OpenCombine VS PromiseKit

The OpenCombine implementation of PassthroughSubject does use a lock, and perhaps the objects needed to be a class to make sure that lock didn't get moved/duplicated.

I have three books on the topic but don't think any of them are that wonderful. One thing that helped a little bit was reading through the OpenCombine source.

Here is an open-source implementation of the @Published property wrapper, it could put you on the right track for getting the same functionality with your @UserDefault wrapper: https://github.com/OpenCombine/OpenCombine/blob/master/Sources/OpenCombine/Published.swift

Do you use PromiseKit?

I use PromiseKit to handle async codes. It's really easy to use for basic uses like this

The most typical way to solve this problem is by using Combine's Future or PromiseKit's Promise, or by using RxSwift's flatMap. If you're interested on how this works under the hood, I plan to write another article for this case, but for now you can check my code for a simple Promise in this gist, or check this quite old but incredibly mind blowing talk from Javi Soto.

Besides that, you can use a promise library, like PromiseKit.