15 Dec 2020

Using runZoned To Catch Unhandled Exceptions

dart
flutter

Handling unhandled exceptions sounds convoluted, but it really isn't!

Default zone

A default zone called Zone.root is created when executing main(). If you don't create any custom zones (like we're going to do in a second), all code gets executed inside the default zone:

void main() {
  print('Hello from the Twilight (default) Zone!');
}

Using runZoned

If you insert runZoned into your app's main() function, a custom zone will be created:

await runZoned<Future<void>>(() async {
  ...
}, onError: (Object obj, StackTrace trace) async {
  // print the error to console
  // send the error to crashlytics etc
});

The lambda passed as the body argument to runZoned will be executed in the newly created custom zone. Any exceptions that occur will bubble up to the onError lambda.

Add runApp to the zoned body and your whole app now captures any uncaught exceptions:

await runZoned<Future<void>>(() async {
  runApp(AcmeApp());
}, onError: (Object error, StackTrace trace) async {
  print(obj);
  print(trace);
});

Pretty neat, huh?

Support Me on Ko-fi