[iOS] Flakey race condition between SetSemanticsTreeEnabled and UpdateSemantics

Steps to reproduce

flutter create my_app
cd my_app
et run --config ios_debug_sim_unopt_arm64
Quit the app once it launches
Open my_app/ios/Runner.xcworkspace
In Xcode do Product > Run

Note: Error does not always happen since it's a race condition, you may need to do step 6 multiple times. It is more likely to happen if you setup UIScene in your Info.plist, but I have seen it happen for both. Also, the crash can also happen when running from the Flutter CLI, but running from Xcode is faster and allows you to see the error log.

Expected results

App should run fine when using a local engine

Actual results

App crashes

[FATAL:flutter/shell/platform/darwin/ios/platform_view_ios.mm(156)] Check failed: accessibility_bridge_.

flutter/engine/src/flutter/shell/platform/darwin/ios/platform_view_ios.mm

Line 154 in 7a13e1b

FML_DCHECK(accessibility_bridge_);

I tried tracing it down and it appears that occasionally FlutterEngine enables semantics before the _handleFrameworkSemanticsEnabledChanged listener is set up.

flutter/engine/src/flutter/shell/platform/darwin/ios/framework/Source/FlutterEngine.mm

Lines 345 to 351 in 7a13e1b

    
           - (void)enableSemantics:(BOOL)enabled withFlags:(int64_t)flags { 
        
             if (!self.platformView) { 
        
               return; 
        
             } 
        
             self.platformView->SetSemanticsEnabled(enabled); 
        
             self.platformView->SetAccessibilityFeatures(flags); 
        
           }

flutter/packages/flutter/lib/src/semantics/binding.dart

Line 42 in 7a13e1b

addSemanticsEnabledListener(_handleFrameworkSemanticsEnabledChanged);

And therefore, the semantics tree is never enabled:

flutter/packages/flutter/lib/src/semantics/binding.dart

Lines 168 to 170 in 7a13e1b

    
           void _handleFrameworkSemanticsEnabledChanged() { 
        
             platformDispatcher.setSemanticsTreeEnabled(semanticsEnabled); 
        
           }

Likely caused by #174163.

Code sample

Use default flutter create sample app

Screenshots or Video

No response

Logs

No response

Flutter Doctor output

Doctor output

Doctor summary (to see all details, run flutter doctor -v):
[✓] Flutter (Channel main, 3.36.0-1.0.pre-348, on macOS 15.6.1 24G90 darwin-arm64, locale en)
[✓] Android toolchain - develop for Android devices (Android SDK version 35.0.0)
[✓] Xcode - develop for iOS and macOS (Xcode 16.3)
[✓] Chrome - develop for the web
[✓] Connected device (4 available)
[✓] Network resources

• No issues found!

@chunhtai

I think this is likely also impacting non-local engine apps, but that's the use case I discovered it with.

cc @chunhtai Do you think this warrants a revert of #174163?

Let me take a look, this seems like a bug that surface by the change. If i can't figure out something quick, I will revert

actually this seems like a corner case in the pr #174163, but should be a easy fix. I will attempt to fix forward due to the initial pr requires g3fix. Let me know otherwise

mark it as P0 since this is serious

Ensures initial semantics state is sent to engine (#174845)

This thread has been automatically locked since there has not been any recent activity after it was closed. If you are still experiencing a similar issue, please open a new bug, including the output of flutter doctor -v and a minimal reproduction of the issue.

Ensures initial semantics state is sent to engine (flutter#174845)

Ensures initial semantics state is sent to engine (#174845)

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[iOS] Flakey race condition between SetSemanticsTreeEnabled and UpdateSemantics #174842

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	- (void)enableSemantics:(BOOL)enabled withFlags:(int64_t)flags {
	if (!self.platformView) {
	return;
	}
	self.platformView->SetSemanticsEnabled(enabled);
	self.platformView->SetAccessibilityFeatures(flags);
	}

	void _handleFrameworkSemanticsEnabledChanged() {
	platformDispatcher.setSemanticsTreeEnabled(semanticsEnabled);
	}

[iOS] Flakey race condition between SetSemanticsTreeEnabled and UpdateSemantics #174842

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vashworth commented on Sep 2, 2025

chunhtai commented on Sep 2, 2025

chunhtai commented on Sep 2, 2025

chunhtai commented on Sep 2, 2025

github-actions commented on Sep 16, 2025

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