关于 swift 中的open ,public ,fileprivate,private, internal的区别 以下按照修饰关键字的访问约束范围 从约束的限定范围大到小的排序进行说明

open,public,fileprivate,private,internal 这几个修饰词的作用是用于修饰访问级别的。

open,public 对应的级别是该模块或者是引用了该模块的模块可以访问 即 a belong to A , B import A 这两种情况都可以对 a进行访问

public： 类用public(或级别更加等级更低的约束(如private等))修饰后只能在本模块（sdk）中被继承，如果public是修饰属性的话也是只能够被这个module(sdk)中的子类重写

open：用open修饰的类可以在本某块(sdk),或者其他引入本模块的(sdk,module)继承，如果是修饰属性的话可以被此模块或引入了此某块(sdk)的模块（sdk）所重写

open 和public 如果是用于继承某个类的话，那么open 和 public 必须是在父类的约束限定之下, 如：不能父类是用public修饰的子类却用open修饰。

、internal 是在模块内部可以访问，在模块外部不可以访问，a belong A , B import A, A 可以访问 a, B 不可以访问a.比如你写了一个sdk。那么这个sdk中有些东西你是不希望外界去访问他，这时候你就需要internal这个关键字（我在导入第三方框架时发现其实没有定义的话sdk里面是默认internal的）

、fileprivate 这个修饰跟名字的含义很像，file private 就是文件之间是private的关系，也就是在同一个source文件中还是可以访问的，但是在其他文件中就不可以访问了  a belong to file A, a not belong to file B , 在 file A 中 可以访问 a，在 file B不可以访问a

、private 这个修饰约束性比fileprivate的约束性更大，private 作用于某个类，也就是说，对于 class A ,如果属性a是private的，那么除了A外其他地方都不能访问了(fileprivate 和private都是一种对某个类的限制性约束。fileprivate的适用场景可以是某个文件下的extension(由于Extention不能存储property所以，这里的fileprivate是针对func有效的)，如果你的类中的变量定义成了private那么这个变量在你这个类在这个类的文件的拓展中就无法访问了，这时就需要定义为fileprivate)

 最后是 Guiding Principle of Access Levels （访问级别的推导原则）：不能在低级别的修饰中定义比自身更高的级别修饰,如public不能修饰在private类中的属性

首先说明一下这里面的区别在文章结尾英文或链接中可以看得到

https://developer.apple.com/library/content/documentation/Swift/Conceptual/Swift_Programming_Language/AccessControl.html#//apple_ref/doc/uid/TP40014097-CH41-ID3

Swift provides five different access levels for entities within your code. These access levels are relative to the source file in which an entity is defined, and also relative to the module that source file belongs to.

• Open access and public access enable entities to be used within any source file from their defining module, and also in a source file from another module that imports the defining module. You typically use open or public access when specifying the public interface to a framework. The difference between open and public access is described below.

• Internal access enables entities to be used within any source file from their defining module, but not in any source file outside of that module. You typically use internal access when defining an app’s or a framework’s internal structure.

• File-private access restricts the use of an entity to its own defining source file. Use file-private access to hide the implementation details of a specific piece of functionality when those details are used within an entire file.

• Private access restricts the use of an entity to the enclosing declaration. Use private access to hide the implementation details of a specific piece of functionality when those details are used only within a single declaration.

Open access is the highest (least restrictive) access level and private access is the lowest (most restrictive) access level.

Open access applies only to classes and class members, and it differs from public access as follows:(public 和 open 的区别)

• Classes with public access, or any more restrictive access level, can be subclassed only within the module where they’re defined.

• Class members with public access, or any more restrictive access level, can be overridden by subclasses only within the module where they’re defined.

• Open classes can be subclassed within the module where they’re defined, and within any module that imports the module where they’re defined.

• Open class members can be overridden by subclasses within the module where they’re defined, and within any module that imports the module where they’re defined.

Marking a class as open explicitly indicates that you’ve considered the impact of code from other modules using that class as a superclass, and that you’ve designed your class’s code accordingly.

Guiding Principle of Access Levels

Access levels in Swift follow an overall guiding principle: No entity can be defined in terms of another entity that has a lower (more restrictive) access level.

For example:

• A public variable cannot be defined as having an internal, file-private, or private type, because the type might not be available everywhere that the public variable is used.

• A function cannot have a higher access level than its parameter types and return type, because the function could be used in situations where its constituent types are not available to the surrounding code.

The specific implications of this guiding principle for different aspects of the language are covered in detail below.