Key–Value Observing
Key–value observing, or KVO, is a mechanism somewhat similar to the target–action mechanism, except that it is not limited to controls. (The KVO mechanism is provided through an informal protocol, NSKeyValueObserving, which is actually a set of categories on NSObject and other classes.) The similarity is that objects register with a particular object to be notified when something happens. The “something” is that a certain value in that object is changed.
Mac OS X Programmer Alert
Mac OS X bindings don’t exist on iOS, but you can sometimes use KVO to achieve similar aims.
KVO can be broken down into three stages:
- Registration
- To hear about a change in a value belonging to object A, object B must be registered with object A.
- Change
- The change takes place in the value belonging to object A, and it must take place in a special way — a KVO compliant way.
- Notification
- Object B is automatically notified that the value in object A has changed and can respond as desired.
Here’s a simple complete example — a rather silly example, but sufficient to demonstrate the KVO mechanism in action. We have a class MyClass1; this will be the class of object A. We also have a class MyClass2; this will be the class of object B. Finally, we have code that creates a MyClass1 instance called objectA
and a MyClass2 instance called objectB
; this code registers objectB
to hear about changes in an instance variable of objectA
called value
, and then changes value
, and sure enough, objectB ...
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