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 Objective-C Runtime (from Apple)
The Objective-C language defers as many decisions as it can from compile time and link time to runtime. Whenever possible, it does things dynamically. This means that the language requires not just a compiler, but also a runtime system to execute the compiled code. The runtime system acts as a kind of operating system for the Objective-C language; it’s what makes the language work.
This document looks at the NSObject class and how Objective-C programs interact with the runtime system. In particular, it examines the paradigms for dynamically loading new classes at runtime, and forwarding messages to other objects. It also provides information about how you can find information about objects while your program is running.
You should read this document to gain an understanding of how the Objective-C runtime system works and how you can take advantage of it. Typically, though, there should be little reason for you to need to know and understand this material to write a Cocoa application.
Objective-C programs interact with the runtime system at three distinct levels: through Objective-C source code; through methods defined in the NSObject class of the Foundation framework; and through direct calls to runtime functions.
Objective-C Source Code
For the most part, the runtime system works automatically and behind the scenes. You use it just by writing and compiling Objective-C source code.
When you compile code containing Objective-C classes and methods, the compiler creates the data structures and function calls that implement the dynamic characteristics of the language. The data structures capture information found in class and category definitions and in protocol declarations; they include the class and protocol objects discussed in Defining a Class and Protocols in The Objective-C Programming Language, as well as method selectors, instance variable templates, and other information distilled from source code. The principal runtime function is the one that sends messages, as described in Messaging. It’s invoked by source-code message expressions.
Most objects in Cocoa are subclasses of the NSObject class, so most objects inherit the methods it defines. (The notable exception is the NSProxy class; see Message Forwarding for more information.) Its methods therefore establish behaviors that are inherent to every instance and every class object. However, in a few cases, the NSObject class merely defines a template for how something should be done; it doesn’t provide all the necessary code itself.
For example, the NSObject class defines a description instance method that returns a string describing the contents of the class. This is primarily used for debugging—the GDB print-object command prints the string returned from this method. NSObject’s implementation of this method doesn’t know what the class contains, so it returns a string with the name and address of the object. Subclasses of NSObject can implement this method to return more details. For example, the Foundation class NSArray returns a list of descriptions of the objects it contains.
Some of the NSObject methods simply query the runtime system for information. These methods allow objects to perform introspection.
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.