Understanding Dart language using Singleton Design Pattern

As of now, Flutter is not mature enough like Xamarin. But it's going to make big differences in cross-platform application development. No worries, this article is nothing to do with Flutter framework but about its language Dart.

I always have believed that whenever we try to apply a design pattern in a new language like Dart, we can able to understand its good sides of it.

As per Wikipedia, the singleton pattern is a software design pattern that restricts the instantiation of a class to one "single" instance. Why the singleton pattern? Singleton is a very simple, powerful and most used design pattern.

The following diagram illustrates the most basic structure of a singleton class:

First direct implementation the Java way. It's the pretty straight forward method.

class Singleton {
  static Singleton _instance;
  
  Singleton._private();
  
  static Singleton getInstance() {
    if (_instance == null) {
      _instance = Singleton._private();
    }
    return _instance;
  }

  void sayHello() {
    print('Hello, world');
  }
}

void main(List<String> arguments) {
    Singleton.getInstance().sayHello();
}

Yes, you're correct the Dart doesn't have the modifier as like in Java or C#. Like python, if an identifier or the named constructor(The named constructors to enable a class to define multiple constructors) starts with an underscore _, it’s private to its class(and its library).

Now the Dart way - Version 1

class Singleton {
  static Singleton _instance;
  
  Singleton._private();

  static Singleton getInstance() {
    _instance ??= Singleton._private();
    return _instance;
  }

  void sayHello() {
    print('Hello, world');
  }
}

void main(List<String> arguments) {
    Singleton.getInstance().sayHello();
}

Now the code is simplified by the using Assignment Operator(??=), which check and assign if the null found.

Now using Dart factory constructor - Version 2

class Singleton {
  static Singleton _instance;

  Singleton._();

  factory Singleton() {
    _instance ??= Singleton._();
    return _instance;
  }

  void sayHello() {
    print('Hello, world');
  }
}

void main(List<String> arguments) {
    Singleton().sayHello();
}

I had used factory constructor now. Because a factory constructor, the return type is fixed to the type of the class while for a static method you can provide your own return type. So factory constructor is more relevant here to use it. A factory constructor can be invoked with new, but that became mostly irrelevant with optional new in Dart 2.

I hope version 2 is simpler than the previous two code.

Summary

1. In dart language, there is no modifier like private, protected, and public. Instead, we can use underscore(_) like python.

2. Dart provides named constructors to enable a class to define multiple constructors

3. Assignment Operator ??=, which check and assign if the null found.

4. The dart language factory constructor, the return type is fixed to the type of the own class return type.

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