Avoid creating a vector when constructing Dart typed data objects for platform messages (flutter#18838)

Author: jason-simmons

lib/ui/window/platform_message_response_dart.cc

@@ -8,43 +8,12 @@
 
 #include "flutter/common/task_runners.h"
 #include "flutter/fml/make_copyable.h"
-#include "flutter/lib/ui/window/window.h"
 #include "third_party/tonic/dart_state.h"
 #include "third_party/tonic/logging/dart_invoke.h"
+#include "third_party/tonic/typed_data/dart_byte_data.h"
 
 namespace flutter {
 
-namespace {
-
-// Avoid copying the contents of messages beyond a certain size.
-const int kMessageCopyThreshold = 1000;
-
-void MessageDataFinalizer(void* isolate_callback_data,
-                          Dart_WeakPersistentHandle handle,
-                          void* peer) {
-  std::vector<uint8_t>* data = reinterpret_cast<std::vector<uint8_t>*>(peer);
-  delete data;
-}
-
-Dart_Handle WrapByteData(std::vector<uint8_t> data) {
-  if (data.size() < kMessageCopyThreshold) {
-    return ToByteData(data);
-  } else {
-    std::vector<uint8_t>* heap_data = new std::vector<uint8_t>(std::move(data));
-    return Dart_NewExternalTypedDataWithFinalizer(
-        Dart_TypedData_kByteData, heap_data->data(), heap_data->size(),
-        heap_data, heap_data->size(), MessageDataFinalizer);
-  }
-}
-
-Dart_Handle WrapByteData(std::unique_ptr<fml::Mapping> mapping) {
-  std::vector<uint8_t> data(mapping->GetSize());
-  memcpy(data.data(), mapping->GetMapping(), mapping->GetSize());
-  return WrapByteData(std::move(data));
-}
-
-}  // anonymous namespace
-
 PlatformMessageResponseDart::PlatformMessageResponseDart(
     tonic::DartPersistentValue callback,
     fml::RefPtr<fml::TaskRunner> ui_task_runner)
@@ -71,7 +40,8 @@ void PlatformMessageResponseDart::Complete(std::unique_ptr<fml::Mapping> data) {
           return;
         tonic::DartState::Scope scope(dart_state);
 
-        Dart_Handle byte_buffer = WrapByteData(std::move(data));
+        Dart_Handle byte_buffer =
+            tonic::DartByteData::Create(data->GetMapping(), data->GetSize());
         tonic::DartInvoke(callback.Release(), {byte_buffer});
       }));
 }

lib/ui/window/window.cc

@@ -166,25 +166,12 @@ void GetPersistentIsolateData(Dart_NativeArguments args) {
                                         persistent_isolate_data->GetSize()));
 }
 
-}  // namespace
-
 Dart_Handle ToByteData(const std::vector<uint8_t>& buffer) {
-  Dart_Handle data_handle =
-      Dart_NewTypedData(Dart_TypedData_kByteData, buffer.size());
-  if (Dart_IsError(data_handle))
-    return data_handle;
-
-  Dart_TypedData_Type type;
-  void* data = nullptr;
-  intptr_t num_bytes = 0;
-  FML_CHECK(!Dart_IsError(
-      Dart_TypedDataAcquireData(data_handle, &type, &data, &num_bytes)));
-
-  memcpy(data, buffer.data(), num_bytes);
-  Dart_TypedDataReleaseData(data_handle);
-  return data_handle;
+  return tonic::DartByteData::Create(buffer.data(), buffer.size());
 }
 
+}  // namespace
+
 WindowClient::~WindowClient() {}
 
 Window::Window(WindowClient* client) : client_(client) {}

lib/ui/window/window.h

@@ -35,8 +35,6 @@ namespace flutter {
 class FontCollection;
 class Scene;
 
-Dart_Handle ToByteData(const std::vector<uint8_t>& buffer);
-
 // Must match the AccessibilityFeatureFlag enum in window.dart.
 enum class AccessibilityFeatureFlag : int32_t {
   kAccessibleNavigation = 1 << 0,

third_party/tonic/typed_data/dart_byte_data.cc

@@ -8,10 +8,32 @@
 
 namespace tonic {
 
+namespace {
+
+// For large objects it is more efficient to use an external typed data object
+// with a buffer allocated outside the Dart heap.
+const int kExternalSizeThreshold = 1000;
+
+void FreeFinalizer(void* isolate_callback_data,
+                   Dart_WeakPersistentHandle handle,
+                   void* peer) {
+  free(peer);
+}
+
+}  // anonymous namespace
+
 Dart_Handle DartByteData::Create(const void* data, size_t length) {
-  auto handle = DartByteData{data, length}.dart_handle();
-  // The destructor should release the typed data.
-  return handle;
+  if (length < kExternalSizeThreshold) {
+    auto handle = DartByteData{data, length}.dart_handle();
+    // The destructor should release the typed data.
+    return handle;
+  } else {
+    void* buf = ::malloc(length);
+    TONIC_DCHECK(buf);
+    ::memcpy(buf, data, length);
+    return Dart_NewExternalTypedDataWithFinalizer(
+        Dart_TypedData_kByteData, buf, length, buf, length, FreeFinalizer);
+  }
 }
 
 DartByteData::DartByteData()