dart - 带有/不带有关键字 `new`的Dart工厂构造函数，有什么区别？

Question

我从flutter_redux示例代码中遇到了以下代码示例。很难理解为什么factory SearchState.initial()用new关键字返回而factory SearchState.loading()和factory SearchState.error()没有返回。

class SearchState {
  final SearchResult result;
  final bool hasError;
  final bool isLoading;

  SearchState({
    this.result,
    this.hasError = false,
    this.isLoading = false,
  });

  factory SearchState.initial() =>
      new SearchState(result: SearchResult.noTerm());

  factory SearchState.loading() => SearchState(isLoading: true);

  factory SearchState.error() => SearchState(hasError: true);
}

刚刚发现Dart语言之旅对这种情况不是很有帮助，而且Dart语言规范过于晦涩。

Answer 1

effective dart指南中的引语:

Dart 2 makes the new keyword optional. Even in Dart 1, its meaning was never clear because factory constructors mean a new invocation may still not actually return a new object.

The language still permits new in order to make migration less painful, but consider it deprecated and remove it from your code.