iOSProgrammingGuidelines

Swift

Type vs instance methods

If a method is independent of instance state it should be declared as type method, preferably by using static keyword (or class in case when the method is designed to be overridden in a subclass).

This is to avoid methods giving a false impression that they are dependent on the instance state while in fact they are not.

Controlling subclassing behavior

Subclassing behavior should be designed with maximal hermetization principle in mind - choosing most restrictive access level possible.

Declare class final unless you explicitly design it to be inheritable. For non final class declare all instance members as final and type members as static except for those you explicitly design for overriding.

When designing a module prefer public for non final class unless you explicitly design it to be inheritable by external clients. For open class prefer public for members except for those you explicitly design for overriding by external clients.

Every point that can be overridden increases complexity, thus it should always be a conscious choice to add it.