Stream and Future in Dart - asynchronous

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;

Answer 1

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