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What Is a Singleton?

Posted at — Dec 03, 2020
#arrow-shape

What Is a Singleton?

Singleton pattern is a way to ensure that the class has only one instance and provide a global point of access to it (the Singleton instance).

Generally, we create an instance of a normal class by using the = operator and the name of the class that we will instantiate (after =).

class HTTPClient {
    ...
}

...

let client = HTTPClient()

But not if we use the Singleton pattern, since a Singleton is a class that has only one instance.

So, how to create a Singleton?

Step 1

To ensure that the class can’t be instantiated more than once from the outside world, the Singleton pattern constructor/initializer should be private.

class HTTPClient {
    private init() {}
}

Step 2

As we can’t instantiate more than once the Singleton class, we should provide a global point of access to the Singleton instance.

class HTTPClient {
    private static let instance = HTTPClient()
    
    private init() {
        
    }
}

Step 3

Create a static function that returns the instance.

class HTTPClient {
    private static let instance = HTTPClient()
    
    private init() {
        
    }
    
    static func getInstance() -> HTTPClient {
        return instance
    }
}

Step 4

From the outside world, create an instance with the getInstance() function.

class HTTPClient {
    private static let instance = HTTPClient()
    
    private init() {
        
    }
    
    static func getInstance() -> HTTPClient {
        return instance
    }
}

let client = HTTPClient.getInstance()

Conclusion

The key point from what we’ve learned is that the Singleton has only one instance. We can use Singleton when we need precisely one instance of a class, and it must be accessible from a well-known access point.

For example, a class that logs messages to the console is a good Singleton candidate, as the system may require access to it from any given point. So, we only need its public API to log messages/events, and we don’t need more than one instance.