Demo that retaining self in a NotificationCenter observer block leads to a reference cycle you have to break manually.

NotificationObserver.swift

// Paste into a macOS playground in Xcode and run.
// Deleting line 46 "c?.stopObserving()" triggers the assertion because
// the reference cycle caused by retaining self in the observer block is never broken.

import Foundation

let myNotification = NSNotification.Name(rawValue: "myNotification")

var cHasBeenDeallocated = false

class C {
  var observer: NSObjectProtocol?

  init() {
    print(#function)
    startObserving()
  }

  deinit {
    print(#function)
    cHasBeenDeallocated = true
    stopObserving()
  }

  func startObserving() {
    print("Starting listening to notification")
    self.observer = NotificationCenter.default.addObserver(forName: myNotification, object: nil, queue: nil) { notification in
      print("- Received notification:", notification)
      print("- Retaining self in notification block:", self)
    }
  }

  func stopObserving() {
    if let observer = self.observer {
      print("Stopping listening to notification")
      NotificationCenter.default.removeObserver(observer)
      self.observer = nil
    }
  }
}

var c: C? = C()
NotificationCenter.default.post(name: myNotification, object: nil)

// Delete this line to trigger the assertion. deinit is never called because we created a reference cycle.
c?.stopObserving()

c = nil
// If the assertion is triggered, it means we have a reference cycle
assert(cHasBeenDeallocated)