Threat-safe Swift singleton example

Swift singleton

class <#ClassName#> : NSObject {
    class var sharedInstance: <#ClassName#> {
        get {
            struct Static {
                static var instance : <#ClassName#>? = nil
                static var token : dispatch_once_t = 0
            }
            
            dispatch_once(&Static.token) {
                Static.instance = <#ClassName#>()
            }
            
            return Static.instance!
        }
    }
}