Reland: Call Dart plugin registrant if available (flutter/engine#25375)

engine/src/flutter/lib/ui/hooks.dart

@@ -132,10 +132,14 @@ typedef _ListStringArgFunction(List<String> args);
 @pragma('vm:entry-point')
 // ignore: unused_element
 void _runMainZoned(Function startMainIsolateFunction,
+                   Function? dartPluginRegistrant,
                    Function userMainFunction,
                    List<String> args) {
   startMainIsolateFunction(() {
     runZonedGuarded<void>(() {
+      if (dartPluginRegistrant != null) {
+        dartPluginRegistrant();
+      }
       if (userMainFunction is _ListStringArgFunction) {
         (userMainFunction as dynamic)(args);
       } else {

engine/src/flutter/runtime/dart_isolate.cc

@@ -700,6 +700,7 @@ bool DartIsolate::MarkIsolateRunnable() {
 
 [[nodiscard]] static bool InvokeMainEntrypoint(
     Dart_Handle user_entrypoint_function,
+    Dart_Handle plugin_registrant_function,
     Dart_Handle args) {
   if (tonic::LogIfError(user_entrypoint_function)) {
     FML_LOG(ERROR) << "Could not resolve main entrypoint function.";
@@ -717,7 +718,8 @@ bool DartIsolate::MarkIsolateRunnable() {
 
   if (tonic::LogIfError(tonic::DartInvokeField(
           Dart_LookupLibrary(tonic::ToDart("dart:ui")), "_runMainZoned",
-          {start_main_isolate_function, user_entrypoint_function, args}))) {
+          {start_main_isolate_function, plugin_registrant_function,
+           user_entrypoint_function, args}))) {
     FML_LOG(ERROR) << "Could not invoke the main entrypoint.";
     return false;
   }
@@ -742,12 +744,34 @@ bool DartIsolate::RunFromLibrary(std::optional<std::string> library_name,
   auto entrypoint_handle = entrypoint.has_value() && !entrypoint.value().empty()
                                ? tonic::ToDart(entrypoint.value().c_str())
                                : tonic::ToDart("main");
+  auto entrypoint_args = tonic::ToDart(args);
   auto user_entrypoint_function =
       ::Dart_GetField(library_handle, entrypoint_handle);
 
-  auto entrypoint_args = tonic::ToDart(args);
+  // The Dart plugin registrant is a function named `_registerPlugins`
+  // generated by the Flutter tool.
+  //
+  // This function binds a plugin implementation to their platform
+  // interface based on the configuration of the app's pubpec.yaml, and the
+  // plugin's pubspec.yaml.
+  //
+  // Since this function may or may not be defined. Check that it is a top
+  // level function, and call it in hooks.dart before the main entrypoint
+  // function.
+  //
+  // If it's not defined, then just call the main entrypoint function
+  // as usual.
+  //
+  // This allows embeddings to change the name of the entrypoint function.
+  auto plugin_registrant_function =
+      ::Dart_GetField(library_handle, tonic::ToDart("_registerPlugins"));
 
-  if (!InvokeMainEntrypoint(user_entrypoint_function, entrypoint_args)) {
+  if (Dart_IsError(plugin_registrant_function)) {
+    plugin_registrant_function = Dart_Null();
+  }
+
+  if (!InvokeMainEntrypoint(user_entrypoint_function,
+                            plugin_registrant_function, entrypoint_args)) {
     return false;
   }
 

engine/src/flutter/runtime/dart_isolate_unittests.cc

@@ -613,5 +613,39 @@ TEST_F(DartIsolateTest, DISABLED_ValidLoadingUnitSucceeds) {
   Wait();
 }
 
+TEST_F(DartIsolateTest, DartPluginRegistrantIsCalled) {
+  ASSERT_FALSE(DartVMRef::IsInstanceRunning());
+
+  std::vector<std::string> messages;
+  fml::AutoResetWaitableEvent latch;
+
+  AddNativeCallback(
+      "PassMessage",
+      CREATE_NATIVE_ENTRY(([&latch, &messages](Dart_NativeArguments args) {
+        auto message = tonic::DartConverter<std::string>::FromDart(
+            Dart_GetNativeArgument(args, 0));
+        messages.push_back(message);
+        latch.Signal();
+      })));
+
+  const auto settings = CreateSettingsForFixture();
+  auto vm_ref = DartVMRef::Create(settings);
+  auto thread = CreateNewThread();
+  TaskRunners task_runners(GetCurrentTestName(),  //
+                           thread,                //
+                           thread,                //
+                           thread,                //
+                           thread                 //
+  );
+  auto isolate = RunDartCodeInIsolate(vm_ref, settings, task_runners,
+                                      "mainForPluginRegistrantTest", {},
+                                      GetFixturesPath());
+  ASSERT_TRUE(isolate);
+  ASSERT_EQ(isolate->get()->GetPhase(), DartIsolate::Phase::Running);
+  latch.Wait();
+  ASSERT_EQ(messages.size(), 1u);
+  ASSERT_EQ(messages[0], "_registerPlugins was called");
+}
+
 }  // namespace testing
 }  // namespace flutter

engine/src/flutter/runtime/fixtures/runtime_test.dart

@@ -10,8 +10,7 @@ import 'dart:ui';
 
 import 'split_lib_test.dart' deferred as splitlib;
 
-void main() {
-}
+void main() {}
 
 @pragma('vm:entry-point')
 void sayHi() {
@@ -115,3 +114,24 @@ void testCanConvertListOfInts(List<int> args){
                 args[2] == 3 &&
                 args[3] == 4);
 }
+
+bool didCallRegistrantBeforeEntrypoint = false;
+
+// Test the Dart plugin registrant.
+// _registerPlugins requires the entrypoint annotation, so the compiler doesn't tree shake it.
+@pragma('vm:entry-point')
+void _registerPlugins() { // ignore: unused_element
+  if (didCallRegistrantBeforeEntrypoint) {
+    throw '_registerPlugins is being called twice';
+  }
+  didCallRegistrantBeforeEntrypoint = true;
+}
+
+@pragma('vm:entry-point')
+void mainForPluginRegistrantTest() { // ignore: unused_element
+  if (didCallRegistrantBeforeEntrypoint) {
+    passMessage('_registerPlugins was called');
+  } else {
+    passMessage('_registerPlugins was not called');
+  }
+}