1.分类里面只新增函数，不要新增变量；虽然新增是语法合法的，但是编译器并不会为你的property合成相应的成员变量、setter和getter

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.

2.扩展的方法在.m文件中的@implementation中实现

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.

3.扩展可以看做是匿名的分类，但是扩展中可以新增变量

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.

4.在.h中将属性声明为readonly，但是在.m中的扩展中声明为readwrite，这样内部可读写，外部只能读

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.

5.Objective-C并不像C++那样，一编译一链接就变成机器码，它执行的时候还需要一个Runtime系统来支持

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.