今天講Objective-C的@interface，講Objective-C的繼承的文法，使用冒号:來表示繼承，相當于java的extends。
The Interface for a Class Defines Expected Interactions
One of the many benefits of object-oriented programming is the idea mentioned earlier—all you need to know in order to use a class is how to interact with its instances. More specifically, an object should be designed to hide the details of its internal implementation.
If you use a standard UIButton in an iOS app, for example, you don’t need to worry about how pixels are manipulated so that the button appears on screen. All you need to know is that you can change certain attributes, such as the button’s title and color, and trust that when you add it to your visual interface, it will be displayed correctly and behave in the way you expect.
When you’re defining your own class, you need to start by figuring out these public attributes and behaviors. What attributes do you want to be accessible publicly? Should you allow those attributes to be changed? How do other objects communicate with instances of your class?
This information goes into the interface for your class—it defines the way you intend other objects to interact with instances of your class. The public interface is described separately from the internal behavior of your class, which makes up the class implementation. In Objective-C, the interface and implementation are usually placed in separate files so that you only need to make the interface public.
Basic Syntax
The Objective-C syntax used to declare a class interface looks like this:
@interface SimpleClass : NSObject
@end
This example declares a class named SimpleClass, which inherits from NSObject.
The public properties and behavior are defined inside the @interface declaration. In this example, nothing is specified beyond the superclass, so the only functionality expected to be available on instances of SimpleClass is the functionality inherited from NSObject.