   void Clear();
   bool IsRecording() const;