今天講Objective-C定義類，Defining Classes。Objective-C和其它OOM語言一樣，你的app将建構在大量的互相關聯的類的基礎上。對于OS X和iOS開發，也提供了大量的類庫，如OSX的Cocoa庫，iOS的Cocoa Touch庫。
When you write software for OS X or iOS, most of your time is spent working with objects. Objects in Objective-C are just like objects in other object-oriented programming languages: they package data with related behavior.
An app is built as a large ecosystem of interconnected objects that communicate with each other to solve specific problems, such as displaying a visual interface, responding to user input, or storing information. For OS X or iOS development, you don’t need to create objects from scratch to solve every conceivable problem; instead you have a large library of existing objects available for your use, provided by Cocoa (for OS X) and Cocoa Touch (for iOS).
Some of these objects are immediately usable, such as basic data types like strings and numbers, or user interface elements like buttons and table views. Some are designed for you to customize with your own code to behave in the way you require. The app development process involves deciding how best to customize and combine the objects provided by the underlying frameworks with your own objects to give your app its unique set of features and functionality.
In object-oriented programming terms, an object is an instance of a class. This chapter demonstrates how to define classes in Objective-C by declaring an interface, which describes the way you intend the class and its instances to be used. This interface includes the list of messages that the class can receive, so you also need to provide the class implementation, which contains the code to be executed in response to each message.
另外，講一下類和執行個體的關系，這個和其它語言定義是一樣的。具體如下：
Classes Are Blueprints for Objects
A class describes the behavior and properties common to any particular type of object. For a string object (in Objective-C, this is an instance of the class NSString), the class offers various ways to examine and convert the internal characters that it represents. Similarly, the class used to describe a number object (NSNumber) offers functionality around an internal numeric value, such as converting that value to a different numeric type.
In the same way that multiple buildings constructed from the same blueprint are identical in structure, every instance of a class shares the same properties and behavior as all other instances of that class. Every NSString instance behaves in the same way, regardless of the internal string of characters it holds.
Any particular object is designed to be used in specific ways. You might know that a string object represents some string of characters, but you don’t need to know the exact internal mechanisms used to store those characters. You don’t know anything about the internal behavior used by the object itself to work directly with its characters, but you do need to know how you are expected to interact with the object, perhaps to ask it for specific characters or request a new object in which all the original characters are converted to uppercase.
In Objective-C, the class interface specifies exactly how a given type of object is intended to be used by other objects. In other words, it defines the public interface between instances of the class and the outside world.