1, private
The private access level modifies properties or methods that can only be accessed in the current class.
2, fileprivate
Fileprivate Access level modifies properties or methods that are accessible in the current Swift source file.
3, internal (default access level, internal modifier can be written or not written)
The internal access level modifies properties or methods that are accessible throughout the module where the source code resides. If it is framework or library code, it is accessible throughout the framework, but not when the framework is referenced by external code. If it’s App code, it’s accessible throughout the App code and within the entire App.
4, public
It can be accessed by anyone. Override and inheritance cannot be applied to other modules, but override and inheritance can be applied to modules.
5, the open
Can be used by anyone, including Override and inheritance.
Access order: The current access permissions are in order: Open, public, internal, Fileprivate, private.