Object Properties and KVC

Central to key-value coding (or KVC) is the general notion of properties. A property refers to a unit of state that an object encapsulates. A property can be one of two general types: an attribute (for example, name, title, subtotal, or textColor) or a relationship to other objects. Relationships can be either to-one or to-many. The value for a to-many relationship is typically an array or set, depending on whether the relationship is ordered or unordered.

KVC locates an object’s property through a key, which is a string identifier. A key usually corresponds to the name of an accessor method or instance variable defined by the object. The key must conform to certain conventions: It must be ASCII encoded, begin with a lowercase letter, and have no whitespace. A key path is a string of dot-separated keys that is used to specify a sequence of object properties to traverse. The property of the first key in the sequence is relative to a specific object (employee1 in the following diagram), and each subsequent key is evaluated relative to the value of the previous property.

Making a Class KVC Compliant

The NSKeyValueCoding informal protocol makes KVC possible. Two of its methods—valueForKey: and setValue:forKey:—are particularly important because they fetch and set a property’s value when given its key. NSObject provides a default implementation of these methods, and if a class is compliant with key-value coding, it can rely on this implementation.

How you make a property KVC compliant depends on whether that property is an attribute, a to-one relationship, or a to-many relationship. For attributes and to-one relationships, a class must implement at least one of the following in the given order of preference (key refers to the property key):

1. The class has a declared property with the name key.

2. It implements accessor methods named key and, if the property is mutable, setKey:. (If the property is a Boolean attribute, the getter accessor method has the form isKey.)

3. It declares an instance variable of the form key or _key.

Implementing KVC compliance for a to-many relationship is a more complicated procedure. Refer to the document that definitively describes key-value coding to learn what this procedure is.