Hi my Flutter dev people, today I am going to share,

• What is a singleton?

• Why singleton?

• How to create a singleton using Dart?

• 1. Ex with explanation.

Initially, during my college days mini projects, I wanted to store some data in my app. I'm not aware of how to store some values and pass them throughout the app. I created a class, created an instance, and used it. But each time it returns a new instance, the new instance has no values. I was totally upset. That time I Googled it, I learned “singleton concept“ in Flutter.

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What is a Singleton?

• Imagine you have a bag. You can put and take things wherever you're from. This is exactly what a singleton is.

• Here, you have a class; we are able to create a multiple instance to access the respective class’s properties and methods. But the magic is, it returns the same instance every time.

• Even if you create 5 instances, all 5 instances will be the same.

• Finally, Singleton ensures that the class has only one instance and provides a global point of access to it.

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Why Singleton?

• If you want to share the data across your entire app, the best choice is Singleton.

• If you have a question like, In which scenarios will it be best to use and more helpful? (Ha.. Haaa.. I got your question!)

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Scenarios:

Scenario 1: Database Connections:

Without using Singleton

• Imagine you are going to connect openDatabase()to a local database.

• Here, each time you will call opneDatabase()multiple times.

• It leads to creating multiple databases in the same location and memory leaks.

Solution

• Use Singleton to fix the issue.

• Why? Because singleton will return only one instance all the time.

Scenario 2: API Clients/Network service (e.g., Dio || Http)

Without using Singleton

• You will create new instance for each API call

• You can not manage the exceptions and tokens wisely for each API call.

• You can not handle the headers and interceptors.

Solution

• If you create the NetworManagerclass with a singleton, the issue will be fixed.

Scenario 3: User Session

Without using Singleton

• We want to share the logged-user information like [name, mail, token] throughout the app. This is what we want.

• But we can't achieve this without a singleton.

Solution

• Use a singleton to share the user details in the entire app.

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How to create a Singleton in Flutter/Dart?

• It’s easy; we need only 3 ingredients.

  1. A static final private instance.

  2. A private named constructor.

  3. A public factory constructor.

//* Creating Singlton
class MySingleton {
  // 1. Create a static private instance
  static final MySingleton _instance = MySingleton._internal();

  // 2. Private constructor
  MySingleton._internal();

  // 3. Public factory constructor that returns the same instance
  factory MySingleton() {
    return _instance;
  }

  void sayHello() {
    print('Hello from Singleton!');
  }
}

//* Usage
void main() {
  var obj1 = MySingleton();
  var obj2 = MySingleton();

  print(obj1 == obj2); // true
  obj1.sayHello(); // Hello from Singleton!
}

As a compiler, I will explain it step by step.

Step 1:

• mainexecutes first.

Step 2:

• MySingleton();will execute.

• Which means it is a factory constructor.

• So it will return the existing instance (not create a new instance)

• But here the _instance is not initially.

Step 3:

• Now, the _instance is not initialized, so the system starts to initialize it (call the MySingleton._internal();private constructor) and store it in a_instance variable.

• This is a static variable. So, it creates an instance only for the class, not for each objects.

• And also, it’s final, so no one is able to reassign the value once it’s initialized.

Step 4:

• Now, the _instance has value, so the factory constructor is ready to return the respective value.

• So, now, the obj1get the value.

Step 5:

• Similarly, the same flow will happen for obj2 as well.

• But, here, the system has the _instance value already.

• So, the system won’t call the private constructor again. Which means the private constructor won’t execute now. It will execute only one time (the very first time).

• Then, obj2 gets the value.

Step 6:

• After that, the system compares obj1andobj2

• Both are the same instance, so the system returns true as a result of the operation.

Step 7:

• Then call the function by using the obj1variable.

• It will return the Hello from Singleton!

• That’s all!

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Conclusion:

• Singleton shares the data all over the app.

• It doesn’t create a new instance for each object creation.

• It only returns the same already existing instance.

• We can't reassign from outside of the class once the instance is created.

• If we create multiple objects the singleton class only returns the same instance, So if we compare it, it will be gets true.