《Programming with Objective-C》第五章 Customizing Existing Classes

Categories can be used to declare either instance methods or class methods but are not usually suitable for declaring additional properties. It’s valid syntax to include a property declaration in a category interface, but it’s not possible to declare an additional instance variable in a category. This means the compiler won’t synthesize any instance variable, nor will it synthesize any property accessor methods. You can write your own accessor methods in the category implementation, but you won’t be able to keep track of a value for that property unless it’s already stored by the original class.

The methods declared by a class extension are implemented in the

@implementation

block for the original class so you can’t, for example, declare a class extension on a framework class, such as a Cocoa or Cocoa Touch class like

NSString

.

class extensions are often referred to as anonymous categories.

Unlike regular categories, a class extension can add its own properties and instance variables to a class.

It’s common, for example, to define a property as

readonly

in the interface, but as

readwrite

in a class extension declared above the implementation, in order that the internal methods of the class can change the property value directly.

Objective-C is more than just a language that is compiled down to machine code. Instead, it requires a runtime system in place to execute that code.