The names of the properties upon which the value of the property identified by dependentKey depends.
dependentKey
The name of a property whose value depends on the properties specified by keys.
Discussion
解説
Invocations of will- and did-change KVO notification methods for any key in keys automatically invokes the corresponding change notification methods for dependentKey. The observed object does not receive willChange or didChange messages to generate the notifications.
Dependencies should be registered before any instances of the receiving class are created, so you typically invoke this method in a class's initialize method, as illustrated in the following example.