Enable Image encoding by leveraging existing Skia functionality

lib/ui/BUILD.gn

@@ -24,6 +24,8 @@ source_set("ui") {
     "painting/gradient.h",
     "painting/image.cc",
     "painting/image.h",
+    "painting/image_encoding.cc",
+    "painting/image_encoding.h",
     "painting/image_filter.cc",
     "painting/image_filter.h",
     "painting/image_shader.cc",

lib/ui/dart_ui.cc

@@ -12,6 +12,7 @@
 #include "flutter/lib/ui/painting/frame_info.h"
 #include "flutter/lib/ui/painting/gradient.h"
 #include "flutter/lib/ui/painting/image.h"
+#include "flutter/lib/ui/painting/image_encoding.h"
 #include "flutter/lib/ui/painting/image_filter.h"
 #include "flutter/lib/ui/painting/image_shader.h"
 #include "flutter/lib/ui/painting/path.h"
@@ -56,6 +57,7 @@ void DartUI::InitForGlobal() {
     Codec::RegisterNatives(g_natives);
     DartRuntimeHooks::RegisterNatives(g_natives);
     FrameInfo::RegisterNatives(g_natives);
+    ImageEncoding::RegisterNatives(g_natives);
     ImageFilter::RegisterNatives(g_natives);
     ImageShader::RegisterNatives(g_natives);
     Paragraph::RegisterNatives(g_natives);

lib/ui/painting.dart

@@ -1316,6 +1316,26 @@ Future<Null> _decodeImageFromListAsync(Uint8List list, ImageDecoderCallback call
   callback(frameInfo.image);
 }
 
+/// The encoding formats supported by the [encodeImage].
+enum EncodedImageFormat {
+  // Be conservative with the formats we expose. We can increase the list in future but reducing
+  // it is more difficult.
+  JPEG,
+  PNG,
+  WEBP,
+}
+
+/// Callback signature for [encodeImage].
+typedef void ImageEncoderCallback(Uint8List result);
+
+/// Convert an [Image] object into a byte array/
+///
+/// [format] is encoding format to be used.
+///
+/// [quality] is a value in [0, 100] where 0 corresponds to the lowest quality.
+void encodeImage(Image image, EncodedImageFormat format, int quality, ImageEncoderCallback callback)
+    native 'encodeImage';
+
 /// Determines the winding rule that decides how the interior of a [Path] is
 /// calculated.
 ///

lib/ui/painting/image_encoding.cc

@@ -0,0 +1,136 @@
+// Copyright 2018 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "flutter/lib/ui/painting/image_encoding.h"
+
+#include "flutter/common/threads.h"
+#include "flutter/lib/ui/painting/image.h"
+#include "flutter/lib/ui/painting/resource_context.h"
+#include "lib/fxl/build_config.h"
+#include "lib/fxl/functional/make_copyable.h"
+#include "lib/tonic/dart_persistent_value.h"
+#include "lib/tonic/dart_state.h"
+#include "lib/tonic/logging/dart_invoke.h"
+#include "lib/tonic/typed_data/uint8_list.h"
+#include "third_party/skia/include/core/SkEncodedImageFormat.h"
+#include "third_party/skia/include/core/SkImage.h"
+
+using tonic::DartInvoke;
+using tonic::DartPersistentValue;
+using tonic::ToDart;
+
+namespace blink {
+namespace {
+
+void InvokeDataCallback(std::unique_ptr<DartPersistentValue> callback,
+                        sk_sp<SkData> buffer) {
+  tonic::DartState* dart_state = callback->dart_state().get();
+  if (!dart_state) {
+    return;
+  }
+  tonic::DartState::Scope scope(dart_state);
+  if (!buffer) {
+    DartInvoke(callback->value(), {Dart_Null()});
+  } else {
+    Dart_Handle dart_data = tonic::DartConverter<tonic::Uint8List>::ToDart(
+        buffer->bytes(), buffer->size());
+    DartInvoke(callback->value(), {dart_data});
+  }
+}
+
+sk_sp<SkData> EncodeImage(sk_sp<SkImage> image,
+                          SkEncodedImageFormat format,
+                          int quality) {
+  if (image == nullptr) {
+    return nullptr;
+  }
+  return image->encodeToData(format, quality);
+}
+
+void EncodeImageAndInvokeDataCallback(
+    std::unique_ptr<DartPersistentValue> callback,
+    sk_sp<SkImage> image,
+    SkEncodedImageFormat format,
+    int quality) {
+  sk_sp<SkData> encoded = EncodeImage(std::move(image), format, quality);
+
+  Threads::UI()->PostTask(
+      fxl::MakeCopyable([callback = std::move(callback), encoded]() mutable {
+        InvokeDataCallback(std::move(callback), std::move(encoded));
+      }));
+}
+
+SkEncodedImageFormat ToSkEncodedImageFormat(int format) {
+  // Map the formats exposed in flutter to formats supported in Skia.
+  // See:
+  // https://github.com/google/skia/blob/master/include/core/SkEncodedImageFormat.h
+  switch (format) {
+    case 0:
+      return SkEncodedImageFormat::kJPEG;
+    case 1:
+      return SkEncodedImageFormat::kPNG;
+    case 2:
+      return SkEncodedImageFormat::kWEBP;
+    default:
+      /* NOTREACHED */
+      return SkEncodedImageFormat::kWEBP;
+  }
+}
+
+void EncodeImage(Dart_NativeArguments args) {
+  Dart_Handle exception = nullptr;
+
+  CanvasImage* canvas_image =
+      tonic::DartConverter<CanvasImage*>::FromArguments(args, 0, exception);
+
+  if (exception) {
+    Dart_ThrowException(exception);
+    return;
+  }
+  if (!canvas_image)
+    Dart_ThrowException(ToDart("encodeImage called with non-genuine Image."));
+
+  int format = tonic::DartConverter<int>::FromArguments(args, 1, exception);
+  if (exception) {
+    Dart_ThrowException(exception);
+    return;
+  }
+  SkEncodedImageFormat image_format = ToSkEncodedImageFormat(format);
+
+  int quality = tonic::DartConverter<int>::FromArguments(args, 2, exception);
+  if (exception) {
+    Dart_ThrowException(exception);
+    return;
+  }
+  if (quality > 100)
+    quality = 100;
+  if (quality < 0)
+    quality = 0;
+
+  Dart_Handle callback_handle = Dart_GetNativeArgument(args, 3);
+  if (!Dart_IsClosure(callback_handle)) {
+    Dart_ThrowException(ToDart("Callback must be a function"));
+    return;
+  }
+
+  auto callback = std::make_unique<DartPersistentValue>(
+      tonic::DartState::Current(), callback_handle);
+  sk_sp<SkImage> image = canvas_image->image();
+
+  Threads::IO()->PostTask(fxl::MakeCopyable(
+      [callback = std::move(callback), image, image_format, quality]() mutable {
+        EncodeImageAndInvokeDataCallback(std::move(callback), std::move(image),
+                                         image_format, quality);
+      }));
+}
+
+}  // namespace
+
+void ImageEncoding::RegisterNatives(tonic::DartLibraryNatives* natives) {
+  natives->Register({
+      {"encodeImage", EncodeImage, 4, true},
+  });
+}
+
+}  // namespace blink

lib/ui/painting/image_encoding.h

@@ -0,0 +1,19 @@
+// Copyright 2018 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef FLUTTER_LIB_UI_PAINTING_IMAGE_ENCODING_H_
+#define FLUTTER_LIB_UI_PAINTING_IMAGE_ENCODING_H_
+
+#include "lib/tonic/dart_library_natives.h"
+
+namespace blink {
+
+class ImageEncoding {
+ public:
+  static void RegisterNatives(tonic::DartLibraryNatives* natives);
+};
+
+}  // namespace blink
+
+#endif  // FLUTTER_LIB_UI_PAINTING_IMAGE_ENCODING_H_