@private
The instance variable is accessible only within the class that declares it.
实例变量只能被声明它的类访问
@protected
The instance variable is accessible within the class that declares it and within classes that inherit it. All instance variables without an explicit scope directive have @protected scope.
实例变量能被声明它的类和子类访问，所有没有显式制定范围的实例变量都是
@public
The instance variable is accessible everywhere.
实例变量可以被在任何地方访问。
@package
Using the modern runtime, an @package instance variable has @public scope inside the executable image that implements the class, but acts like @private outside.使用modern运行时，一个@package实例变量在实现这个类的可执行文件镜像中实际上是@public的，但是在外面就是@private[runtime需要再看一下苹果文档Runtime Programming Guide]
The @package scope for Objective-C instance variables is analogous to private_extern for C variables and functions. Any code outside the class implementation’s image that tries to use the instance variable gets a link error.
Objective-C中的@package与C语言中变量和函数的private_extern类似。任何在实现类的镜像之外的代码想使用这个实例变量都会引发link error.
This scope is most useful for instance variables in framework classes, where @private may be too restrictive but @protected or @public too permissive.
这个类型最常用于框架类的实例变量，使用 @private 太限制，使用 @protected 或者 @public 又太开放.
