Brief on Singletons and Mixins in dart/flutter

main.dart

//Run this code snippet in dartPad

void main() {
  // Example of creating Singletons in dart & Use of mixins.
  // Singleton creates only one instance/object.
  // Hence less memory allocation.

  //Below we have created two instances but its initialised only once.
  ApiService service1 = ApiService.instance;
  ApiService service2 = ApiService.instance;

  service1.fetchCart();
  service2.callAllMethods();
  service1.logout();
}

class ApiService with CartApiService, AuthApiService {
  // We will be assigning a instance of this class within the class itself
  // We use a private constructor for this [_privateConstructor]
  // Note: Use _ in dart to make something private in dart;
  static ApiService? _instance;
  late String name;
  // late Dio dio;

  ApiService._privateConstructor() {
    // Initialise everything here
    // Usually in Api Service we initialise the  network client here.
    // dio = Dio.instance
    name = "Hello Lokam!";
    print("constructor initialised for one time only");
  }

  // static getter which returns the _instance;
  static ApiService get instance {
    if (_instance == null) {
      // Null then ->
      // initilise a instance by calling the private constructor
      _instance = ApiService._privateConstructor();
      return _instance!;
    }
    // Not Null ->
    // Return the already created instance;
    return _instance!;
  }

  void callAllMethods() {
    print("\n\n==================>");
    signUp();
    login();
    fetchCart();
    addToCart();
    deleteFromCart();
    print("<===================\n\n");
  }
}

// In Dart we can only inherit from a single class i.e only one super class
// But lets say you want multiple hierarchies
// Like in this example ApiService inherits from both AuthApiService and CartApiService

mixin AuthApiService {
  signUp() {
    print("Successfully Registered!");
  }

  logout() {
    print("Logged out! Login to continue");
  }

  login() {
    print("Successfully logged in");
  }
}

mixin CartApiService {
  fetchCart() {
    print("Fetching products in cart");
  }

  addToCart() {
    print("Adding item to cart");
  }

  deleteFromCart() {
    print("Deleting item from cart");
  }
}