Explain implicit copying of variables on 'Isolates' page

[mmcdon20]

Page URL

https://dart.dev/language/isolates

Page source

https://github.com/dart-lang/site-www/tree/main/src/content/language/isolates.md

Describe the problem

The current section titled "Sending closures with isolates" does not explain that variables captured by the closure are implicitly copied into the new isolate.

See the following example:

import 'dart:isolate';

void main() async {
  int i = 5;
  print(i); // 5
  await Isolate.run(() {
    print(i); // 5
    i = 10;
    print(i); // 10
  });
  print(i); // 5 "i" is unchanged in main isolate
}

Note that modifying i in the new isolate does not affect the value of i in the main isolate, because i was implicitly copied into the new isolate.

---

One issue that people sometimes run into is that when refencing a class property in a closure sent to an isolate, the rest of the class instance gets copied as well. This can especially be an issue if the class contains an object of a type that is not allowed to be sent to an isolate (see https://api.dart.dev/stable/3.4.0/dart-isolate/SendPort/send.html).

See the following example:

import 'dart:isolate';

class Example {
  ReceivePort port = ReceivePort();
  int i = 5;

  Future<void> run() async {
    await Isolate.run(() {
      print(i);
    });
  }
}

void main() async {
  await Example().run();
}

The above code fails with the following exception:

Unhandled exception:
Invalid argument(s): Illegal argument in isolate message: object is unsendable - Library:'dart:isolate' Class: _ReceivePortImpl@1026248 (see restrictions listed at `SendPort.send()` documentation for more information)

A ReceivePort is one of the types of objects restricted from being sent to another isolate. Even though the port variable was not explicitly referenced, it was still implicitly copied over due to i being referenced. I believe this happens because print(i); is shorthand for print(this.i); in the above example.

Expected fix

Add some explanation about implicit copying of variables, and potential issues.

Additional context

No response

I would like to fix this problem.

• I will try and fix this problem on dart.dev.