To enforce the ability of an object to hide its data, the compiler limits the scope of instance variables—that is, limits their visibility within the program.
为了强制一个对象隐藏其数据，编译器限制实例变量范围以限制其在程序中的可见性
But to provide flexibility, it also lets you explicitly set the scope at four levels. Each level is marked by a compiler directive:
但是为了提供灵活性，苹果也让开发者显式设置范围（四选一）
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Directive |
Meaning |
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@private |
The instance variable is accessible only within the class that declares it. |
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@protected |
实例变量只能被声明它的类访问 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@protected
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@public |
The instance variable is accessible everywhere. 实例变量可以被在任何地方访问。
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@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 |