In Dart, is it possible to pass an argument in a singleton?

Sure! You need to pass the arguments to the factory method then you need to update the properties USING the referenced instance.

For example, you had

class Peoples {  int id;  String name;  static final Peoples _inst = Peoples._internal();  Peoples._internal();  factory Peoples() {    return _inst;  }}

If you apply my solution then you have

class Peoples {  int id;  String name;  static final Peoples _inst = Peoples._internal();  Peoples._internal();  factory Peoples({int id, String name}) {    _inst.id = id    _inst.name = name    return _inst;  }}

with this your question should be answeredfor more info about factory and params visit

https://dart.dev/guides/language/language-tour

Working Example

class Peoples {  int id;  String name;  static final Peoples _inst = Peoples._internal();  Peoples._internal();  factory Peoples(int id, String name) {    _inst.id = id;    _inst.name = name;    return _inst;  }}void main() {  print("Instance of = " + Peoples(0, "Dylan").name);  print("Instance of = " + Peoples(1, "Joe").name);  print("Instance of = " + Peoples(2, "Maria").name);}

I'd like to answer showing a way to create a singleton by passing arguments to it and how to "lock" its values after creating it for the first time.

class People {  static final People _inst = People._internal();  People._internal();  factory People(int id, String name) {    assert(!_inst._lock, "it's a singleton that can't re-defined");    _inst.id = id;    _inst.name = name;    _inst._lock = true;    return _inst;  }    int id;  String name;  bool _lock = false;}void main() {  var people = People(0, 'Dylan');  try{    print('Instance of = ' + People(0, 'Joe').name);    print('Instance of = ' + People(1, 'Maria').name);    print('Instance of = ' + People(2, 'Ete sech').name);  } finally {    print('Instance of = ' + people.name);  }}