LoopUsage.swift

var first: First!

let third = Third(first: first)
let second = Second(third: third)
first = First(second: second)

print(first.second.third.first == first)

class First: NSObject {
   let second: Second

   init(second: Second) {
      self.second = second
   }
}

class Second {
   let third: Third

   init(third: Third) {
      self.third = third
   }
}

class Third {
   var first: First? {
      return weakFirst.value
   }

   private let weakFirst: DeferredWeakVar<First>

   init(first availableLater: @escaping @autoclosure () -> First?) {
      self.weakFirst = DeferredWeakVar(availableLater)
   }
}

final class DeferredWeakVar<T: AnyObject> {
   var value: T? {
      if let value = _value {
         return value
      } else if let initializer = initializer {
         let value = initializer()
         _value = value
         self.initializer = nil
         return value
      } else {
         return nil
      }
   }

   private var initializer: (() -> T?)?
   private(set) weak var _value: T?

   init(_ initializer: @escaping () -> T?) {
      // force initialize in other case we can have strong reference inside initializer forever if we did not call it
      DispatchQueue.main.async {
         _ = self.value
      }

      self.initializer = initializer
   }
}