Explaining the Singleton Design Pattern

The singleton design pattern is a creational design pattern that ensures that a class has only one instance and provides a global point of access to that instance. This pattern is often used to implement global objects or services that should be shared across an application.

In Swift, the singleton design pattern is typically implemented using a static property and a private initializer. Here's an example to illustrate how this works:

// This is a simple class that represents a logger.
// The logger has a single instance, which can be accessed
// using the "shared" static property.
class Logger {
  static let shared = Logger()

  private init() {}

  func log(message: String) {
    print(message)
  }
}

// Now we can access the shared logger instance and use it
// to log messages.
Logger.shared.log(message: "Hello world!")

In this example, the Logger class has a static shared property that holds the single instance of the class. The Logger class also has a private initializer, which prevents other objects from creating instances of the class.

To use the Logger class, we access the shared property, which returns the single instance of the class. We can then call the log(message:) method on the instance to log a message.

This example shows how the singleton pattern can be used to ensure that a class has only one instance and provides a global point of access to that instance.