How to access one class method from another class in dart?

You could ensure a singleton instance of the class using a public factory constructor with a private regular constructor:

class Helper {
  static Helper _instance;
  factory Helper() => _instance ??= new Helper._();

  Helper._();

  ...
}

If you call new Helper(), you'll always get the same instance.

You need to import the file that contains class Helper {} everywhere where you want to use it.

??= means new Helper._() is only executed when _instance is null and if it is executed the result will be assigned to _instance before it is returned to the caller.

update

getUserDetailsFromSharedPreference is async and can therefore not be used in the way you use it, at least it will not lead to the expected result. getUserDetailsFromSharedPreference returns a Future that provides the result when the Future completes.

class UserProfileState extends State<UserProfile> {
    Helper helper = new Helper();
    Future<Map> _userData; // this with ??= of the next line is to prevent `getUserDetailsFromSharedPreference` to be called more than once 
    Future<Map> get userData => _userData ??= helper.getUserDetailsFromSharedPreference();
}

If you need to access userData you need to mark the method where you do with async and use await to get the result.

  foo() async {
    var ud = await userData;
    print(ud);
  }

To access other class method you can simply put static on the method.

class Helper {
  static printing(String someText){
    print(someText);
  }
}


void main() {
  Helper.printing('Hello World!');
}