Closures over-capture, cannot be sent to other isolate.

[Silfalion]

Hello! I'm trying to create a FormData from a Map<String, dynamic> but I'm faced with the error:

Invalid argument(s): Illegal argument in isolate message: object is unsendable - Library:'dart:async' Class: _Future@4048458 (see restrictions listed at `SendPort.send()` documentation for more information)
I/flutter (18310):  <- Instance of '_HttpClientConnection' (from dart:_http)
I/flutter (18310):  <- Instance of '_HashSetEntry<_HttpClientConnection>' (from dart:collection)
I/flutter (18310):  <- _List len:8 (from dart:core)
I/flutter (18310):  <- Instance of '_HashSet<_HttpClientConnection>' (from dart:collection)
I/flutter (18310):  <- Instance of '_ConnectionTarget' (from dart:_http)
I/flutter (18310):  <- Instance of '_HashMapEntry<String, _ConnectionTarget>' (from dart:collection)
I/flutter (18310):  <- _List len:8 (from dart:core)
I/flutter (18310):  <- Instance of '_HashMap<String, _ConnectionTarget>' (from dart:collection)
I/flutter (18310):  <- Instance of '_HttpClient' (from dart:_http)

Very sorry, the only code I'm able to share is the following:

`final result = await compute((m)=>exportToFormData(mapOfFields), 'hi');

The function returns a Future.

I'm not able to share much more, but the case is very similar to this one(which has a code sample):

flutter/flutter#126520

[lrhn]

This sounds like your closure is over-capturing, which is a known problem with the VM's implementation of closures.
Try moving the creation of the closure into another, function, say:

// Declared somewhere, top-level or static.
Future<WhateverItIs> _computeExport(Map<String, String> data) => compute((_) => exportToForm(data), 'hi');
...
  // Then use as:
  final result = await _computeExport(mapOfFields);

[Silfalion]

Got it, will give it a shot. Thank you

[mkustermann]

We have an older open issue regarding the over-capturing of closures. Will close this in favor of #36983.