feat: Expose PostPlatformThreadTask and IsPlatformThread to PluginRegistrarWindows by Gustl22 · Pull Request #191140 · flutter/flutter

Gustl22 · 2026-08-14T19:32:30Z

Plugins also should be able to post to the platform thread on windows.
This is especially important for platform channel event handling, as Windows platform events are mostly emitted by another thread.

Closes #190089

Formerly blocked by #193411

For transparency: Claude assisted code

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Code Review

This pull request introduces APIs to check if the current thread is the platform thread and to post tasks to the platform thread from the Windows plugin registrar. Feedback suggests adding defensive null checks for the registrar parameter in the newly added C APIs to prevent potential null-pointer dereferences and memory leaks.

Gustl22 · 2026-10-09T05:49:08Z

+          delete static_cast<std::function<void()>*>(user_data);
+        },
+        /*user_data=*/new std::function<void()>(std::move(callback)));
+  }


Copied from flutter_engine.cc

Gustl22 · 2026-10-09T06:07:30Z

+TEST(PluginRegistrarWindowsTest, IsPlatformThread) {
+  auto windows_api = std::make_unique<TestWindowsApi>();
+  EXPECT_CALL(*windows_api, PluginRegistrarGetView)
+      .WillRepeatedly(Return(nullptr));


The check for PluginRegistrarGetView is probably irrelevant for this test, I left it in, because the other tests define it, too. But we can also remove it from all thread tests.

loic-sharma · 2026-10-09T21:37:06Z

  EXPECT_EQ(result, 7);
 }

+TEST(PluginRegistrarWindowsTest, IsPlatformThread) {


Could you also add tests on the C APIs directly? See:

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

Lines 406 to 573 in 999dffe

// Verify IsPlatformThread returns true on the platform thread.

TEST_F(WindowsTest, IsPlatformThread) {

auto& context = GetContext();

WindowsConfigBuilder builder(context);

EnginePtr engine{builder.RunHeadless()};

ASSERT_NE(engine, nullptr);

EXPECT_TRUE(FlutterDesktopEngineIsPlatformThread(engine.get()));

}

// Verify IsPlatformThread returns false on a background thread.

TEST_F(WindowsTest, IsNotPlatformThread) {

auto& context = GetContext();

WindowsConfigBuilder builder(context);

EnginePtr engine{builder.RunHeadless()};

ASSERT_NE(engine, nullptr);

bool result = true;

std::thread background(

[&]() { result = FlutterDesktopEngineIsPlatformThread(engine.get()); });

background.join();

EXPECT_FALSE(result);

}

// Verify a task can be posted to the platform thread while on the platform

// thread.

TEST_F(WindowsTest, PostPlatformThreadTaskFromPlatformThread) {

auto& context = GetContext();

WindowsConfigBuilder builder(context);

EnginePtr engine{builder.RunHeadless()};

ASSERT_NE(engine, nullptr);

struct Captures {

std::thread::id thread_id;

bool done = false;

} captures;

FlutterDesktopEnginePostPlatformThreadTask(

engine.get(),

[](void* user_data) {

auto captures = static_cast<Captures*>(user_data);

captures->thread_id = std::this_thread::get_id();

captures->done = true;

},

/*on_cancel=*/nullptr, &captures);

while (!captures.done) {

PumpMessage();

}

EXPECT_EQ(captures.thread_id, std::this_thread::get_id());

}

// Verify a task can be posted to the platform thread while on a background

// thread.

TEST_F(WindowsTest, PostPlatformThreadTaskFromBackgroundThread) {

auto& context = GetContext();

WindowsConfigBuilder builder(context);

EnginePtr engine{builder.RunHeadless()};

ASSERT_NE(engine, nullptr);

std::mutex background_thread_id_mutex;

struct Captures {

std::thread::id background_thread_id;

std::thread::id platform_thread_id;

bool done = false;

} captures;

std::thread background([&]() {

{

std::scoped_lock lock{background_thread_id_mutex};

captures.background_thread_id = std::this_thread::get_id();

}

FlutterDesktopEnginePostPlatformThreadTask(

engine.get(),

[](void* user_data) {

auto captures = static_cast<Captures*>(user_data);

captures->platform_thread_id = std::this_thread::get_id();

captures->done = true;

},

/*on_cancel=*/nullptr, &captures);

});

background.join();

while (!captures.done) {

PumpMessage();

}

std::scoped_lock lock{background_thread_id_mutex};

EXPECT_NE(captures.background_thread_id, std::thread::id{});

EXPECT_NE(captures.background_thread_id, captures.platform_thread_id);

EXPECT_EQ(captures.platform_thread_id, std::this_thread::get_id());

}

// Verify that destroying the engine after posting a task invokes the cancel

// callback instead of the task callback.

TEST_F(WindowsTest, PostPlatformThreadTaskCancelledOnEngineDestroy) {

auto& context = GetContext();

WindowsConfigBuilder builder(context);

EnginePtr engine{builder.RunHeadless()};

ASSERT_NE(engine, nullptr);

struct Captures {

bool callback_called = false;

bool cancel_called = false;

} captures;

FlutterDesktopEnginePostPlatformThreadTask(

engine.get(),

[](void* user_data) {

static_cast<Captures*>(user_data)->callback_called = true;

},

[](void* user_data) {

static_cast<Captures*>(user_data)->cancel_called = true;

},

&captures);

// Destroy the engine before the task has a chance to run. The cancel

// callback should be called and the task callback should not.

engine.reset();

EXPECT_FALSE(captures.callback_called);

EXPECT_TRUE(captures.cancel_called);

}

// Verify that the cancel callback is not invoked for a task that runs.

TEST_F(WindowsTest, PostPlatformThreadTaskNotCancelledWhenRun) {

auto& context = GetContext();

WindowsConfigBuilder builder(context);

EnginePtr engine{builder.RunHeadless()};

ASSERT_NE(engine, nullptr);

struct Captures {

bool callback_called = false;

bool cancel_called = false;

} captures;

FlutterDesktopEnginePostPlatformThreadTask(

engine.get(),

[](void* user_data) {

static_cast<Captures*>(user_data)->callback_called = true;

},

[](void* user_data) {

static_cast<Captures*>(user_data)->cancel_called = true;

},

&captures);

EXPECT_FALSE(captures.cancel_called);

while (!captures.callback_called) {

PumpMessage();

}

// Ensures that the cancel callback is not called,

// in case the engine was shut down after running the task.

engine.reset();

EXPECT_TRUE(captures.callback_called);

EXPECT_FALSE(captures.cancel_called);

}

@loic-sharma
Yikes, sorry my tests were sloppy. To be honest I am not that familiar with C(++), as I would wish to be.

I added:

C API tests for PluginRegistrarIsPlatformThread

C API tests for PluginRegistrarPostPlatformThreadTaskFromPlatformThread. I purposely did not duplicate all existing tests, as it is basically only a forward

Also I did ensure on existing tests, that

on_cancel is not called, when task runs successfully

if on_cancel is called, the callback does not leak into memory

Thanks for the updates!

And no worries. I'm also not as familiar with C++ as I'd like after almost 15 years 😂

loic-sharma

LGTM, thanks for the wonderful contribution!

Gustl22 · 2026-10-20T14:22:17Z

@mattkae can you have a look for the second review? Thx for the time.

mattkae

Nice, thanks!

auto-submit · 2026-10-21T14:25:30Z

autosubmit label was removed for flutter/flutter/191140, because The base commit of the PR is older than 7 days and can not be merged. Please merge the latest changes from the main into this branch and resubmit the PR.

Gustl22 · 2026-10-21T14:41:39Z

Thank you all! Unfortunately I cannot merge back from master, otherwise my approvals get revoked.

Gustl22 · 2026-10-21T17:53:23Z

Sorry I am so chatty today. I think master should be merged before reapplying autosubmit again. The test probably fails due to trying getting an engine build which doesn't exist (from a failed mac build?).

…rarWindows.

mattkae · 2026-10-21T18:25:13Z

Sorry I am so chatty today. I think master should be merged before reapplying autosubmit again. The test probably fails due to trying getting an engine build which doesn't exist (from a failed mac build?).

No worries! I will merge up, then retry. Fickle CI 😅

…istrarWindows (flutter#191140) Plugins also should be able to post to the platform thread on windows. This is especially important for platform channel event handling, as Windows platform events are mostly emitted by another thread. Closes flutter#190089 Formerly blocked by flutter#193411 For transparency: Claude assisted code ## Pre-launch Checklist - [x] I read the [Contributor Guide] and followed the process outlined there for submitting PRs. - [x] I read the [AI contribution guidelines] and understand my responsibilities, or I am not using AI tools. - [x] I read the [Tree Hygiene] wiki page, which explains my responsibilities. - [x] I read and followed the [Flutter Style Guide], including [Features we expect every widget to implement]. - [x] I signed the [CLA]. - [x] I listed at least one issue that this PR fixes in the description above. - [x] I updated/added relevant documentation (doc comments with `///`). - [x] I added new tests to check the change I am making, or this PR is [test-exempt]. - [x] I followed the [breaking change policy] and added [Data Driven Fixes] where supported. - [ ] All existing and new tests are passing. If you need help, consider asking for advice on the #hackers-new channel on [Discord]. If this change needs to override an active code freeze, provide a comment explaining why. The code freeze workflow can be overridden by code reviewers. See pinned issues for any active code freezes with guidance. **Note**: The Flutter team is currently trialing the use of [Gemini Code Assist for GitHub](https://developers.google.com/gemini-code-assist/docs/review-github-code). Comments from the `gemini-code-assist` bot should not be taken as authoritative feedback from the Flutter team. If you find its comments useful you can update your code accordingly, but if you are unsure or disagree with the feedback, please feel free to wait for a Flutter team member's review for guidance on which automated comments should be addressed.  [Contributor Guide]: https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#overview [AI contribution guidelines]: https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#ai-contribution-guidelines [Tree Hygiene]: https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md [test-exempt]: https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#tests [Flutter Style Guide]: https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md [Features we expect every widget to implement]: https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md#features-we-expect-every-widget-to-implement [CLA]: https://cla.developers.google.com/ [flutter/tests]: https://github.com/flutter/tests [breaking change policy]: https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#handling-breaking-changes [Discord]: https://github.com/flutter/flutter/blob/main/docs/contributing/Chat.md [Data Driven Fixes]: https://github.com/flutter/flutter/blob/main/docs/contributing/Data-driven-Fixes.md

github-actions Bot added engine flutter/engine related. See also e: labels. platform-windows Building on or for Windows specifically a: desktop Running on desktop team-windows Owned by the Windows platform team labels Aug 14, 2026

Gustl22 changed the title ~~feat: Add PostPlatformThreadTask and IsPlatformThread to PluginRegistrarWindows~~ feat: Expose PostPlatformThreadTask and IsPlatformThread to PluginRegistrarWindows Aug 14, 2026

Gustl22 force-pushed the 190089-platform-thread branch 2 times, most recently from 7139599 to 517ef7f Compare September 8, 2026 22:33

Gustl22 force-pushed the 190089-platform-thread branch from 517ef7f to 562a8d2 Compare September 25, 2026 20:27

Gustl22 force-pushed the 190089-platform-thread branch from 562a8d2 to 643ca13 Compare October 8, 2026 18:49

Gustl22 marked this pull request as ready for review October 8, 2026 18:49

Gustl22 requested a review from a team as a code owner October 8, 2026 18:49

gemini-code-assist Bot reviewed Oct 8, 2026

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Comment thread engine/src/flutter/shell/platform/windows/flutter_windows.cc Resolved

Gustl22 commented Oct 9, 2026

View reviewed changes

Comment thread ...src/flutter/shell/platform/windows/client_wrapper/include/flutter/plugin_registrar_windows.h Resolved

Gustl22 commented Oct 9, 2026

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loic-sharma reviewed Oct 9, 2026

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Gustl22 force-pushed the 190089-platform-thread branch from 538bf40 to 6e7e232 Compare October 10, 2026 17:00

Gustl22 requested a review from loic-sharma October 10, 2026 17:14

loic-sharma requested a review from mattkae October 10, 2026 21:43

loic-sharma added the CICD Run CI/CD label Oct 10, 2026

loic-sharma approved these changes Oct 10, 2026

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mattkae approved these changes Oct 21, 2026

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mattkae added the autosubmit Merge PR when tree becomes green via auto submit App label Oct 21, 2026

auto-submit Bot removed the autosubmit Merge PR when tree becomes green via auto submit App label Oct 21, 2026

mattkae force-pushed the 190089-platform-thread branch from 6e7e232 to 3e37fcf Compare October 21, 2026 14:46

Gustl22 added 5 commits October 21, 2026 14:25

feat: Add PostPlatformThreadTask and IsPlatformThread to PluginRegist…

0b2b2ac

…rarWindows.

test: Verify posted task is not run before execution or on cancel.

a87bc56

test: Verify on_cancel frees the posted callback.

ebe0974

C tests for PluginRegistrarIsPlatformThread

75dbf62

C tests for PluginRegistrarPostPlatformThreadTaskFromPlatformThread

5f6b178

mattkae force-pushed the 190089-platform-thread branch from 3e37fcf to 5f6b178 Compare October 21, 2026 18:25

flutter-dashboard Bot removed the CICD Run CI/CD label Oct 21, 2026

mattkae added CICD Run CI/CD autosubmit Merge PR when tree becomes green via auto submit App labels Oct 21, 2026

auto-submit Bot added this pull request to the merge queue Oct 21, 2026

Merged via the queue into flutter:master with commit e194458 Oct 22, 2026
40 of 41 checks passed

flutter-dashboard Bot removed the autosubmit Merge PR when tree becomes green via auto submit App label Oct 22, 2026

justinmc mentioned this pull request Oct 23, 2026

Flutter Notable Commits - 2026 #181433

Open

Gustl22 deleted the 190089-platform-thread branch October 24, 2026 20:56

Gustl22 mentioned this pull request Oct 25, 2026

fix(windows): Run registrar platform events on platform thread bluefireteam/audioplayers#2032

Draft

5 of 7 tasks

	// Verify IsPlatformThread returns true on the platform thread.
	TEST_F(WindowsTest, IsPlatformThread) {
	auto& context = GetContext();
	WindowsConfigBuilder builder(context);

	EnginePtr engine{builder.RunHeadless()};
	ASSERT_NE(engine, nullptr);

	EXPECT_TRUE(FlutterDesktopEngineIsPlatformThread(engine.get()));
	}

	// Verify IsPlatformThread returns false on a background thread.
	TEST_F(WindowsTest, IsNotPlatformThread) {
	auto& context = GetContext();
	WindowsConfigBuilder builder(context);

	EnginePtr engine{builder.RunHeadless()};
	ASSERT_NE(engine, nullptr);

	bool result = true;
	std::thread background(
	[&]() { result = FlutterDesktopEngineIsPlatformThread(engine.get()); });
	background.join();

	EXPECT_FALSE(result);
	}

	// Verify a task can be posted to the platform thread while on the platform
	// thread.
	TEST_F(WindowsTest, PostPlatformThreadTaskFromPlatformThread) {
	auto& context = GetContext();
	WindowsConfigBuilder builder(context);

	EnginePtr engine{builder.RunHeadless()};
	ASSERT_NE(engine, nullptr);

	struct Captures {
	std::thread::id thread_id;
	bool done = false;
	} captures;

	FlutterDesktopEnginePostPlatformThreadTask(
	engine.get(),
	[](void* user_data) {
	auto captures = static_cast<Captures*>(user_data);
	captures->thread_id = std::this_thread::get_id();
	captures->done = true;
	},
	/on_cancel=/nullptr, &captures);

	while (!captures.done) {
	PumpMessage();
	}

	EXPECT_EQ(captures.thread_id, std::this_thread::get_id());
	}

	// Verify a task can be posted to the platform thread while on a background
	// thread.
	TEST_F(WindowsTest, PostPlatformThreadTaskFromBackgroundThread) {
	auto& context = GetContext();
	WindowsConfigBuilder builder(context);

	EnginePtr engine{builder.RunHeadless()};
	ASSERT_NE(engine, nullptr);

	std::mutex background_thread_id_mutex;
	struct Captures {
	std::thread::id background_thread_id;
	std::thread::id platform_thread_id;
	bool done = false;
	} captures;

	std::thread background([&]() {
	{
	std::scoped_lock lock{background_thread_id_mutex};
	captures.background_thread_id = std::this_thread::get_id();
	}

	FlutterDesktopEnginePostPlatformThreadTask(
	engine.get(),
	[](void* user_data) {
	auto captures = static_cast<Captures*>(user_data);
	captures->platform_thread_id = std::this_thread::get_id();
	captures->done = true;
	},
	/on_cancel=/nullptr, &captures);
	});
	background.join();

	while (!captures.done) {
	PumpMessage();
	}

	std::scoped_lock lock{background_thread_id_mutex};
	EXPECT_NE(captures.background_thread_id, std::thread::id{});
	EXPECT_NE(captures.background_thread_id, captures.platform_thread_id);
	EXPECT_EQ(captures.platform_thread_id, std::this_thread::get_id());
	}

	// Verify that destroying the engine after posting a task invokes the cancel
	// callback instead of the task callback.
	TEST_F(WindowsTest, PostPlatformThreadTaskCancelledOnEngineDestroy) {
	auto& context = GetContext();
	WindowsConfigBuilder builder(context);

	EnginePtr engine{builder.RunHeadless()};
	ASSERT_NE(engine, nullptr);

	struct Captures {
	bool callback_called = false;
	bool cancel_called = false;
	} captures;

	FlutterDesktopEnginePostPlatformThreadTask(
	engine.get(),
	[](void* user_data) {
	static_cast<Captures*>(user_data)->callback_called = true;
	},
	[](void* user_data) {
	static_cast<Captures*>(user_data)->cancel_called = true;
	},
	&captures);

	// Destroy the engine before the task has a chance to run. The cancel
	// callback should be called and the task callback should not.
	engine.reset();

	EXPECT_FALSE(captures.callback_called);
	EXPECT_TRUE(captures.cancel_called);
	}

	// Verify that the cancel callback is not invoked for a task that runs.
	TEST_F(WindowsTest, PostPlatformThreadTaskNotCancelledWhenRun) {
	auto& context = GetContext();
	WindowsConfigBuilder builder(context);

	EnginePtr engine{builder.RunHeadless()};
	ASSERT_NE(engine, nullptr);

	struct Captures {
	bool callback_called = false;
	bool cancel_called = false;
	} captures;

	FlutterDesktopEnginePostPlatformThreadTask(
	engine.get(),
	[](void* user_data) {
	static_cast<Captures*>(user_data)->callback_called = true;
	},
	[](void* user_data) {
	static_cast<Captures*>(user_data)->cancel_called = true;
	},
	&captures);

	EXPECT_FALSE(captures.cancel_called);

	while (!captures.callback_called) {
	PumpMessage();
	}

	// Ensures that the cancel callback is not called,
	// in case the engine was shut down after running the task.
	engine.reset();

	EXPECT_TRUE(captures.callback_called);
	EXPECT_FALSE(captures.cancel_called);
	}

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