Stream and Future in Dart

Question

I've been using basic async/await for some time without many problems and I thought I understood how it worked. Can't say I'm an expert in it, but I understadn the gist of it. I just can't get my head around Streams though. Before today I thought I understood how they worked (basically ala Reactive Programming), but I can't get them to work in Dart.

I'm working on a persistance layer with the possibility of saving and retrieving (json) files. I've been using the fileManager example as a guideline.

import 'dart:io';
import 'dart:async';
import 'package:intl/intl.dart'; //date
import 'package:markdowneditor/model/note.dart';//Model
import 'package:path_provider/path_provider.dart';
import 'package:path/path.dart' as p;
import 'package:flutter/foundation.dart'; //log
import 'package:simple_permissions/simple_permissions.dart';//OS permissions

class FileManager {
  static final FileManager _singleton = new FileManager._internal();

  factory FileManager() {
    return _singleton;
  }

  FileManager._internal();

  Future<String> get _localPath async {
    final directory = (await getApplicationDocumentsDirectory()).toString();
    return p.join(directory, "notes"); //path takes strings and not Path objects
  }

  Future<File> writeNote(Note note) async {
    var file = await _localPath;
    file = p.join(
        file,
        DateFormat('kk:mm:ssEEEMMd').format(DateTime.now()) +
            " " +
            note.title); //add timestamp to title
    // Write the file

    SimplePermissions.requestPermission(Permission.WriteExternalStorage)
        .then((value) {
      if (value == PermissionStatus.authorized) {
        return File(file).writeAsString('$note');
      } else {
        SimplePermissions.openSettings();
        return null;
      }
    });

  }

  Future<List<Note>> getNotes() async {
    //need file access permission on android. use https://pub.dartlang.org/packages/simple_permissions#-example-tab-
    final file = await _localPath;

    SimplePermissions.requestPermission(Permission.ReadExternalStorage)
        .then((value) {
      if (value == PermissionStatus.authorized) {
        try {
          Stream<FileSystemEntity> fileList =
              Directory(file).list(recursive: false, followLinks: false);

          // await for(FileSystemEntity s in fileList) { print(s); }
          List<Note> array = [];
          fileList.forEach((x) {

            if (x is File) {
              var res1 = ((x as File).readAsString()).then((value2) {
                Note note = Note.fromJsonResponse(value2);
                return note;
              }).catchError((error) {
                debugPrint('is not file content futurestring getNoteError: $x');
                return null;
              });
              var array2 = res1.then((value3) {
                array.add(value3);
                return array;
              });
            //?
            } else {
              debugPrint('is not file getNoteError: $x');
            }
          });


          // Add the file to the files array
          //Return the Future<List<Note>>
          return array;

        } catch (e) {
          debugPrint('getNoteError: $e');
          // If encountering an error, return 0
          return null;
        }
      } else {
        SimplePermissions.openSettings();
        return null;
      }
    });
  }
}

Obviously as it is it won't work, but even trying to await the loop using the commented out parts raises an error.

In "getNotes", after checking the permissions I want to get an array of all the files in the directory, parse them as Note objects and return the resulting array.

I get the list of files:

Stream<FileSystemEntity> fileList =
          Directory(file).list(recursive: false, followLinks: false);

And for each one of them in the stream I want to parse the file into an object and append it to an array to return at the end.

       List<Note> array = [];
      fileList.forEach((x) {

        if (x is File) {
          var res1 = ((x as File).readAsString()).then((value2) {
            Note note = Note.fromJsonResponse(value2);
            return note;
          }).catchError((error) {
            debugPrint('is not file content futurestring getNoteError: $x');
            return null;
          });
          var array2 = res1.then((value3) {
            array.add(value3);
            return array;
          });
        //?
        } else {
          debugPrint('is not file getNoteError: $x');
        }
      });


      // Add the file to the files array
      //Return the Future<List<Note>>
      return array;

Solution

Stream.forEach() returns a Future. Your last return statement runs immediately after the for-each call, but should await it.

await fileList.forEach((x) {
  ...

https://api.dartlang.org/stable/2.2.0/dart-async/Stream/forEach.html

Comments

[vale]

Edited the question to make it clearer. Let me know if you have any other question.

[vale]

Allright got it. You can convert a stream to a Future if you need all elements and don't want them one by one by using .toList(); at the end and return that future in the end. Thanks.

[pskink]

sure, your welcome, Stream#asyncMap is very handy if you have to perform some Future actions on the elements of your Stream

[Jacob Phillips]

What is the error? Include it in the post please.