How to Build a Singleton In Dart?

• General

Only one instance of a class is created in a singleton pattern. So in this article, we will go through how to Build a Singleton In Dart.

How to Build a Singleton In Dart?

If you use Dart’s factory constructors, it’s easy to build a Singleton:

class Singleton {
  static final Singleton _singleton = Singleton._internal();
  factory Singleton() {
    return _singleton;
  }
  Singleton._internal();
}

Firstly, You can construct it like this

main() {
  var s1 = Singleton();
  var s2 = Singleton();
  print(identical(s1, s2));  // true
  print(s1 == s2);           // true
}

But what about just using a global variable within your library.

Single.dart will have a code snippet like the below:

library singleton;
var singleton = Impl();
class Impl {
  int? i;
}

main.dart

import 'single.dart';

void main() {
  var a = Singleton;
  var b = Singleton;
  a.i = 2;
  print(b.i);
}

Also, look at the Dart singleton by const constructor & factory

class Singleton {
  factory Singleton() =>
    Singleton._internal_();
  Singleton._internal_();
}
 
void main() {
  print(new Singleton() == new Singleton());
  print(identical(new Singleton() , new Singleton()));
}

However, here is another possible way:

void main() {
  var s1 = Singleton.instance;
  s1.somedata = 123;
  var s2 = Singleton.instance;
  print(s2.somedata); // 123
  print(identical(s1, s2));  // true
  print(s1 == s2); // true
  //var s3 = new Singleton(); //produces a warning re missing default constructor and breaks on execution
}

class Singleton {
  static final Singleton _singleton = new Singleton._internal();
  Singleton._internal();
  static Singleton get instance => _singleton;
  var somedata;
}

Moreover, Accessing the singleton can be done by:

• Using a Singleton global variable that points to the instance.
• The common Singleton.instance pattern.
• Using the default constructor, which is a factory that returns the instance.

Also, consider a code snippet like the below:

Singleton get singleton => Singleton.instance;
ComplexSingleton get complexSingleton => ComplexSingleton._instance;

class Singleton {
  static final Singleton instance = Singleton._private();
  Singleton._private();
  factory Singleton() => instance;
}

class ComplexSingleton {
  static ComplexSingleton _instance;
  static ComplexSingleton get instance => _instance;
  static void init(arg) => _instance ??= ComplexSingleton._init(arg);

  final property;
  ComplexSingleton._init(this.property);
  factory ComplexSingleton() => _instance;
}

But what if you need to do complex initialization? You’ll just have to do so before using the instance later in the program.

Example:

void main() {
 print(identical(singleton, Singleton.instance));      // true
 print(identical(singleton, Singleton()));             // true
 print(complexSingleton == null);                      // true
 ComplexSingleton.init(0); 
 print(complexSingleton == null);                      // false
 print(identical(complexSingleton, ComplexSingleton()));// true
}

• Factory constructor

  class SingletonOne {
    SingletonOne._privateConstructor();
    static final SingletonOne _instance = SingletonOne._privateConstructor();
    factory SingletonOne() {
      return _instance;
    }
  }

• Static field with getter

  class SingletonTwo {
    SingletonTwo._privateConstructor();
    static final SingletonTwo _instance = SingletonTwo._privateConstructor();
    static SingletonTwo get instance => _instance;
  }

• Static field

  class SingletonThree {
    SingletonThree._privateConstructor();
    static final SingletonThree instance = SingletonThree._privateConstructor();
    
  }

  How to instantiate

  The above singletons are instantiated as below:

  SingletonOne one = SingletonOne();
  SingletonTwo two = SingletonTwo.instance;
  SingletonThree three = SingletonThree.instance;

Conclusion:

Thanks for Reading!

So, In this article, we have been through how to Build a Singleton In Dart.

So, Keep Learning! Keep Fluttering!

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Written by Abhishek Dhanani

Abhishek Dhanani, a skilled software developer with 3+ years of experience, masters Dart, JavaScript, TypeScript, and frameworks like Flutter and NodeJS. Proficient in MySQL, Firebase, and cloud platforms AWS and GCP, he delivers innovative digital solutions.