Example of building an inheritance-friendly singleton class in Swift

singleton.swift

#!/usr/bin/xcrun -sdk macosx swift

import Foundation

public protocol SharedInstanceType
{
    init()
}

private struct TokenKey
{
    static let tokenKey = "tokenKey"
}

private struct InstanceKey
{
    static let instanceKey = UnsafePointer<Void>()
}

private extension SharedInstanceType where Self: AnyObject
{
    static var tokenData: NSMutableData {
        get {
            var result = objc_getAssociatedObject(self, TokenKey.tokenKey)
            if result == nil {
                result = NSMutableData(length: sizeof(dispatch_once_t))
                objc_setAssociatedObject(self, TokenKey.tokenKey, result, .OBJC_ASSOCIATION_RETAIN_NONATOMIC)
            }

            return result as! NSMutableData
        }
    }

    static var _instance: Self? {
        get {
            if let instance = objc_getAssociatedObject(self, InstanceKey.instanceKey),
                let result = instance as? Self
            {
                return result
            }
            else {
                return nil
            }
        }
        set {
            if let value = newValue
            {
                objc_setAssociatedObject(self, InstanceKey.instanceKey, value, .OBJC_ASSOCIATION_RETAIN_NONATOMIC)
            }
        }
    }
}

public extension SharedInstanceType where Self: AnyObject
{
    static var sharedInstance:Self {
        get {
            dispatch_once(UnsafeMutablePointer<dispatch_once_t>(self.tokenData.mutableBytes)) {
                self._instance = Self()
            }

            return _instance ?? Self()
        }
    }
}

class FooBar: SharedInstanceType
{
    required init() {}
}

class Foo: FooBar
{

}

class Baz: Foo
{

}

print("\(FooBar.sharedInstance.dynamicType)")
print("\(Foo.sharedInstance.dynamicType)")
print("\(Baz.sharedInstance.dynamicType)")

Can you help in translating to Swift 3.
Thanks in advance