flutter - 什么是 Dart Flutter 中的零参数构造函数

Question

Dart Flutter 中的零参数构造函数是什么？
在 flutter 中创建一个块时，我收到以下错误

  The superclass 'Bloc<QuoteEvent, QuoteState>' doesn't have a zero argument constructor.
Try declaring a zero argument constructor in 'Bloc<QuoteEvent, QuoteState>', or explicitly invoking a different constructor in 'Bloc<QuoteEvent, QuoteState>'.

请指导如何修复它。谢谢
下面是代码

import 'package:meta/meta.dart';
import 'package:bloc/bloc.dart';

import 'package:random_quote/repositories/repositories.dart';
import 'package:random_quote/models/models.dart';
import 'package:random_quote/bloc/bloc.dart';

class QuoteBloc extends Bloc<QuoteEvent, QuoteState> {
  final QuoteRepository repository;

  QuoteBloc({@required this.repository}) : assert(repository != null);

  @override
  QuoteState get initialState => QuoteEmpty();

  @override
  Stream<QuoteState> mapEventToState(QuoteEvent event) async* {
    if (event is FetchQuote) {
      yield QuoteLoading();
      try {
        final Quote quote = await repository.fetchQuote();
        yield QuoteLoaded(quote: quote);
      } catch (_) {
        yield QuoteError();
      }
    }
  }
}

Answer 1

initialState属性已从 flutter_bloc 中删除自 v5.0.0.这是migration guide .
您应该改用 super() 构造函数:

class QuoteBloc extends Bloc<QuoteEvent, QuoteState> {
  final QuoteRepository repository;

  QuoteBloc({@required this.repository}) : 
    assert(this.repository != null),
    super(QuoteEmpty());

  ...