Expose engineId on native plugin registrars on Linux and Windows by tarrinneal · Pull Request #196032 · flutter/flutter

tarrinneal · 2026-11-05T08:44:16Z

Follow-up to #195881, completing platform parity by exposing the engine identifier (corresponding to PlatformDispatcher.instance.engineId in Dart) directly on native plugin registrars on Linux and Windows so that FFI plugins can associate and clean up per-engine native state during plugin registration and detachment without access to internal engine APIs:

Linux: Adds fl_plugin_registrar_get_engine_id(FlPluginRegistrar*) to FlPluginRegistrar (fl_plugin_registrar.h).
Windows: Adds FlutterDesktopPluginRegistrarGetEngineId(FlutterDesktopPluginRegistrarRef) to the C API (flutter_windows.h) and PluginRegistrarWindows::GetEngineId() to the C++ client wrapper (plugin_registrar_windows.h).

Fixes #195876

Breaking Change Assessment

Unlike the @required Objective-C protocol property added on iOS and macOS in #195881 (which required updating custom FlutterPluginRegistrar test stubs in flutter/packages#13574 and has been tagged c: API break on #195881 / #195876 for release notes), this PR is non-breaking for both production plugins and custom test fakes/mocks:

Linux (FlPluginRegistrar): get_engine_id is appended to _FlPluginRegistrarInterface. GLib zero-initializes GTypeInterface structs prior to iface_init, and fl_plugin_registrar_get_engine_id guards against iface->get_engine_id == nullptr (returning 0), so existing GObject mock/fake FlPluginRegistrar implementations that do not assign iface->get_engine_id continue to compile and run without changes (verified by FlPluginRegistrarTest.LegacyFakeWithoutGetEngineId).
Windows (FlutterDesktopPluginRegistrarRef / PluginRegistrarWindows): FlutterDesktopPluginRegistrarGetEngineId is a standalone C function and PluginRegistrarWindows::GetEngineId() is a concrete, non-virtual method on a concrete class, requiring no overrides in existing test doubles.

Pre-launch Checklist

gemini-code-assist

Code Review

This pull request introduces APIs to retrieve the unique engine identifier from the plugin registrar on both Linux and Windows platforms, along with corresponding unit tests. The review feedback points out a portability issue in fl_engine.cc, fl_view.cc, and flutter_windows.cc, where pointers are directly cast to int64_t using reinterpret_cast. To align with the Google C++ Style Guide, these pointers should be cast to intptr_t or uintptr_t first to avoid compilation issues on 32-bit platforms.

gemini-code-assist · 2026-11-05T08:45:24Z

  return fl_plugin_registrar_new(nullptr, self->binary_messenger,
-                                 self->texture_registrar);
+                                 self->texture_registrar,
+                                 reinterpret_cast<int64_t>(self));


According to the Google C++ Style Guide (referenced by the Repository Style Guide), when casting a pointer to an integer, you should only cast to/from intptr_t or uintptr_t. Direct casting to int64_t using reinterpret_cast can lead to compilation errors or warnings on 32-bit platforms where pointer sizes differ from 64-bit integers. Casting to intptr_t first is portable and safe, and it will be implicitly promoted to int64_t.

reinterpret_cast<intptr_t>(self));

References

Google C++ Style Guide: When casting a pointer to an integer, use reinterpret_cast to intptr_t or uintptr_t. ^(link)

gemini-code-assist · 2026-11-05T08:45:25Z

                                 fl_engine_get_binary_messenger(self->engine),
-                                 fl_engine_get_texture_registrar(self->engine));
+                                 fl_engine_get_texture_registrar(self->engine),
+                                 reinterpret_cast<int64_t>(self->engine));


According to the Google C++ Style Guide (referenced by the Repository Style Guide), when casting a pointer to an integer, you should only cast to/from intptr_t or uintptr_t. Direct casting to int64_t using reinterpret_cast can lead to compilation errors or warnings on 32-bit platforms where pointer sizes differ from 64-bit integers. Casting to intptr_t first is portable and safe, and it will be implicitly promoted to int64_t.

reinterpret_cast<intptr_t>(self->engine));

References

Google C++ Style Guide: When casting a pointer to an integer, use reinterpret_cast to intptr_t or uintptr_t. ^(link)

gemini-code-assist · 2026-11-05T08:45:25Z

+  if (!registrar) {
+    return 0;
+  }
+  return reinterpret_cast<int64_t>(registrar->engine);


According to the Google C++ Style Guide (referenced by the Repository Style Guide), when casting a pointer to an integer, you should only cast to/from intptr_t or uintptr_t. Direct casting to int64_t using reinterpret_cast can lead to compilation errors or warnings on 32-bit platforms where pointer sizes differ from 64-bit integers. Casting to intptr_t first is portable and safe, and it will be implicitly promoted to int64_t.

return reinterpret_cast<intptr_t>(registrar->engine);

References

Google C++ Style Guide: When casting a pointer to an integer, use reinterpret_cast to intptr_t or uintptr_t. ^(link)

…a intptr_t

loic-sharma · 2026-11-06T00:30:51Z

+  if (!registrar) {
+    return 0;
+  }
+  return reinterpret_cast<intptr_t>(registrar->engine);


Could we centralize the casting logic to ensure it doesn't drift? Currently both FlutterDesktopPluginRegistrarGetEngineId and FlutterWindowsEngine have logic to convert a FlutterWindowsEngine* into an ID. I'd consider:

Adding a int64_t FlutterWindowsEngine::id()

Updating this to call registrar->engine->id()

Updating this line to call id():

flutter/engine/src/flutter/shell/platform/windows/flutter_windows_engine.cc

Line 404 in c0921ab

args.engine_id = reinterpret_cast<int64_t>(this);

loic-sharma · 2026-11-06T00:31:08Z

+//
+// This corresponds to PlatformDispatcher.instance.engineId in Dart.
+FLUTTER_EXPORT int64_t FlutterDesktopPluginRegistrarGetEngineId(
+    FlutterDesktopPluginRegistrarRef registrar);


Nit: could you introduce a FlutterDesktopEngineId typedef, similar to the existing FlutterDesktopViewId above:

https://github.com/tarrinneal/flutter/blob/c19d3980351ceb6e2a22f76cbfea5efca97eb2bf/engine/src/flutter/shell/platform/windows/public/flutter_windows.h#L35-L36

You'll get some type safety benefits if we ever add an API that accepts an engine ID.

loic-sharma · 2026-11-06T00:37:46Z

  return fl_plugin_registrar_new(nullptr, self->binary_messenger,
-                                 self->texture_registrar);
+                                 self->texture_registrar,
+                                 reinterpret_cast<intptr_t>(self));


I'd consider adding a private int64_t fl_engine_get_id(FlEngine*) helper function so that we can keep this logic unified. This would be a good spot for it:

https://github.com/tarrinneal/flutter/blob/76395508545ff366c171bf5ab39fc893a9ca3e71/engine/src/flutter/shell/platform/linux/fl_engine_private.h#L656-L668

loic-sharma · 2026-11-06T00:40:36Z


+// Returns the unique identifier for the engine associated with this registrar.
+//
+// This corresponds to PlatformDispatcher.instance.engineId in Dart.


Let's call out thread-safety here since this can be used from any thread:

Suggested change

// This corresponds to PlatformDispatcher.instance.engineId in Dart.

// This corresponds to PlatformDispatcher.instance.engineId in Dart.

// This can be called on any thread.

mattkae

One nit on Linux, and I +1 Loic's Windows review

mattkae · 2026-11-12T14:45:15Z

+
+  FlPluginRegistrarInterface* iface = FL_PLUGIN_REGISTRAR_GET_IFACE(self);
+  if (iface->get_engine_id == nullptr) {
+    return 0;


Is there a constant that we can pin this too somewhere, e.g. kDefaultEngineId if that exists in the repo?

Expose engineId on native plugin registrars on Linux and Windows

9f4e666

tarrinneal requested review from a team as code owners November 5, 2026 08:44

flutter-dashboard Bot added the CICD Run CI/CD label Nov 5, 2026

gemini-code-assist Bot reviewed Nov 5, 2026

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Test legacy FlPluginRegistrar fake compatibility and cast pointers vi…

c19d398

…a intptr_t

loic-sharma reviewed Nov 6, 2026

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loic-sharma requested review from knopp and robert-ancell November 6, 2026 00:42

mattkae reviewed Nov 12, 2026

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Expose engineId on native plugin registrars on Linux and Windows - #196032

Expose engineId on native plugin registrars on Linux and Windows#196032
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	// This corresponds to PlatformDispatcher.instance.engineId in Dart.
	// This corresponds to PlatformDispatcher.instance.engineId in Dart.
	// This can be called on any thread.

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