Scoping

Also called “visibility” or “access level”, it’s important to use the proper scope to ensure your code is used properly, both by others and by Future You.

Prefer more-private scopes first, and only make something more public if necessary.

In case it’s unclear, here are the visibility scopes as of Swift 5:

• private: Only the current context can see this
  • This is our preferred default scope
  • Top-level in file: Only visible in this file; synonymous with fileprivate
  • In class, struct, enum, extension, etc.: Only visible in this type, and only in this file. When used in a nested type, it’s only visible in the nested one, not its parent.
• fileprivate: Only (and all) things in this file can see this
  • Top-level in file: Avoid this; synonymous with private
  • In class, struct, enum, extension, etc.: This is okay; use this for utilities only needed in this file
• internal: Only (and all) things in this Swift module can see this
  • You may use this liberally or judicously, to your preference
• public: Everything can see this
  • This is okay, but only if necessary.
• open: By itself, this is public, but subclassable/overridable.
  • If subclassing/overriding is the goal, great. Otherwise, avoid this, or at least pair it with another scope to shield it from the public.
• unspecified
  • Assume surrounding context’s visibility. This is OK.
  • If top-level (no surrounding context), synonymous with internal. Avoid this.