[dart:ui] add an explicit API for loading assets

lib/ui/painting.dart

@@ -4960,6 +4960,22 @@ class ImmutableBuffer extends NativeFieldWrapperClass2 {
   }
   void _init(Uint8List list, _Callback<void> callback) native 'ImmutableBuffer_init';
 
+  /// Create a buffer from the asset with key [assetKey].
+  ///
+  /// Returns `null` if the asset does not exist.
+  static ImmutableBuffer? fromAsset(String assetKey) {
+    final ImmutableBuffer buffer = ImmutableBuffer._(0);
+    bool success = false;
+    _fromAsset(assetKey, buffer, (void result) {
+      success = true;
+    });
+    if (!success) {
+      return null;
+    }
+    return buffer;
+  }
+  static void _fromAsset(String assetKey, ImmutableBuffer buffer, _Callback<void> callback) native 'ImmutableBuffer_fromAsset';
+
   /// The length, in bytes, of the underlying data.
   final int length;
 

lib/ui/painting/immutable_buffer.cc

@@ -10,6 +10,8 @@
 #include "third_party/tonic/converter/dart_converter.h"
 #include "third_party/tonic/dart_args.h"
 #include "third_party/tonic/dart_binding_macros.h"
+#include "flutter/lib/ui/ui_dart_state.h"
+#include "flutter/lib/ui/window/platform_configuration.h"
 
 #if OS_ANDROID
 #include <sys/mman.h>
@@ -30,6 +32,9 @@ ImmutableBuffer::~ImmutableBuffer() {}
 void ImmutableBuffer::RegisterNatives(tonic::DartLibraryNatives* natives) {
   natives->Register({{"ImmutableBuffer_init", ImmutableBuffer::init, 3, true},
                      FOR_EACH_BINDING(DART_REGISTER_NATIVE)});
+  natives->Register({{"ImmutableBuffer_fromAsset", ImmutableBuffer::fromAsset, 3, true},
+                    FOR_EACH_BINDING(DART_REGISTER_NATIVE)});
+
 }
 
 void ImmutableBuffer::init(Dart_NativeArguments args) {
@@ -53,6 +58,49 @@ size_t ImmutableBuffer::GetAllocationSize() const {
   return sizeof(ImmutableBuffer) + data_->size();
 }
 
+void ImmutableBuffer::fromAsset(Dart_NativeArguments args) {
+  UIDartState::ThrowIfUIOperationsProhibited();
+  Dart_Handle callback_handle = Dart_GetNativeArgument(args, 2);
+  if (!Dart_IsClosure(callback_handle)) {
+    Dart_SetReturnValue(args, tonic::ToDart("Callback must be a function"));
+    return;
+  }
+  Dart_Handle asset_name_handle = Dart_GetNativeArgument(args, 0);
+  uint8_t* chars = nullptr;
+  intptr_t asset_length = 0;
+  Dart_Handle result =
+      Dart_StringToUTF8(asset_name_handle, &chars, &asset_length);
+  if (Dart_IsError(result)) {
+    Dart_PropagateError(result);
+    return;
+  }
+  Dart_Handle immutable_buffer = Dart_GetNativeArgument(args, 1);
+
+  std::string asset_name = std::string{reinterpret_cast<const char*>(chars),
+                                       static_cast<size_t>(asset_length)};
+
+  std::shared_ptr<AssetManager> asset_manager = UIDartState::Current()
+                                                    ->platform_configuration()
+                                                    ->client()
+                                                    ->GetAssetManager();
+  std::unique_ptr<fml::Mapping> data = asset_manager->GetAsMapping(asset_name);
+
+  if (data == nullptr) {
+    return;
+  }
+  const void* bytes = static_cast<const void*>(data->GetMapping());
+  void* peer = reinterpret_cast<void*>(data.release());
+  auto size = data->GetSize();
+  SkData::ReleaseProc proc = [](const void* ptr, void* context) {
+    // TODO: what do
+  };
+
+  auto sk_data = SkData::MakeWithProc(bytes, size, proc, peer);
+  auto buffer = fml::MakeRefCounted<ImmutableBuffer>(sk_data);
+  buffer->AssociateWithDartWrapper(immutable_buffer);
+  tonic::DartInvoke(callback_handle, {Dart_TypeVoid()});
+}
+
 #if OS_ANDROID
 
 // Compressed image buffers are allocated on the UI thread but are deleted on a

lib/ui/painting/immutable_buffer.h

@@ -39,6 +39,15 @@ class ImmutableBuffer : public RefCountedDartWrappable<ImmutableBuffer> {
   /// when the copy has completed.
   static void init(Dart_NativeArguments args);
 
+  /// Create a new ImmutableData from a Dart asset key.
+  ///
+  /// The zero indexed argument is the UTF8 encoded string which corresponds to
+  /// the asset manager key.
+  ///
+  /// The first indexed argument is a callback that can optionally be invoked
+  /// with the resulting ImmutableData.
+  static void fromAsset(Dart_NativeArguments args);
+
   /// The length of the data in bytes.
   size_t length() const {
     FML_DCHECK(data_);

lib/ui/window/platform_configuration.h

@@ -11,6 +11,7 @@
 #include <unordered_map>
 #include <vector>
 
+#include "flutter/assets/asset_manager.h"
 #include "flutter/fml/time/time_point.h"
 #include "flutter/lib/ui/semantics/semantics_update.h"
 #include "flutter/lib/ui/window/platform_message.h"
@@ -111,6 +112,8 @@ class PlatformConfigurationClient {
   ///             creation.
   virtual FontCollection& GetFontCollection() = 0;
 
+  virtual std::shared_ptr<AssetManager> GetAssetManager() = 0;
+
   //--------------------------------------------------------------------------
   /// @brief      Notifies this client of the name of the root isolate and its
   ///             port when that isolate is launched, restarted (in the

lib/web_ui/lib/src/ui/assets.dart

@@ -0,0 +1,17 @@
+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// @dart = 2.10
+part of ui;
+
+/// Load the asset bytes specified by [assetKey].
+///
+/// The [assetKey] is generally the filepath of the asset which is bundled
+/// into the flutter application. This API is not supported on the
+/// Web.
+///
+/// If the [assetKey] does not correspond to a real asset, returns `null`.
+ByteData? loadAsset(String assetKey) {
+  return null;
+}

lib/web_ui/lib/ui.dart

@@ -35,6 +35,7 @@ part 'src/ui/test_embedding.dart';
 part 'src/ui/text.dart';
 part 'src/ui/tile_mode.dart';
 part 'src/ui/window.dart';
+part 'src/ui/assets.dart';
 
 /// Provides a compile time constant to customize flutter framework and other
 /// users of ui engine for web runtime.

runtime/runtime_controller.cc

@@ -3,6 +3,7 @@
 // found in the LICENSE file.
 
 #include "flutter/runtime/runtime_controller.h"
+#include "flutter/assets/asset_manager.h"
 
 #include "flutter/fml/message_loop.h"
 #include "flutter/fml/trace_event.h"
@@ -335,6 +336,11 @@ FontCollection& RuntimeController::GetFontCollection() {
   return client_.GetFontCollection();
 }
 
+// |PlatfromConfigurationClient|
+std::shared_ptr<AssetManager> RuntimeController::GetAssetManager() {
+  return client_.GetAssetManager();
+}
+
 // |PlatformConfigurationClient|
 void RuntimeController::UpdateIsolateDescription(const std::string isolate_name,
                                                  int64_t isolate_port) {

runtime/runtime_controller.h

@@ -8,6 +8,7 @@
 #include <memory>
 #include <vector>
 
+#include "flutter/assets/asset_manager.h"
 #include "flutter/common/task_runners.h"
 #include "flutter/flow/layers/layer_tree.h"
 #include "flutter/fml/macros.h"
@@ -508,6 +509,9 @@ class RuntimeController : public PlatformConfigurationClient {
   // |PlatformConfigurationClient|
   FontCollection& GetFontCollection() override;
 
+  // |PlatformConfigurationClient|
+  std::shared_ptr<AssetManager> GetAssetManager() override;
+
   // |PlatformConfigurationClient|
   void UpdateIsolateDescription(const std::string isolate_name,
                                 int64_t isolate_port) override;

runtime/runtime_delegate.h

@@ -8,6 +8,7 @@
 #include <memory>
 #include <vector>
 
+#include "flutter/assets/asset_manager.h"
 #include "flutter/flow/layers/layer_tree.h"
 #include "flutter/lib/ui/semantics/custom_accessibility_action.h"
 #include "flutter/lib/ui/semantics/semantics_node.h"
@@ -32,6 +33,8 @@ class RuntimeDelegate {
 
   virtual FontCollection& GetFontCollection() = 0;
 
+  virtual std::shared_ptr<AssetManager> GetAssetManager() = 0;
+
   virtual void UpdateIsolateDescription(const std::string isolate_name,
                                         int64_t isolate_port) = 0;
 

shell/common/engine.cc

@@ -515,6 +515,10 @@ FontCollection& Engine::GetFontCollection() {
   return font_collection_;
 }
 
+std::shared_ptr<AssetManager> Engine::GetAssetManager() {
+  return asset_manager_;
+}
+
 void Engine::DoDispatchPacket(std::unique_ptr<PointerDataPacket> packet,
                               uint64_t trace_flow_id) {
   animator_->EnqueueTraceFlowId(trace_flow_id);

shell/common/engine.h

@@ -734,6 +734,9 @@ class Engine final : public RuntimeDelegate,
   // |RuntimeDelegate|
   FontCollection& GetFontCollection() override;
 
+  // |RuntimeDelegate|
+  std::shared_ptr<AssetManager> GetAssetManager() override;
+
   // |PointerDataDispatcher::Delegate|
   void DoDispatchPacket(std::unique_ptr<PointerDataPacket> packet,
                         uint64_t trace_flow_id) override;

shell/common/engine_unittests.cc

@@ -49,6 +49,7 @@ class MockRuntimeDelegate : public RuntimeDelegate {
                void(SemanticsNodeUpdates, CustomAccessibilityActionUpdates));
   MOCK_METHOD1(HandlePlatformMessage, void(fml::RefPtr<PlatformMessage>));
   MOCK_METHOD0(GetFontCollection, FontCollection&());
+  MOCK_METHOD0(GetAssetManager, std::shared_ptr<AssetManager>());
   MOCK_METHOD2(UpdateIsolateDescription, void(const std::string, int64_t));
   MOCK_METHOD1(SetNeedsReportTimings, void(bool));
   MOCK_METHOD1(ComputePlatformResolvedLocale,

testing/dart/assets_test.dart

@@ -0,0 +1,18 @@
+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// @dart = 2.6
+import 'dart:ui' as ui;
+
+import 'package:test/test.dart';
+
+void main() {
+  test('Loading an asset that does not exist returns null', () {
+    expect(ui.loadAsset('ThisDoesNotExist'), null);
+  });
+
+  test('returns the bytes of a bundled asset', () {
+    expect(ui.loadAsset('FontManifest.json'), isNotNull);
+  });
+}