UE4 Live Coding for Unit Testing

TL;DR: Use the new Live Coding feature in UE4.22 as a workaround for the fact that automation unit tests cannot be Hot Reloaded.

My goal for the day was to write a high score system for my summer project, which I am creating in Unreal Engine 4.22. As I started thinking about the requirements, I realized that TDD would be a good approach. Of course, I've spent more hours diving into how to make this work than I would have spent on a brute force approach, but I'm hoping I'll recoup the investment in the future.

The Automation Technical Guide provided enough scaffolding for me to write a single unit test. I took some time then to pull this out into its own module—this seemed like a good idea, although developing with multiple modules is something else I've also never done. Orfeas Eleftheriou's blog post was instrumental in my pulling my unit tests into their own module.

I remembered reading months ago that unit tests in UE4 are not Hot Reloaded, and this is confirmed as expected behavior in UE-25350. The frustrating fallout of this is that the editor has to be reloaded in order to see changes made in the C++ test implementation. This slow and tedious process is anathema to good TDD rhythm. I came across Vhite Rabbit's workaround, but I could not get it to jive with the modular decomposition I gleaned from Eleftheriou.

Then I remembered that 4.22 shipped with an experimental new Live Coding feature, which promises to allow changes from C++ to be brought into a running game session, as long as they are not structural changes. I wondered, if this works for running games, would it work for unit tests?

The happy answer here is Yes. I had to restart the editor because Live Coding cannot be used after any Hot Reloading. The editor opens with a separate window that seems to be managing the Live Coding feature. I went into my unit test and turned its simple "return false" into "return true", hit the magic Ctrl-Alt-F11 combo, waited just a few seconds for the Live Coding system to run, and then re-ran my test. Sure enough, now the test passes.

I have essentially no progress on a high score feature after the morning's work, but hopefully by documenting my findings here, I can help others move forward in their unit testing adventures.