Add Picture.toImage (#3657)

Fixes https://github.com/flutter/flutter/issues/6774

lib/ui/painting.dart

@@ -1649,6 +1649,15 @@ abstract class Picture extends NativeFieldWrapperClass2 {
   /// object. To create a Picture object, use a [PictureRecorder].
   Picture(); // (this constructor is here just so we can document it)
 
+  /// Creates an image from this picture.
+  /// 
+  /// The picture is rasterized using the number of pixels specified by the
+  /// given width and height.
+  /// 
+  /// Although the image is returned synchronously, the picture is actually
+  /// rasterized the first time the image is drawn and then cached.
+  Image toImage(int width, int height) native "Picture_toImage";
+
   /// Release the resources used by this object. The object is no longer usable
   /// after this method is called.
   void dispose() native "Picture_dispose";

lib/ui/painting/picture.cc

@@ -7,16 +7,19 @@
 #include "flutter/common/threads.h"
 #include "flutter/lib/ui/painting/canvas.h"
 #include "flutter/lib/ui/painting/utils.h"
+#include "lib/tonic/converter/dart_converter.h"
 #include "lib/tonic/dart_args.h"
 #include "lib/tonic/dart_binding_macros.h"
-#include "lib/tonic/converter/dart_converter.h"
 #include "lib/tonic/dart_library_natives.h"
+#include "third_party/skia/include/core/SkImage.h"
 
 namespace blink {
 
 IMPLEMENT_WRAPPERTYPEINFO(ui, Picture);
 
-#define FOR_EACH_BINDING(V) V(Picture, dispose)
+#define FOR_EACH_BINDING(V) \
+  V(Picture, toImage)       \
+  V(Picture, dispose)
 
 DART_BIND_ALL(Picture, FOR_EACH_BINDING)
 
@@ -32,6 +35,15 @@ Picture::~Picture() {
   SkiaUnrefOnIOThread(&picture_);
 }
 
+ftl::RefPtr<CanvasImage> Picture::toImage(int width, int height) {
+  ftl::RefPtr<CanvasImage> image = CanvasImage::Create();
+  // TODO(abarth): We should pass in an SkColorSpace at some point.
+  image->set_image(
+      SkImage::MakeFromPicture(picture_, SkISize::Make(width, height), nullptr,
+                               nullptr, SkImage::BitDepth::kU8, nullptr));
+  return image;
+}
+
 void Picture::dispose() {
   ClearDartWrapper();
 }

lib/ui/painting/picture.h

@@ -5,6 +5,7 @@
 #ifndef FLUTTER_LIB_UI_PAINTING_PICTURE_H_
 #define FLUTTER_LIB_UI_PAINTING_PICTURE_H_
 
+#include "flutter/lib/ui/painting/image.h"
 #include "lib/tonic/dart_wrappable.h"
 #include "third_party/skia/include/core/SkPicture.h"
 
@@ -26,6 +27,8 @@ class Picture : public ftl::RefCountedThreadSafe<Picture>,
 
   const sk_sp<SkPicture>& picture() const { return picture_; }
 
+  ftl::RefPtr<CanvasImage> toImage(int width, int height);
+
   void dispose();
 
   static void RegisterNatives(tonic::DartLibraryNatives* natives);