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Singleton in Swift

If you’re not familiar with singleton, in software engineering, the singleton pattern is an object which is instantiated exactly once. This is useful when exactly one object is needed to coordinate actions across the system. But this can be abused to form poorly constructed code.

There are several ways to implement singleton in swift. There are three approaches to implement the Singleton pattern that support lazy initialization and thread safety.

1. dispatch_once The traditional Objective-C approach ported to Swift.

class Singleton { class var sharedInstance: Singleton> { struct Static { static var onceToken: dispatch_once_t = 0 static var instance: Singleton? = nil } dispatch_once(&Static.onceToken) { Static.instance = Singleton() } return Static.instance! } }

This way is a straight port of Objective-C's singleton implementation over to Swift. Bad in my opinion because Swift was meant to be terse and expressive.

2. Nested Struct

class Singleton { class var sharedInstance: Singleton { struct Static { static let instance = Singleton() } return Static.instance } }

This way was how we had to do it in Swift 1.0 since classes still didn't support static class variables back then. Structs, however, did support them! Because of these restrictions on static variables, we were forced into a model that looked like this. It's better than the straight Objective-C port but still not good enough.

3. One Line Singleton

class Singleton { static let sharedInstance = Singleton() }

One line singleton?? Yeah! Just one line singleton!

And don’t forget to private init.

class Singleton { static let sharedInstance = Singleton() private init() {} //This prevents others from using the default '()' initializer for this class. }

As of Swift 1.2 and static class variables, implementing a singleton has gotten significantly easier as shown above. It’s worth keeping in mind what a property marked static actually is: it’s a shared property between all objects of that class that can’t be overridden by subclasses (unlike using the class keyword). It’s usage extends beyond just the singleton pattern!

Have fun and Happy coding!

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