Record it:

The overview of iOS you need to fast read:

https://developer.apple.com/library/ios/#referencelibrary/GettingStarted/RoadMapiOS/chapters/Introduction.html

First Step:

Object-C

https://developer.apple.com/library/ios/#documentation/Cocoa/Conceptual/ObjectiveC/Introduction/introObjectiveC.html#//apple_ref/doc/uid/TP30001163

https://developer.apple.com/library/ios/#referencelibrary/GettingStarted/RoadMapiOS/chapters/WriteObjective-CCode/WriteObjective-CCode/WriteObjective-CCode.html

Someting new for Object-C

A protocol declares methods that can be implemented by any class, even if those classes implementing the protocol don’t have a common superclass.

From a practical perspective, a protocol defines a list of methods that establishes a contract between objects without requiring them to be instances of any specific class. This contract enables communication between those objects. One object wants to tell another object about the events it’s encountering, or perhaps it wants to ask for advice about those events.

A category is a feature of the Objective-C language that enables you to extend the interface of a class without having to subclass it. You can send a message to any instance of the class (or its subclasses) to invoke a method defined in the category.

When you want to call a method, you do so by sending a message to the object that implements the method. (Although the phrase "sending a message” is commonly used as a synonym for “calling a method,” the Objective-C runtime does the actual sending.) A message is the method name along with the parameter information the method needs (properly conforming to type). All messages you send to an object are dispatched dynamically, thus facilitating the polymorphic behavior of Objective-C classes. (Polymorphism refers to the ability of different types of objects to respond to the same message.)

https://developer.apple.com/library/ios/#referencelibrary/GettingStarted/RoadMapiOS/chapters/AcquireBasicProgrammingSkills/AcquireBasicSkills/AcquireBasicSkills.html

Cocoa:

https://developer.apple.com/library/ios/#documentation/Miscellaneous/Conceptual/iPhoneOSTechOverview/PortingfromCocoa/PortingfromCocoa.html#//apple_ref/doc/uid/TP40007898-CH8