Using Enums as Namespaces

A few code-bases I’ve come across have used instance properties to hold constants, as follows.

class LoginViewController: UIViewController {
    private let headerSpace: CGFloat = 24
}

This is convenient since the value can be referenced within the class without any additional prefix or scoping.

However this approach is problematic because it suggests that the value varies between instances of the type when in fact it does not. A static property better represents the real semantics of the situation.

class LoginViewController: UIViewController {
    private static let headerSpace: CGFloat = 24
}

This can then be referenced by qualifying the identifier with the classname or Self, e.g. LoginViewController.headerSpace or Self.headerSpace.

A nice alternative is to embed the property within a case-less enum, as follows:

class LoginViewController: UIViewController {
    private enum LayoutMetric {
        static let headerSpace: CGFloat = 24
    }
}

Then the property can be referenced internally as LayoutMetric.headerSpace. With this convention we can group semantically related constants and the qualified name tells us what it relates to.

Using an enum in this way is similar to the namespace concept in C++ which provides a first class way of collecting together type names.

In Swift an enum with no cases is known as an uninhabited type. It is a type that cannot be instantiated. As a result there’s no way a sleep deprived developer could start using LayoutMetric variables by mistake.