Objective-C Categories - raywenderlich.com

Categories allow you to add behavior to your classes even though you may not have access to the original source. In this video, you'll be learning all about them.

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About Categories (from Apple)

https://developer.apple.com/library/content/releasenotes/ObjectiveC/RN-TransitioningToARC/Introduction/Introduction.html

Categories Add Methods to Existing Classes

If you need to add a method to an existing class, perhaps to add functionality to make it easier to do something in your own application, the easiest way is to use a category.

The syntax to declare a category uses the @interface keyword, just like a standard Objective-C class description, but does not indicate any inheritance from a subclass. Instead, it specifies the name of the category in parentheses.

A category can be declared for any class, even if you don’t have the original implementation source code (such as for standard Cocoa or Cocoa Touch classes). Any methods that you declare in a category will be available to all instances of the original class, as well as any subclasses of the original class. At runtime, there’s no difference between a method added by a category and one that is implemented by the original class.

Consider the XYZPerson class from the previous chapters, which has properties for a person’s first and last name. If you’re writing a record-keeping application, you may find that you frequently need to display a list of people by last name.

As well as just adding methods to existing classes, you can also use categories to split the implementation of a complex class across multiple source code files. You might, for example, put the drawing code for a custom user interface element in a separate file to the rest of the implementation if the geometrical calculations, colors, and gradients, etc, are particularly complicated. Alternatively, you could provide different implementations for the category methods, depending on whether you were writing an app for OS X or iOS.

Categories can be used to declare either instance methods or class methods but are not usually suitable for declaring additional properties. It’s valid syntax to include a property declaration in a category interface, but it’s not possible to declare an additional instance variable in a category. This means the compiler won’t synthesize any instance variable, nor will it synthesize any property accessor methods. You can write your own accessor methods in the category implementation, but you won’t be able to keep track of a value for that property unless it’s already stored by the original class.

Avoid Category Method Name Clashes
Because the methods declared in a category are added to an existing class, you need to be very careful about method names.

If the name of a method declared in a category is the same as a method in the original class, or a method in another category on the same class (or even a superclass), the behavior is undefined as to which method implementation is used at runtime. This is less likely to be an issue if you’re using categories with your own classes, but can cause problems when using categories to add methods to standard Cocoa or Cocoa Touch classes.

An application that works with a remote web service, for example, might need an easy way to encode a string of characters using Base64 encoding. It would make sense to define a category on NSString to add an instance method to return a Base64-encoded version of a string, so you might add a convenience method called base64EncodedString.

A problem arises if you link to another framework that also happens to define its own category on NSString, including its own method called base64EncodedString. At runtime, only one of the method implementations will “win” and be added to NSString, but which one is undefined.

About Objective C (from Wikipedia)

Objective-C is a general-purpose, object-oriented programming language that adds Smalltalk-style messaging to the C programming language. It was the main programming language used by Apple for the OS X and iOS operating systems, and their respective application programming interfaces (APIs) Cocoa and Cocoa Touch prior to the introduction of Swift.

The programming language Objective-C was originally developed in the early 1980s. It was selected as the main language used by NeXT for its NeXTSTEP operating system, from which OS X and iOS are derived.