Objects and classes

A class consists primarily of two things: variables that can store data and methods

that can perform operations.

Subclasses

Classes can also inherit functionality from an existing class

Instance and class variables

Changing the value of an instance variable in one object

will not affect the value of the same variable in all the other objects of that class.

Conversely, only a single copy of a class variable exists. If you change the value of

a class variable from one object, the value of that variable will change for all the

objects of that class

Accessor methods

Accessor methods, sometimes called getters and setters, are usually fairly simple

methods used to get and set instance variables in a class

Class methods

Class methods (also known as static methods) are  methods that are associated directly

with the class rather than the object instance

Events and messages

An event is a message generated by the user interacting with your application’s

controls. For instance, if you tap the screen of your iOS device, this generates a UI

event in your application that is passed via a message from the application to an

object that has been delegated to deal with that specific type of event

Protocols

A protocol definition declares methods that any class can implement. If your class

declares that it abides by a particular protocol definition, you are announcing that

you have implemented the minimum mandatory methods declared in the protocol

definition, and may optionally have implemented some nonmandatory methods

Delegate classes

A delegate class is a class that implements a protocol for handling events.

Event loop

The main event loop is the principal control loop for your application.

Reference Counting

Reference counting is a form of garbage collection, which counts the number of references

to an object (or portion of memory) and frees the associated memory when the

number of references reaches zero. The main advantage of reference counting over

"classic" garbage collection is that memory is freed as soon as it is no longer in use