April 2019 Posts

Object oriented programming is great, but sometimes things don’t fit neatly into a superclass/subclass hierarchy. You may have a piece of code that would be needed in several contexts, but for technical reasons beyond your control you cannot merge them into a single hierarchy. Some languages have the concept of multiple inheritence, where a subclass can specifically inherit from several parents. But this has it’s own set of problems. Many other languages, however, solve this through the use of traits or mixins. These allow us to have a set of methods that are basically copied into the object at compile time. This way they can be used anywhere they are needed. Swift doesn’t have the concept of mixins or traits per se. But, starting with Swift 3, you can get very equivalent functionality using protocol default implementations.

Somewhat related to my previous post about responder chains, sometimes it is useful to be able to debug what all is in the responder chain at any given time. As a good rule of thumb, all ancestor views of a view are in that view’s responder chain, as well as (usually) the related controllers.

The responder chain is one of those parts of macOS and iOS development that may seem a little strange if you have not done any GUI programming before. Briefly, a responder chain is a hierarichy of objects that can respond to events. So, for example, a click or a tap might be passed up the responder chain until something responds to the action. But, the responder chain is more than just UI events. We can pass our own custom events up the responder chain as well!