docs should clearly detail that objective-c callbacks are scoped to an isolate #3264

Activity

1. brianquinlan

   added

   package:ffi

   needs-triageNeeds triage from the team. Remove this label once triaged.Needs triage from the team. Remove this label once triaged.

   lang-objective_cRelated to Objective C supportRelated to Objective C support

   type-documentationA request to add or improve documentationA request to add or improve documentation

   on Mar 25, 2026
2. liamappelbe

   removed

   needs-triageNeeds triage from the team. Remove this label once triaged.Needs triage from the team. Remove this label once triaged.

   on Mar 26, 2026
3. liamappelbe

   added this to ObjC/Swift interopon Mar 26, 2026
4. liamappelbe

   added this to the ffigen 21.0.0 (2026 Q1) milestone on Mar 26, 2026
5. dcharkes commented on Mar 26, 2026

   

   dcharkes

   on Mar 26, 2026

   Collaborator

   More actions

   Yes, and the documentation should state that you should unregister your native callbacks on isolate exit. This can be done via NativeFinalizer.

   E.g. if you do an HTTP request, and the native code is going to stream packets to Dart. In your NativeFinalizer you need to call your native cancel_http_request so that it stops streaming packets to Dart and stops doing callbacks.

   Side note: The native finalizer approach will not work properly yet on Flutter with hot restart on helper isolates if you have singletons. E.g. if you have a native_register and native_unregister, and you call unregister on a helper isolate when it is shut down, then a new helper isolate might be started before the old helper isolate has shut down. Calling you to call native_register first, which is probably undefined behavior for your native code. More details: flutter/flutter#75528 (comment)
6. brianquinlan commented on Mar 27, 2026

   

   brianquinlan

   on Mar 27, 2026

   ContributorAuthor

   More actions

   Yes, and the documentation should state that you should unregister your native callbacks on isolate exit. This can be done via NativeFinalizer.

   E.g. if you do an HTTP request, and the native code is going to stream packets to Dart. In your NativeFinalizer you need to call your native cancel_http_request so that it stops streaming packets to Dart and stops doing callbacks.

   1. I think that the premise of our objective-c interop is that the user does not have to write native code
   2. Wouldn't cancel_http_request (or whatever) be racy, i.e., if the callback is dispatched after the isolate becomes unuseable before the finalizer completes?

   Side note: The native finalizer approach will not work properly yet on Flutter with hot restart on helper isolates if you have singletons. E.g. if you have a native_register and native_unregister, and you call unregister on a helper isolate when it is shut down, then a new helper isolate might be started before the old helper isolate has shut down. Calling you to call native_register first, which is probably undefined behavior for your native code. More details: flutter/flutter#75528 (comment)
7. liamappelbe commented on Mar 30, 2026

   

   liamappelbe

   on Mar 30, 2026

   Contributor

   More actions

   1. I think that the premise of our objective-c interop is that the user does not have to write native code

   That's more of an aspiration than a premise. If this is a case where native code is necessary, so be it.

   2. Wouldn't cancel_http_request (or whatever) be racy, i.e., if the callback is dispatched after the isolate becomes unuseable before the finalizer completes?

   You'd need to make the finalizer blocking. The metadata entries aren't cleaned up until after the finalizers run, so blocking the final isolate cleanup until your HTTP client is confirmed closed should at least prevent crashes ¹.

   You could still have a memory leak if the callback is sending an object, and it never arrives (because the caller increments the ref count and the receiver decrements it). Hopefully this would be a rare edge case, because I'm struggling to think of a solution. Everything I think of just makes the edge case a bit more edgy.

   Footnotes

   1. The crash happens during callback dispatch, on the caller thread, where the callback's target port etc is looked up. If its metadata entry still exists then this will work fine even if the target isolate is mid-shutdown. The message will just silently never be handled. ↩
8. liamappelbe

   closed this as completedin #3692on Sep 28, 2026
9. github-project-automation

   moved this to Done in ObjC/Swift interopon Sep 28, 2026