Support late observables

[tomwyr]

Currently, it appears not to be possible to declare a store observable that uses the late keyword. An attempt to set the observable value for the first time causes mobx to read it first throwing LateInitializationError.

class CounterStore extends _CounterStore with _$CounterStore {}

abstract class _CounterStore with Store {
  @observable
  late int counter;
}

The above will generate:

mixin _$CounterStore on _CounterStore, Store {
  late final _$counterAtom =
      Atom(name: '_CounterStore.counter', context: context);

  @override
  int get counter {
    _$counterAtom.reportRead();
    return super.counter;
  }

  @override
  set counter(int value) {
    _$counterAtom.reportWrite(value, super.counter, () { // <- reads value before the initialization has had a chance to happen
      super.counter = value;
    });
  }

  @override
  String toString() {
    return '''
counter: ${counter}
    ''';
  }
}

Consider generating code in a way, that reads the current value only if the observable is not late or it's already been initialized.

[amondnet]

mobx_codegen: 2.5.0