Is cleaning up strong references in deinit a correct pattern?

问题

There are several resources (blog, SO question, plus I've seen it used everywhere) that recommend removing an observer from the NotificationCenter in the deinit of the UIViewController, e.g.:

deinit {
    NotificationCenter.default.removeObserver(self, name: NSNotification.Name.UIKeyboardWillShow, object: nil)
    NotificationCenter.default.removeObserver(self, name: NSNotification.Name.UIKeyboardWillHide, object: nil)
}

Now while according to another blog entry I don't have to care about removing an observer from NotificationCenter since it uses weak references, I've seen the same pattern used with other references.

The question that bugs me. According to official documentation:

A deinitializer is called immediately before a class instance is deallocated. You write deinitializers with the deinit keyword, similar to how initializers are written with the init keyword. Deinitializers are only available on class types.

Doesn't this mean that if there still is a strong reference to the class, deinit will not get called, thus rendering the deinit references cleanup useless? If there is still a strong reference to the viewController from the NotificationCenter, then viewController's deinit will never get called, right? So removing the strong refenreces in deinit can never really work.

Am I missing something here?

回答1:

This statement

[...] that recommend removing an observer from the NotificationCenter in the deinit of the UIViewController [...]

was true in the past.

And your statement

[...] if there still is a strong reference to the class, deinit will not get called.

is correct.

Observers have weak reference

An observer holds a weak reference to the target object.

This explain why the deinit of an object will be called even if there are multiple active observers.

So why do we want to remove the observers in the deinit?

This was needed prior to iOS 9 to prevent an observer from invoking a method of a deallocated object.

However unregistering an observer is no longer needed from macOS 10.11 and iOS 9.0

In OS X 10.11 and iOS 9.0 NSNotificationCenter and NSDistributedNotificationCenter will no longer send notifications to registered observers that may be deallocated.

Source

来源：https://stackoverflow.com/questions/47741859/is-cleaning-up-strong-references-in-deinit-a-correct-pattern