Varieties of @interface declarations, some with parentheses

I've noticed a variety of @interface declarations for Objective-c classes. I'd like to understand why developers declare @interface in the following ways:

// in the .h file
@interface MyClass : NSObject
// ...
@end

// in the .m file (what's the purpose of the parens?)
@interface MyClass ()
// more property declarations which seem like they can go in the .h file
@end

// again in the .m file (what's the purpose of private?)
@in开发者_运维百科terface MyClass (Private)
// some method declarations
@end

This is just a normal class interface, inheriting from NSObject, where you declare ivars, properties and methods

// in the .h file
@interface MyClass : NSObject
// ...
@end

The following two are categories, which allow you to add methods to a class. It is not a subclass however (do not declare a method with the same name, as you won't be able to access the original one). If you have a named category of the interface (like @interface MyClass (Private)), then the implementation should be provided in @implementation MyClass (Private), in the case of unnamed categories (also called extensions), the implementation can be provided as usual. Note that extensions also allow you to add ivars to the class while (named) categories do not.

// in the .m file (what's the purpose of the parens?)
@interface MyClass ()
// more property declarations which seem like they can go in the .h file
@end

// again in the .m file (what's the purpose of private?)
@interface MyClass (Private)
// some method declarations
@end

It is used to declared private methods.

This response explain this in details: What are best practices that you use when writing Objective-C and Cocoa?

What ever goes in the .m file is private. the parens are for categories so you can segment your code into categories to make it more readable. because the code is in .m and private, they called the category Private.