CODEWITHSUNDEEP
flutterdartasync-awaittry-catch
user1085665
user1085665

Reputation:

Thrown exception does not get catched in `on` clause (type seems to be wrong)

I have an asynchronous method with a try catch block and it calls another async method that throws. My problem is that the thrown exception is not catched in the catch block.

Future savePlayer(Player player) async {
  try {
    var store = await SharedPreferences.getInstance();
    await store.setString(_playerId, jsonEncode(player.toJson()));   // exception in player.toJson() "NoSuchMethod"
  } on Exception catch (err) {
    throw LocalStorageException(                                     // no catch here
        message: TextConstants.exception.localStorageSaveException,
        hint: TextConstants.exception.localStorageSaveExceptionHint,
        originalExceptionText: err.toString());
  } catch (err) {
    throw LocalStorageException(                                     // catch here, but why?
        message: TextConstants.exception.localStorageSaveException,
        hint: TextConstants.exception.localStorageSaveExceptionHint,
        originalExceptionText: err.toString());
  }
}

If I remove on Exception and just use the catch, then it works. This would be a valid workaround for me. But as I am using the Effective Dart coding rules I get a warning when omitting the on clause.

https://dart-lang.github.io/linter/lints/avoid_catches_without_on_clauses.html

What is the correct way to catch ANY exception in a catch block with an on clause?

Upvotes: 0

Views: 295

Answers (1)

jamesdlin
jamesdlin

Reputation: 89965

on Exception catches objects of type Exception.

What is being thrown is a NoSuchMethodError, which is an Error. Errors are not Exceptions (nor vice-versa).

Dart distinguishes between runtime errors and logical errors (programmer errors). Runtime errors should derive from Exception; logical errors should derive from Error.

Effective Dart discourages catching Error. They usually represent a programming mistake that could have and should have been avoided (e.g. calling a non-existent function).

Upvotes: 1

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