Nested struct with static constants inside of a class is a neat way of managing magic string constants, like identifiers, user defaults or other dictionary keys, etc…

keys.swift

class Foo {
  struct Keys {
    static let keychainGroup = "com.foo.keychain-group"
    static let enabled = "foo.enabled"
  }
  
  // now you can just write `Keys.enabled`, `Keys.keychainGroup` inside of the Foo class.
}