Added drawArc to Canvas. (#2995)

* Added drawArc to Canvas.

Added a way to call SkCanvas::drawArc from Dart. This method is
optimized for the arc case and should be faster that drawing paths.

* Removed implementation details from docs.

* Added more details to the docs.

lib/ui/painting.dart

@@ -1297,6 +1297,30 @@ class Canvas extends NativeFieldWrapperClass2 {
                    List<dynamic> paintObjects,
                    ByteData paintData) native "Canvas_drawCircle";
 
+  /// Draw an arc scaled to fit inside the given rectangle. It starts from
+  /// startAngle radians around the oval up to startAngle + sweepAngle
+  /// radians around the oval, with zero radians being the point on
+  /// the right hand side of the oval that crosses the horizontal line
+  /// that intersects the center of the rectangle and with positive
+  /// angles going clockwise around the oval. If useCenter is true, the arc is
+  /// closed back to the center, forming a circle sector. Otherwise, the arc is
+  /// not closed, forming a circle segment.
+  ///
+  /// This method is optimized for drawing arcs and should be faster than [Path.arcTo].
+  void drawArc(Rect rect, double startAngle, double sweepAngle, bool useCenter, Paint paint) {
+    _drawArc(rect.left, rect.top, rect.right, rect.bottom, startAngle,
+             sweepAngle, useCenter, paint._objects, paint._data);
+  }
+  void _drawArc(double left,
+                double top,
+                double right,
+                double bottom,
+                double startAngle,
+                double sweepAngle,
+                bool useCenter,
+                List<dynamic> paintObjects,
+                ByteData paintData) native "Canvas_drawArc";
+
   /// Draws the given [Path] with the given [Paint]. Whether this shape is
   /// filled or stroked (or both) is controlled by [Paint.style]. If the path is
   /// filled, then subpaths within it are implicitly closed (see [Path.close]).

lib/ui/painting/canvas.cc

@@ -46,6 +46,7 @@ IMPLEMENT_WRAPPERTYPEINFO(ui, Canvas);
   V(Canvas, drawDRRect)             \
   V(Canvas, drawOval)               \
   V(Canvas, drawCircle)             \
+  V(Canvas, drawArc)                \
   V(Canvas, drawPath)               \
   V(Canvas, drawImage)              \
   V(Canvas, drawImageRect)          \
@@ -242,6 +243,24 @@ void Canvas::drawCircle(double x,
   canvas_->drawCircle(x, y, radius, *paint.paint());
 }
 
+void Canvas::drawArc(double left,
+             double top,
+             double right,
+             double bottom,
+             double startAngle,
+             double sweepAngle,
+             bool useCenter,
+             const Paint& paint,
+             const PaintData& paint_data) {
+  if (!canvas_)
+    return;
+  canvas_->drawArc(SkRect::MakeLTRB(left, top, right, bottom),
+                   startAngle * 180.0 / M_PI,
+                   sweepAngle* 180.0 / M_PI,
+                   useCenter,
+                   *paint.paint());
+}
+
 void Canvas::drawPath(const CanvasPath* path,
                       const Paint& paint,
                       const PaintData& paint_data) {

lib/ui/painting/canvas.h

@@ -100,6 +100,15 @@ class Canvas : public ftl::RefCountedThreadSafe<Canvas>,
                   double radius,
                   const Paint& paint,
                   const PaintData& paint_data);
+  void drawArc(double left,
+               double top,
+               double right,
+               double bottom,
+               double startAngle,
+               double sweepAngle,
+               bool useCenter,
+               const Paint& paint,
+               const PaintData& paint_data);
   void drawPath(const CanvasPath* path,
                 const Paint& paint,
                 const PaintData& paint_data);