如何在 Dart 中遍历 Stream

Question

在学习了多年的 Python 之后，我正在使用 Dart 学习绳索。 我对 Streams 有点困惑，如果有任何其他特定的解决方案可以用于我的具体情况。

我正在构建一个 API 驱动的应用程序，我需要动态填充 ListView 而无需等待所有 HTTP 调用完成。 目前我正在使用一个 Stream 来填充一个 List 并返回：

类似这样的：

var scraperList = List<Api> ();

Stream<List<Api>> get getFiles (String id) async* {
    var res = await http.post('${baseUrl}/folder/list/{id}');
    if (res.statusCode == 200) {
      var jsonResult = jsonDecode(res.body);
      jsonResult = jsonResult['content'];
      List<dynamic> body = jsonResult;
      List<Api> result = body.map((dynamic item) => ApiModel.apiFiles(item),).toList();
}

现在，这是我迷路的地方，因为 API 充当一种文件/文件夹存储，如果它们是“文件”，我需要产生结果，并使用文件夹的 id 重复调用并获取该文件夹中的“文件”并继续这样做，直到我递归地获取每个文件夹中的所有文件。

关键是我想继续提供该 Stream 并在结果到达时获得结果，而无需等待所有调用完成。

我现在迷失的地方是这部分

      // THIS IS THE PART I NEED TO UNDERSTAND YET
      for (item in result){
        if (item.type == 'file'){
          yield filelist.add(0, item);}
        else { I NEED TO MAKE A CALL AGAIN TO THIS FUNCTION TO GET THE FOLDER CONTENT
             THE CALL AGAIN }

我如何迭代这个 API，递归地获取每个文件夹中的所有文件并继续提供该 Stream？ 有一个更好的方法吗？ 在 python 中，有很多方法可以跨类共享列表等，但我仍然对如何在 Dart 中动态实现这一点感到困惑。

Answer 1

你需要的是 yield* 和 await

检查一下

import 'dart:async';

void main() {

  final folder1 = Future.value({"a","b","c","d"});
  final folder2 = Future.value({"a","b","c","d",folder1});
  final folder3 = {"a","b","c","d",folder1,folder2}; //what you get initially

  Stream printElement(list) async*{

     for(dynamic element in list){

      if(element is String){
        yield element;
       }else{
        yield* printElement(
        await(element));  //make your http call here for your nested folder
        }

     };
  }

  printElement(folder3).listen((element){
      print(element);
  });

}

您还可以在dartz packageEither 中使用更多类型安全代码

import 'dart:async';
import 'package:dartz/dartz.dart';

class MyFile{

  final String fileName ;

  MyFile(this.fileName);

  @override
  String toString() {
    return this.fileName;
  }
}

class MYFolder{

  final String folderName;
  final List<Either<Future<MYFolder>,MyFile>> files; //files can be Either file or Future of folder from HTTP

  MYFolder(this.folderName, this.files);

  @override
  String toString() {
    return this.folderName;
  }
}

void main() {

  //leaf folders

  final foldera = MYFolder("foldera",[Right(MyFile("a")),Right(MyFile("a")),Right(MyFile("a")),Right(MyFile("a"))]);
  final folderb = MYFolder("folderb",[Right(MyFile("b")),Right(MyFile("b")),Right(MyFile("b")),Right(MyFile("b"))]);
  final folderc = MYFolder("folderc",[Right(MyFile("c")),Right(MyFile("c")),Right(MyFile("c")),Right(MyFile("c"))]);
  final folderd = MYFolder("folderd",[Right(MyFile("d")),Right(MyFile("d")),Right(MyFile("d")),Right(MyFile("d"))]);

  //                    ...foldera
  //                   - folderb
  //        -  folder1
  //
  //folder3 -  folder2
  //                  ... folderc
  //                  - folderd
  //
  // --triple dot notation for folding elements in lists ,it is just a shortcut for populating lists

  final  folder1 = Future.value(MYFolder("folder1", [...foldera.files,Left(Future.value(folderb))]));
  final folder2 = Future.value(MYFolder("folder2", [...folderc.files, Left(Future.value(folderd))]));
  final folder3 = MYFolder("folder3",[Left(Future.value(folder1)),Left(Future.value(folder2))]); //what you get initially

  Stream printFilesIn(MYFolder folder) async*{

    print("Current folder is $folder");

     for(Either file in folder.files){
      if(file is Right){
        yield file.value;
      }else{
        yield* printFilesIn(
        await(file as Left).value); //make your http call here for your nested folder
        }
      };
  }

  printFilesIn(folder3).listen(print);

}