Registering for Key-Value Observing

In order to receive key-value observing notifications for a property, three things are required:

• The observed class must be key-value observing compliant for the property that you wish to observe.
• You must register the observing object with the observed object, using the method addObserver:forKeyPath:options:context:.
• The observing class must implement observeValueForKeyPath:ofObject:change:context:.

Important: Not all classes are KVO-compliant for all properties. You can ensure your own classes are KVO-compliant by following the steps described in KVO Compliance. Typically properties in Apple-supplied frameworks are only KVO-compliant if they are documented as such.

Key-Value Observing

• Наблюдатель (шаблон проектирования) — Википедия

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• Geoff’s Primer On iOS Observers
• Using Observers on iOS | Thoughts from Geoff Kratz @ FarWest Software
• iOS Patterns. Observer | Димин Блог
• Cocoa with Love: Five approaches to listening, observing and notifying in Cocoa.

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• Key Value Observing (KVO) - Tutorial — sonducngo.com
• Key-Value Observing - NSHipster
• NSKeyValueObserving Protocol Reference
• KVO – Observing any change in Object state | Thought Repository

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• Key-Value Observing for the mainstream | Damien DeVille
• A note about KVO dependent keys | Damien DeVille
• Key-Value Coding and Key-Value Observing: A Basic Tutorial.

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• Преодолеваем скрытые опасности KVO в Objective C / Блог компании EastBanc Technologies / Хабрахабр

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• Key Value Observation in Swift | Venkat Peri