13.1 Understanding Property Wrappers

When values are assigned or accessed via a property within a class or structure instance it is sometimes necessary to perform some form of transformation or validation on that value before it is stored or read. As outlined in the chapter entitled “The Basics of Swift Object-Oriented Programming”, this type of behavior can be implemented through the creation of computed properties. Frequently, patterns emerge where a computed property is common to multiple classes or structures. Prior to the introduction of Swift 5.1, the only way to share the logic of a computed property was to duplicate the code and embed it into each class or structure implementation. Not only is this inefficient, but a change in the behavior of the computation must be manually propagated across all the entities that use it.

To address this shortcoming, Swift 5.1 introduced a feature known as property wrappers. Property wrappers essentially allow the capabilities...