If you're on other platforms or if you need to support versions of iOS earlier that 13, you should look at RxSwift. Combine is simply Apple's take on ReactiveX, and Rx is available for Java, JavaScript, and many other languages.

Should also note the the delegate pattern we all know and love was designed for inter-object communication between Objective-C reference types in a time even before closures were a twinkle in a developer's eye.