英文:
Are imports in Dart not recursive?
问题
我有一个项目文件，其中包括class MyFileStream extends Stream<someList>。该文件以import 'dart:io';开始。
当我尝试在class MyFileStream内部实现函数.listen()时，我发现我还需要在同一文件中导入import 'dart:async';。
问题是，dart:async已经在文件*"/dart-sdk/lib/io/io.dart"*中导入，跟在library dart.io;这一行后面。
那么，为什么我需要导入async，而它已经在io中导入了呢？这似乎与C语言不同。在Dart语言中，导入是私有的吗？这背后的想法是什么？
英文:
I have a project file which includes class MyFileStream extends Stream<someList>. The file begins with import 'dart:io';.
When I try to implement the function .listen() inside class MyFileStream I found out that I needed to import 'dart:async'; as well in the same file.
The thing is, dart:async is already imported in file "/dart-sdk/lib/io/io.dart" following the line library dart.io;.
So, why would I import async while it is already imported in io? This seems to be different than C language. Are imports private in Dart language? What is the idea behind?
答案1
得分: 3
如果A导入了B，而B导入了C，C的公共符号在B中可见，但不会重新导出到A。要实现这一点，B必须使用export 'C.dart';而不仅仅是import 'C.dart';。
因此，回答你的问题，导入默认情况下不是递归的，但可以根据请求进行递归。这可以非常强大，还可以使用适当的show和hide操作重新导出使用的接口的子集。
一个示例是flutter_riverpod如何完全重新发布riverpod类：
export 'package:riverpod/riverpod.dart';
以及Riverpod如何谨慎地在https://github.com/rrousselGit/riverpod/blob/master/packages/riverpod/lib/riverpod.dart中导出其内部符号的一部分。
英文:
If A imports B, and B imports C, the public symbols of C are visible in B, but not re-exported to A. To do that, B would have to say export 'C.dart'; instead of just import 'C.dart';
So, to answer your question, imports are not recursive by default, but they can be, on request. This can be quite powerful, to also re-export a subset of a consumed interface using the proper show and hide operations.
An example of that is how flutter_riverpod completely re-publishes the riverpod classes:
export 'package:riverpod/riverpod.dart';
and how Riverpod carefully exports a portion of its internal symbols in https://github.com/rrousselGit/riverpod/blob/master/packages/riverpod/lib/riverpod.dart.
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