KVO (Key Value Observing) in Swift

Key-value observing is a mechanism that enables an object to be notified directly when a property of another object changes.

It is a mode of communication between objects in applications designed in conformance with the Model-View-Controller design pattern.

Controller can Observe Model and View can Observe Controller and Model. So, change in model object property can be handled gracefully using KVO.

Key-Value Observing requires your Object to conform to NSKeyValueObserving protocol. Any Class which inherits NSObject automatically implements NSKeyValueObserving protocol as NSObject conforms to NSKeyValueObserving.

Note: NSKeyValueObserving APIs are not the cleanest to use(in terms of syntax and arguments). But having said that it’s powerful and developer should use it.

NSKeyValueObserving: An informal protocol that objects adopt to be notified of changes to the specified properties of other objects.

How to register and observe another object property?

Registering for Observation:

Observation

Registers the observer object to receive KVO notifications for the key path relative to the object receiving this message.

func addObserver(_ observer: NSObject, 
      forKeyPath keyPath: String, 
         options: NSKeyValueObservingOptions = [], 
         context: UnsafeMutableRawPointer?)

Registers the observer object to receive KVO notifications for the key path relative to the object receiving this message.

Observe Notification method:

func observeValue(forKeyPath keyPath: String?, 
               of object: Any?, 
           change: [NSKeyValueChangeKey : Any]?, 
          context: UnsafeMutableRawPointer?)

Removing an Observer:

Another major issue when working with Key-Value Observing is memory management. Observers need to be explicitly removed when they are no longer interested in receiving notifications for a particular key path. You have two options.

deinit {
    removeObserver(self, forKeyPath: #keyPath(property))
}

It’s very simple to understand and use, still developers don’t use it frequently.

Hope you understood this pattern and will use it in your project as well.