Allow shadows to be colored when physical layer is drawn by engine (#4812)

* pass shadow_color from dart:ui to physical layer

* default shadow color to SK_ColorBLACK

* add doc comments to pushPhysicalShape

flow/layers/default_layer_builder.cc

@@ -109,6 +109,7 @@ void DefaultLayerBuilder::PushShaderMask(sk_sp<SkShader> shader,
 void DefaultLayerBuilder::PushPhysicalShape(const SkPath& sk_path,
                                             double elevation,
                                             SkColor color,
+                                            SkColor shadow_color,
                                             SkScalar device_pixel_ratio) {
   SkRect cullRect;
   if (!cullRect.intersect(sk_path.getBounds(), cull_rects_.top())) {
@@ -118,6 +119,7 @@ void DefaultLayerBuilder::PushPhysicalShape(const SkPath& sk_path,
   layer->set_path(sk_path);
   layer->set_elevation(elevation);
   layer->set_color(color);
+  layer->set_shadow_color(shadow_color);
   layer->set_device_pixel_ratio(device_pixel_ratio);
   PushLayer(std::move(layer), cullRect);
 }

flow/layers/default_layer_builder.h

@@ -50,6 +50,7 @@ class DefaultLayerBuilder final : public LayerBuilder {
   void PushPhysicalShape(const SkPath& path,
                          double elevation,
                          SkColor color,
+                         SkColor shadow_color,
                          SkScalar device_pixel_ratio) override;
 
   // |flow::LayerBuilder|

flow/layers/layer_builder.h

@@ -50,6 +50,7 @@ class LayerBuilder {
   virtual void PushPhysicalShape(const SkPath& path,
                                  double elevation,
                                  SkColor color,
+                                 SkColor shadow_color,
                                  SkScalar device_pixel_ratio) = 0;
 
   virtual void PushPerformanceOverlay(uint64_t enabled_options,

flow/layers/physical_shape_layer.cc

@@ -82,7 +82,7 @@ void PhysicalShapeLayer::Paint(PaintContext& context) const {
   FXL_DCHECK(needs_painting());
 
   if (elevation_ != 0) {
-    DrawShadow(&context.canvas, path_, SK_ColorBLACK, elevation_,
+    DrawShadow(&context.canvas, path_, shadow_color_, elevation_,
                SkColorGetA(color_) != 0xff, device_pixel_ratio_);
   }
 

flow/layers/physical_shape_layer.h

@@ -18,6 +18,7 @@ class PhysicalShapeLayer : public ContainerLayer {
 
   void set_elevation(float elevation) { elevation_ = elevation; }
   void set_color(SkColor color) { color_ = color; }
+  void set_shadow_color(SkColor shadow_color) { shadow_color_ = shadow_color; }
   void set_device_pixel_ratio(SkScalar dpr) { device_pixel_ratio_ = dpr; }
 
   static void DrawShadow(SkCanvas* canvas,
@@ -38,6 +39,7 @@ class PhysicalShapeLayer : public ContainerLayer {
  private:
   float elevation_;
   SkColor color_;
+  SkColor shadow_color_;
   SkScalar device_pixel_ratio_;
   SkPath path_;
   bool isRect_;

lib/ui/compositing.dart

@@ -130,13 +130,16 @@ class SceneBuilder extends NativeFieldWrapperClass2 {
   /// Pushes a physical layer operation for an arbitrary shape onto the
   /// operation stack.
   ///
-  /// Rasterization will be clipped to the given shape.
+  /// Rasterization will be clipped to the given shape defined by [path]. If
+  /// [elevation] is greater than 0.0, then a shadow is drawn around the layer.
+  /// [shadowColor] defines the color of the shadow if present and [color] defines the 
+  /// color of the layer background.
   ///
   /// See [pop] for details about the operation stack.
-  void pushPhysicalShape({ Path path, double elevation, Color color }) {
-    _pushPhysicalShape(path, elevation, color.value);
+  void pushPhysicalShape({ Path path, double elevation, Color color, Color shadowColor}) {
+    _pushPhysicalShape(path, elevation, color.value, shadowColor?.value ?? 0xFF000000);
   }
-  void _pushPhysicalShape(Path path, double elevation, int color) native 
+  void _pushPhysicalShape(Path path, double elevation, int color, int shadowColor) native 
     'SceneBuilder_pushPhysicalShape';
 
   /// Ends the effect of the most recently pushed operation.

lib/ui/compositing/scene_builder.cc

@@ -102,11 +102,13 @@ void SceneBuilder::pushShaderMask(Shader* shader,
 
 void SceneBuilder::pushPhysicalShape(const CanvasPath* path,
                                      double elevation,
-                                     int color) {
+                                     int color,
+                                     int shadow_color) {
   layer_builder_->PushPhysicalShape(
       path->path(),                 //
       elevation,                    //
       static_cast<SkColor>(color),  //
+      static_cast<SkColor>(shadow_color),
       UIDartState::Current()->window()->viewport_metrics().device_pixel_ratio);
 }
 

lib/ui/compositing/scene_builder.h

@@ -47,7 +47,7 @@ class SceneBuilder : public fxl::RefCountedThreadSafe<SceneBuilder>,
                       double maskRectTop,
                       double maskRectBottom,
                       int blendMode);
-  void pushPhysicalShape(const CanvasPath* path, double elevation, int color);
+  void pushPhysicalShape(const CanvasPath* path, double elevation, int color, int shadowColor);
 
   void pop();