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Singleton Pattern in Dart

· 2 min read

Introduction​

Singleton is a design pattern and is one of the simplest ones. It is useful in the cases when we want only a single instance of any object in our application. In this tutorial we will learn about the singleton pattern in Dart and how we can create a singletons in Dart. So let's get started

If you would rather watch a video

What is singleton?​

  • One of the simplest design pattern
  • There can only be one instance of the Class
  • Class itself is responsible to keep track of its sole instance
  • The instance is globally accessible through static method
  • Constructor is private so it cannot be instantiated

When to use singleton?​

  • Mostly useful when multiple instantiation is costly in terms of configurations required
  • Also useful when we want to access same object over and over across our app

Some considerations​

  • Lazy construction
  • Singleton class shouldn’t require any params
  • Should be thread safe Dart is a single threaded language

Creating singleton class in dart​

Create a class and then follow the steps to make it a singleton pattern.

1. Make constructor private​

Singleton._internal() {}

2. Create instance variable​

Create a private static instance variable, that keeps track of the sole instance.

static Singleton _instance;

3. Static getter​

Finally, create a static getter method, that allows access of the instance globally. It is responsible to create the new instance if it's not previously created.

static Singleton get instance {
  if(_instance == null) {
    _instance = Singleton._internal();
  }
  return _instance;
}

Final Code​

class Singleton {
  String name;
  static Singleton _instance;
  
  Singleton._internal() {
    
    name="Singleton pattern";
  }
  
  static Singleton get instance {
    if(_instance == null) {
      _instance = Singleton._internal();
    }
    return _instance;
  }
}