Swift - Singleton Class

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A Singleton Class is a special class which have only one object and will make sure that your application or program uses that object. If you try to create another variable of singleton class you will get an error. Singleton class is easy to use but it has a major drawback which is global access, which means you can access singleton class globally by sharing the instance of the class.

Which makes it hard to maintain, can lead to concurrency issues, etc. So to avoid excess use of singleton object we use dependency injection. Dependency injection passes the singleton object to the object where it is required.

If a class depends upon the singleton and they are present in different contexts, then you have to import a singleton into that context without importing the class cannot access the singleton.

Syntax

Following is the syntax of the Singleton Class −

class Name{
   // body
}

Creating and Accessing Singleton Class

We can create a singleton class just like the normal class using the “class” keyword but with some different rules and they are −

• A singleton class should have a private initializer.

• A singleton class should have a static-type object.

Private Initializer

This private initializer prevents the object creation from outside the class. If you try to create an object outside the class you will get an error. To create a private initializer we will use a “private” keyword.

Syntax

Following is the syntax for private initializer −

private init(){}

Example

Swift program to create a singleton class with a private initializer.

import Foundation
// Outer class
class SingletonClass{

   // Creating a private initializer
   private init(){}
    
   func show(){
      print("Hello")
   }
}

// Creating object outside the class
// We will get an error
let myObject = SingletonClass()
myObject.show()

Output

It will produce the following output −

main.swift:16:16: error: 'SingletonClass' initializer is inaccessible due to 'private' protection level
let myObject = SingletonClass()

Here the private protection level prevents us from creating an object outside the class. So to overcome this error we have to create an object inside the class.

Static Object

If we have a static object inside the single class we can easily access the object by using the class name. To create a static object we use the “Static” keyword.

Syntax

Following is the syntax for static objects −

static let objectName = className()

Following is the syntax for accessing static objects −

let variableName = className.objectName

Example

Swift program to create a singleton class with an object.

import Foundation
// Singleton class
class SingletonClass{

   // Creating object outside the class
   static let myObject = SingletonClass()

   // Creating a private initializer
   private init(){}
    
   // Method
   func show(){
      print("Hello I am Singleton Class")
   }
}
// Accessing the object 
let ob = SingletonClass.myObject

// Accessing the method
ob.show()

Output

It will produce the following output −

Hello I am Singleton Class