Subclass, Category and Extensions in Objective C

Hi guys, hope you find the articles i have posted before, interesting and useful. I always welcome your feedback to make the future articles more interesting.Today lets see what is subclassing, categories and extensions in Objective C, and where, when and how to use these concepts.

1) Subclass in Objective C

Every object you create in your Cocoa application descends from the 'NSObject' foundation class. The NSObject class identifies properties and methods which apply to all objects. The NSObject class is divided into smaller groups of objects, called subclasses. Objects in these subclasses not only conform to the protocol of NSObject, they are also defined more precisely by the methods that govern their subclass. Every object class inherits from the superclasses above it in the object hierarchy, and also declares the methods which make it a unique class.

Subclassing in simple words is changing the behaviour of properties or methods of an existing class or in other words subclassing is inheriting a class and modifying the methods or properties of super class however you want. Additionally subclassing lets you add state. when subclassing, (sometimes you intently need to override the existing behavior/methods or you can add extra functionality too.) you explicitly declare that Object with the type like

MyCustomString *string;

and then all the methods written in your subclass become visible.

2) Categories in Objective C

An Objective C category provide the ability to add functionality to an object without subclassing or changing the actual object. Categories let  you expand the API of existing classes without changing their type such as NSString or your own custom objects & allows you add your own methods to an existing class. Categories are also called as "informal protocols".

Suppose take an example, since Foundation Framework classes such as NSString, NSArray, NSDate etc… doesn’t have any access to modify, you can add your own methods in to these classes by the help of a category.

Consider NSString Class and if suppose we want to add a reverse string method to NSString class, so that in our application at any point of time any NSString object can call this category method and get a reversed string as a result. We can do this as below,

 Usually naming convention for category file is like OriginalClassName+CategoryName 

Ex:  NSString+ReverseNSString                                                                             

Note: in a category you can’t add an instance variable, since methods within a category are added to a class at runtime.

3) Extensions in Objective C

A class extension is used to declare additional interface -- methods and properties -- whose implementation contract will be met within the class's primary @implementaiton.

The docs state:

Class extensions are like anonymous categories, except that the methods they declare must be implemented in the main @implementation block for the corresponding class.

Extensions are similar to categories but the need of extension is different. 

• Class extensions are often used to extend the public interface with additional private methods or properties for use within the implementation of the class. 
• Extensions can only be added to a class for which you have the source code at compile time (the class is compiled at the same time as the class extension).
• Extensions will be local to a class file.

The syntax to declare class extension looks like,

@interface ClassName()                                                                                                

@end

since no name is given in the parentheses, class extensions are often referred to as anonymous categories. Usually people will use extensions to hide private information of a class without exposing them to access from any other class.

Note: Extensions can add instance variables.

Finally the simple point to remember is , Subclassing is better option if you want to customize an existing stuffs or functionalities, and Category is a best option if you want to add additional functionalities to an existing class