What are private APIs

Question

What does Apple mean when they refer to private APIs?

Answer 1

A private method is one that is used as an implementation detail rather than a [public] interface detail. In other languages where public and private methods are more enforceable, private methods typically cannot be called from anything other than the class that they are contained within. The purpose of which is to hide implementation details, or to prevent external reliance on implementation details. For example, NSArray probably has a number of private methods that deal with memory allocation and optimised storage for efficient access.

Objective-C does not have truly private methods; you are free to send whatever message you want to any object, and it may respond to it or it may not. At runtime, you are also able to inspect exactly which messages a class (and its instances) will respond to through a series of Objective-C Runtime API calls [that are publicly documented].

Some people attempt to use private methods to obtain program behaviour that is not possible with the publicly documented interface; perhaps as an optimisation, perhaps to do something that the API was never meant to do. It is easily possible because of Objective-C's dynamic nature and lack of true private methods.

As a side note; Apple typically use a leading underscore in method names to denote that it is private. Apple also state that method names beginning with an underscore are reserved for Apple only.