How can I implement the observer pattern in Rust?

The Observer pattern, depending on implementation choices, may pose an ownership challenge.

In garbage collected languages it is typical to have the Observable referring to the Observer (to notify it) and the Observer referring to the Observable (to unregister itself)… this causes some challenges in terms of ownership (who outlives whom?) and there is this whole “notification on un-registering” thing.

In Rust (and C++), I advise avoiding cycles.

Simple solution

The Observable and Observer have distinct lifetimes, none owning the other or being expected to outlive the other.

use std::rc::Weak;

struct Event;

trait Observable {
    fn register(&mut self, observer: Weak<dyn Observer>);
}

trait Observer {
    fn notify(&self, event: &Event);
}

The key is to allocate the Observer into a Rc and then hand over Weak (weak references) to the Observable.

If the Observer needs be modified on the Event, then either it needs internal mutability or it needs to be wrapped into a RefCell (passing Weak<RefCell<dyn Observer>> to the Observable).

When notifying, the Observable will regularly realize that there are dead weak-references (the Observer has disappeared), it can remove those then, lazily.

There are other solutions, such as using a Broker (quite similar to an event loop), moving from push mode to pull mode (i.e. (1) generate all events, (2) treat all of them), however these depart a bit from the traditional Observer Pattern and have different pluses/minuses so I will not attempt to treat them all here.