How to Define a Protocol With @Published Property Wrapper Type

A simple and elegant workaround

This article is originally published at https://swiftsenpai.com on May 17, 2020.

The Combine framework was introduced in WWDC 2019 and it is mainly used alongside SwiftUI. However, this does not limit us to use the Combine framework on our UIKit apps.

In fact, the @Published property wrapper introduced in Combine is the perfect fit for any UIKit apps with MVVM architecture. We can use @Published to elegantly link up the view controller with its view model so that the view controller can be notified automatically by any changes made on the view model.

Everything works nicely until the day where I wanted to define a protocol for my view model in order to achieve polymorphism using protocol-oriented programming. The problem arises because the current Swift version (5.2) does not support property wrapper definition in a protocol.

How should we go about defining @Published property wrapper type in a protocol? Read on to find out more.

The Problem

Let’s say we have a view controller and a view model with a @Published variable called name. Whenever name is set or updated, the name publisher will notify MyViewController to print out a…