The singleton pattern 

The singleton pattern is one of the most used (and misused) creational patterns. It is seen throughout the Cocoa libraries, UIKit, AppKit, and many other Apple frameworks. Be aware that its use can lead to a central state point that gets mutated across your whole program, which can lead to nasty bugs.

In this section, we'll go back to basics with the singleton and focus on its use cases.

Using singletons

Singletons are fairly easy in Swift. A singleton is an object of which there can never be more than one instance in your program. The first instance of a singleton object will be the last. As a corollary, singletons never die, and their lifespan is always your whole program.

In Swift, we have...