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/painting.dart

@@ -1196,6 +1196,16 @@ class Paint {
   }
 }
 
+/// The encoding formats supported by the [Image.toByteData].
+// These enum values must be kept in sync with the logic in ToSkEncodedImageFormat.
+enum EncodeFormat {
+  // Be conservative with the formats we expose. It is easy to add new formats in future but
+  // difficult to remove.
+  JPEG,
+  PNG,
+  WEBP,
+}
+
 /// Opaque handle to raw decoded image data (pixels).
 ///
 /// To obtain an [Image] object, use [instantiateImageCodec].
@@ -1215,6 +1225,25 @@ class Image extends NativeFieldWrapperClass2 {
   /// The number of image pixels along the image's vertical axis.
   int get height native 'Image_height';
 
+  /// Converts the [Image] object into a byte array.
+  ///
+  /// The [format] is encoding format to be used.
+  ///
+  /// The [quality] is a value in the range 0 to 100. [quality] is a format specific metric trading
+  /// off size and encoding error. When used, 100 encodes with highest quality (i.e., least error)
+  /// and 0 encodes with lowest quality (i.e., smallest size).
+  ///
+  /// Returns a future which complete with the binary image data (e.g a PNG or JPEG binary data) or
+  /// an error if encoding fails.
+  Future<Uint8List> toByteData({EncodeFormat format: EncodeFormat.JPEG, int quality: 80}) {
+    return _futurize(
+          (_Callback<Uint8List> callback) => _toByteData(format.index, quality, callback)
+    );
+  }
+
+  /// Returns an error message on failure, null on success.
+  String _toByteData(int format, int quality, _Callback<Uint8List> callback) native 'Image_toByteData';
+
   /// Release the resources used by this object. The object is no longer usable
   /// after this method is called.
   void dispose() native 'Image_dispose';

lib/ui/painting/image.cc

@@ -5,6 +5,7 @@
 #include "flutter/lib/ui/painting/image.h"
 
 #include "flutter/common/threads.h"
+#include "flutter/lib/ui/painting/image_encoding.h"
 #include "flutter/lib/ui/painting/utils.h"
 #include "lib/tonic/converter/dart_converter.h"
 #include "lib/tonic/dart_args.h"
@@ -20,6 +21,7 @@ IMPLEMENT_WRAPPERTYPEINFO(ui, Image);
 #define FOR_EACH_BINDING(V) \
   V(Image, width)           \
   V(Image, height)          \
+  V(Image, toByteData)      \
   V(Image, dispose)
 
 FOR_EACH_BINDING(DART_NATIVE_CALLBACK)
@@ -36,6 +38,12 @@ CanvasImage::~CanvasImage() {
   SkiaUnrefOnIOThread(&image_);
 }
 
+Dart_Handle CanvasImage::toByteData(int format,
+                                    int quality,
+                                    Dart_Handle callback) {
+  return EncodeImage(this, format, quality, callback);
+}
+
 void CanvasImage::dispose() {
   ClearDartWrapper();
 }

lib/ui/painting/image.h

@@ -27,6 +27,7 @@ class CanvasImage final : public fxl::RefCountedThreadSafe<CanvasImage>,
 
   int width() { return image_->width(); }
   int height() { return image_->height(); }
+  Dart_Handle toByteData(int format, int quality, Dart_Handle callback);
   void dispose();
 
   const sk_sp<SkImage>& image() const { return image_; }

lib/ui/painting/image_encoding.cc

@@ -0,0 +1,113 @@
+// 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;
+  }
+}
+
+}  // namespace
+
+Dart_Handle EncodeImage(CanvasImage* canvas_image,
+                        int format,
+                        int quality,
+                        Dart_Handle callback_handle) {
+  if (!canvas_image)
+    return ToDart("encode called with non-genuine Image.");
+
+  if (!Dart_IsClosure(callback_handle))
+    return ToDart("Callback must be a function.");
+
+  SkEncodedImageFormat image_format = ToSkEncodedImageFormat(format);
+
+  if (quality > 100)
+    quality = 100;
+  if (quality < 0)
+    quality = 0;
+
+  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);
+      }));
+
+  return Dart_Null();
+}
+
+}  // namespace blink

lib/ui/painting/image_encoding.h

@@ -0,0 +1,21 @@
+// 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 CanvasImage;
+
+Dart_Handle EncodeImage(CanvasImage* canvas_image,
+                        int format,
+                        int quality,
+                        Dart_Handle callback_handle);
+
+}  // namespace blink
+
+#endif  // FLUTTER_LIB_UI_PAINTING_IMAGE_ENCODING_H_

testing/dart/encoding_test.dart

@@ -0,0 +1,61 @@
+// 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.
+
+import 'dart:async';
+import 'dart:ui';
+import 'dart:typed_data';
+import 'dart:io';
+
+import 'package:test/test.dart';
+import 'package:path/path.dart' as path;
+
+void main() {
+
+  Image createSquareTestImage() {
+    PictureRecorder recorder = new PictureRecorder();
+    Canvas canvas = new Canvas(recorder, new Rect.fromLTWH(0.0, 0.0, 10.0, 10.0));
+
+    var black = new Paint()
+      ..strokeWidth = 1.0
+      ..color = const Color.fromRGBO(0, 0, 0, 1.0);
+    var green = new Paint()
+      ..strokeWidth = 1.0
+      ..color = const Color.fromRGBO(0, 255, 0, 1.0);
+
+    canvas.drawRect(new Rect.fromLTWH(0.0, 0.0, 10.0, 10.0), black);
+    canvas.drawRect(new Rect.fromLTWH(2.0, 2.0, 6.0, 6.0), green);
+    return recorder.endRecording().toImage(10, 10);
+  }
+
+  Uint8List readFile(fileName) {
+    final file = new File(path.join('flutter', 'testing', 'resources', fileName));
+    return file.readAsBytesSync();
+  }
+
+  final Image testImage = createSquareTestImage();
+
+  test('Encode with default arguments', () async {
+    Uint8List data = await testImage.toByteData();
+    Uint8List expected = readFile('square-80.jpg');
+    expect(data, expected);
+  });
+
+  test('Encode JPEG', () async {
+    Uint8List data = await testImage.toByteData(format: EncodeFormat.JPEG, quality:80);
+    Uint8List expected = readFile('square-80.jpg');
+    expect(data, expected);
+  });
+
+  test('Encode PNG', () async {
+    Uint8List data = await testImage.toByteData(format: EncodeFormat.PNG, quality: 80);
+    Uint8List expected = readFile('square-80.png');
+    expect(data, expected);
+  });
+
+  test('Encode WEBP', () async {
+    Uint8List data = await testImage.toByteData(format: EncodeFormat.WEBP, quality: 80);
+    Uint8List expected = readFile('square-80.webp');
+    expect(data, expected);
+  });
+}

travis/licenses_golden/licenses_flutter

@@ -1642,6 +1642,8 @@ ORIGIN: ../../../flutter/assets/asset_provider.h + ../../../LICENSE
 TYPE: LicenseType.bsd
 FILE: ../../../flutter/assets/asset_provider.h
 FILE: ../../../flutter/flutter_kernel_transformers/lib/track_widget_constructor_locations.dart
+FILE: ../../../flutter/lib/ui/painting/image_encoding.cc
+FILE: ../../../flutter/lib/ui/painting/image_encoding.h
 FILE: ../../../flutter/shell/platform/android/apk_asset_provider.h
 FILE: ../../../flutter/shell/platform/darwin/ios/framework/Source/accessibility_text_entry.h
 FILE: ../../../flutter/shell/platform/darwin/ios/framework/Source/accessibility_text_entry.mm