ClinetExamplePlayground.swift

//: Playground - noun: a place where people can play

import UIKit

class ClinetSession {
    static func shared() -> ClinetSession { return ClinetSession() }
    private init() { }
}

class Clinet {
    weak var delegate: ClinetDelgate?
    init(_ session: ClinetSession) { }
}

protocol ClinetDelgate: class { }

class Foo: ClinetDelgate {

    lazy var str: String! = "Bar"

    lazy var aClient: Clinet! = {
        var _aClient = Clinet(ClinetSession.shared())
        _aClient.delegate = self
        return _aClient
    }()

// ...

    func bar() {
        print("pre nil: \(aClient) \(str)") // pre nil: Optional(Clinet) Optional("Bar")
        aClient = nil // Foo
        print("post nil: \(aClient) \(str)") // post nil: Optional(Clinet) Optional("Bar")
    }

}

let foo = Foo() // Foo
foo.str // "Bar"
foo.bar() // Foo