Reference ownership and closure cycles

The idea

Automatic reference counting keeps a class instance alive while strong references to it remain. A cycle can prevent instances from being released even after outside owners disappear. For example, an owner may retain a callback that strongly captures the owner itself. Draw the ownership arrows to identify the cycle instead of assuming that leaving a local scope always destroys every reachable object.

A weak reference does not keep its target alive and becomes nil when the target is released, so it uses optional handling. A weak self capture can break an owner-callback cycle, but the callback must define what happens when the owner is gone. unowned expresses a stronger lifetime assumption and is unsafe if that assumption fails. Select the relationship from the real lifetime contract rather than applying weak mechanically to every reference.

class Token {}
var strong:Token?=Token()
weak var observer=strong
print(observer != nil)
strong=nil
print(observer == nil)

true
true

The weak observer does not retain the token, so removing its only strong owner releases it and clears the observer.