Singleton with Swift 3.0

Question

I had this implementation with Swift 2.0 and the Xcode suggestion is not only baffling but causes compilation error as well. it\'s a library where users are passing callfunc

Answer 1

Create a singleTon class as follows :

     class mySingleTon{
        
        //1. gives you SingleTon object which is created only once.
        static let sharedInstance = mySingleTon() 
        
        //2. make init private so it  prevents others from using the default '()' initializer for this class.    
        private init(){
        print("i am born")
        }
    
        func foo()
        {
        print("hello")
        }
    }

Example usage :

class A{
func bar()
{
  mySingleTon.sharedInstance.foo()

//3. Uncomment to get error : initializer is inaccessible due to 'private' protection level.
// let newSharedInstance = mySingleTon()
}

let a = A()
let b = A()
a.bar()
b.bar()

Output :

i am born

hello

hello

As you noticed Initializer is only called once. As per apple docs :

“The lazy initializer for a global variable (also for static members of structs and enums) is run the first time that global is accessed, and is launched as dispatch_once to make sure that the initialization is atomic. This enables a cool way to use dispatch_once in your code: just declare a global variable with an initializer and mark it private.”

Explaining Comments :

1. Static member of class implicitly calls "dispatch_once" hence it is thread safe.

2. Making init private prevents initialisation again.

3. Test code line to prove initialisation is private.