From 72cc1d198c954f5748cd1881b948d68ebd2d2d26 Mon Sep 17 00:00:00 2001
From: Jonah Williams <jonahwilliams@google.com>
Date: Wed, 19 Feb 2025 14:52:07 -0800
Subject: [PATCH] [iOS] always post new task during gesture dispatch. (#163646)

Fixes https://github.com/flutter/flutter/issues/163429

If we synchronously call into dart ui when dispatching pointer events,
then we only end up scheduling about ~half as many frames as we should.
Nothing is blocked, no task starvation, et cetera. It should always be
safe to post pointer events as a new task instead.
---
 .../shell/common/fixtures/shell_test.dart     |  7 ++++
 engine/src/flutter/shell/common/shell.cc      |  4 +--
 .../flutter/shell/common/shell_unittests.cc   | 36 +++++++++++++++++++
 3 files changed, 45 insertions(+), 2 deletions(-)

diff --git a/engine/src/flutter/shell/common/fixtures/shell_test.dart b/engine/src/flutter/shell/common/fixtures/shell_test.dart
index c217114682..c4fd1f2cd3 100644
--- a/engine/src/flutter/shell/common/fixtures/shell_test.dart
+++ b/engine/src/flutter/shell/common/fixtures/shell_test.dart
@@ -625,3 +625,10 @@ void testPointerActions() {
     });
   };
 }
+
+@pragma('vm:entry-point')
+void testDispatchEvents() {
+  PlatformDispatcher.instance.onPointerDataPacket = (PointerDataPacket pointer) {
+    notifyNative();
+  };
+}
diff --git a/engine/src/flutter/shell/common/shell.cc b/engine/src/flutter/shell/common/shell.cc
index e8c8f3e846..3c37ad0ac7 100644
--- a/engine/src/flutter/shell/common/shell.cc
+++ b/engine/src/flutter/shell/common/shell.cc
@@ -1095,8 +1095,8 @@ void Shell::OnPlatformViewDispatchPointerDataPacket(
   TRACE_FLOW_BEGIN("flutter", "PointerEvent", next_pointer_flow_id_);
   FML_DCHECK(is_set_up_);
   FML_DCHECK(task_runners_.GetPlatformTaskRunner()->RunsTasksOnCurrentThread());
-  fml::TaskRunner::RunNowAndFlushMessages(
-      task_runners_.GetUITaskRunner(),
+
+  task_runners_.GetUITaskRunner()->PostTask(
       fml::MakeCopyable([engine = weak_engine_, packet = std::move(packet),
                          flow_id = next_pointer_flow_id_]() mutable {
         if (engine) {
diff --git a/engine/src/flutter/shell/common/shell_unittests.cc b/engine/src/flutter/shell/common/shell_unittests.cc
index f6d9751bb4..c8f1c796cd 100644
--- a/engine/src/flutter/shell/common/shell_unittests.cc
+++ b/engine/src/flutter/shell/common/shell_unittests.cc
@@ -4376,6 +4376,42 @@ TEST_F(ShellTest, PointerPacketFlushMessageLoop) {
   ASSERT_FALSE(DartVMRef::IsInstanceRunning());
 }
 
+// Verifies a pointer event will flush the dart event loop.
+TEST_F(ShellTest, PointerPacketsAreDispatchedWithTask) {
+  Settings settings = CreateSettingsForFixture();
+  ThreadHost thread_host("io.flutter.test." + GetCurrentTestName() + ".",
+                         ThreadHost::Type::kPlatform);
+  auto task_runner = thread_host.platform_thread->GetTaskRunner();
+  TaskRunners task_runners("test", task_runner, task_runner, task_runner,
+                           task_runner);
+
+  EXPECT_EQ(task_runners.GetPlatformTaskRunner(),
+            task_runners.GetUITaskRunner());
+  auto shell = CreateShell(settings, task_runners);
+  auto configuration = RunConfiguration::InferFromSettings(settings);
+  configuration.SetEntrypoint("testDispatchEvents");
+
+  RunEngine(shell.get(), std::move(configuration));
+  fml::CountDownLatch latch(1);
+  bool did_invoke_callback = false;
+  AddNativeCallback(
+      // The Dart native function names aren't very consistent but this is
+      // just the native function name of the second vm entrypoint in the
+      // fixture.
+      "NotifyNative", CREATE_NATIVE_ENTRY([&](auto args) {
+        did_invoke_callback = true;
+        latch.CountDown();
+      }));
+
+  DispatchFakePointerData(shell.get(), 23);
+  EXPECT_FALSE(did_invoke_callback);
+  latch.Wait();
+  EXPECT_TRUE(did_invoke_callback);
+
+  DestroyShell(std::move(shell), task_runners);
+  ASSERT_FALSE(DartVMRef::IsInstanceRunning());
+}
+
 TEST_F(ShellTest, DiesIfSoftwareRenderingAndImpellerAreEnabledDeathTest) {
 #if defined(OS_FUCHSIA)
   GTEST_SKIP() << "Fuchsia";