Allow hot reload without syncing all asset files to devFS

shell/common/fixtures/shell_test.dart

@@ -163,3 +163,19 @@ void localtimesMatch() {
   // formatting since package:intl is not available.
   notifyLocalTime(timeStr.split(":")[0]);
 }
+
+void notifyCanAccessResource(bool success) native 'NotifyCanAccessResource';
+
+void notifySetAssetBundlePath() native 'NotifySetAssetBundlePath';
+
+@pragma('vm:entry-point')
+void canAccessResourceFromAssetDir() async {
+  notifySetAssetBundlePath();
+  window.sendPlatformMessage(
+    'flutter/assets',
+    Uint8List.fromList(utf8.encode('kernel_blob.bin')).buffer.asByteData(),
+    (ByteData byteData) {
+      notifyCanAccessResource(byteData != null);
+    },
+  );
+}

shell/common/shell.cc

@@ -1404,6 +1404,7 @@ bool Shell::OnServiceProtocolRunInView(
   configuration.AddAssetResolver(
       std::make_unique<DirectoryAssetBundle>(fml::OpenDirectory(
           asset_directory_path.c_str(), false, fml::FilePermission::kRead)));
+  configuration.AddAssetResolver(RestoreOriginalAssetResolver());
 
   auto& allocator = response->GetAllocator();
   response->SetObject();
@@ -1516,6 +1517,7 @@ bool Shell::OnServiceProtocolSetAssetBundlePath(
 
   auto asset_manager = std::make_shared<AssetManager>();
 
+  asset_manager->PushFront(RestoreOriginalAssetResolver());
   asset_manager->PushFront(std::make_unique<DirectoryAssetBundle>(
       fml::OpenDirectory(params.at("assetDirectory").data(), false,
                          fml::FilePermission::kRead)));
@@ -1622,4 +1624,16 @@ void Shell::OnDisplayUpdates(DisplayUpdateType update_type,
   display_manager_->HandleDisplayUpdates(update_type, displays);
 }
 
+// Add the original asset directory to the resolvers so that unmodified assets
+// bundled with the application specific format (APK, IPA) can be used without
+// syncing to the Dart devFS.
+std::unique_ptr<DirectoryAssetBundle> Shell::RestoreOriginalAssetResolver() {
+  if (fml::UniqueFD::traits_type::IsValid(settings_.assets_dir)) {
+    return std::make_unique<DirectoryAssetBundle>(
+        fml::Duplicate(settings_.assets_dir));
+  }
+  return std::make_unique<DirectoryAssetBundle>(fml::OpenDirectory(
+      settings_.assets_path.c_str(), false, fml::FilePermission::kRead));
+};
+
 }  // namespace flutter

shell/common/shell.h

@@ -10,6 +10,7 @@
 #include <string_view>
 #include <unordered_map>
 
+#include "flutter/assets/directory_asset_bundle.h"
 #include "flutter/common/settings.h"
 #include "flutter/common/task_runners.h"
 #include "flutter/flow/surface.h"
@@ -612,6 +613,10 @@ class Shell final : public PlatformView::Delegate,
       const ServiceProtocol::Handler::ServiceProtocolMap& params,
       rapidjson::Document* response);
 
+  // Creates an asset bundle from the original settings asset path or
+  // directory.
+  std::unique_ptr<DirectoryAssetBundle> RestoreOriginalAssetResolver();
+
   // For accessing the Shell via the raster thread, necessary for various
   // rasterizer callbacks.
   std::unique_ptr<fml::TaskRunnerAffineWeakPtrFactory<Shell>> weak_factory_gpu_;

shell/common/shell_unittests.cc

@@ -2135,5 +2135,54 @@ TEST_F(ShellTest, IgnoresInvalidMetrics) {
   DestroyShell(std::move(shell), std::move(task_runners));
 }
 
+TEST_F(ShellTest, OnServiceProtocolSetAssetBundlePathWorks) {
+  Settings settings = CreateSettingsForFixture();
+  std::unique_ptr<Shell> shell = CreateShell(settings);
+  RunConfiguration configuration =
+      RunConfiguration::InferFromSettings(settings);
+  configuration.SetEntrypoint("canAccessResourceFromAssetDir");
+
+  // Verify isolate can load a known resource with the
+  // default asset directory - kernel_blob.bin
+  fml::AutoResetWaitableEvent latch;
+
+  // Callback used to signal whether the resource was loaded successfully.
+  bool can_access_resource = false;
+  auto native_can_access_resource = [&can_access_resource,
+                                     &latch](Dart_NativeArguments args) {
+    Dart_Handle exception = nullptr;
+    can_access_resource =
+        tonic::DartConverter<bool>::FromArguments(args, 0, exception);
+    latch.Signal();
+  };
+  AddNativeCallback("NotifyCanAccessResource",
+                    CREATE_NATIVE_ENTRY(native_can_access_resource));
+
+  // Callback used to delay the asset load until after the service
+  // protocol method has finished.
+  auto native_notify_set_asset_bundle_path =
+      [&shell](Dart_NativeArguments args) {
+        // Update the asset directory to a bonus path.
+        ServiceProtocol::Handler::ServiceProtocolMap params;
+        params["assetDirectory"] = "assetDirectory";
+        rapidjson::Document document;
+        OnServiceProtocol(shell.get(), ServiceProtocolEnum::kSetAssetBundlePath,
+                          shell->GetTaskRunners().GetUITaskRunner(), params,
+                          &document);
+        rapidjson::StringBuffer buffer;
+        rapidjson::Writer<rapidjson::StringBuffer> writer(buffer);
+        document.Accept(writer);
+      };
+  AddNativeCallback("NotifySetAssetBundlePath",
+                    CREATE_NATIVE_ENTRY(native_notify_set_asset_bundle_path));
+
+  RunEngine(shell.get(), std::move(configuration));
+
+  latch.Wait();
+  ASSERT_TRUE(can_access_resource);
+
+  DestroyShell(std::move(shell));
+}
+
 }  // namespace testing
 }  // namespace flutter