`History` takes its observer by value and never hands it back, so a
caller who wanted to read what an observer recorded had no way to keep a
handle to it. The natural spelling did not compile:
let recorder = Arc::new(Recorder::default());
History::builder().observer(Arc::clone(&recorder))
// error[E0277]: `Arc<Recorder>: Observer<i64>` is not satisfied
The workaround was for every observer to wrap each of its own fields in
an `Arc` and derive `Clone` — one allocation and one lock per field, a
pattern each implementor had to rediscover, and nothing documenting it.
Adds blanket `Observer` impls for `Arc<O>`, `Box<O>` and `&O`. All are
`?Sized`, so `Arc<dyn Observer<T>>` and `Box<dyn Observer<T>>` work too
and an observer can be chosen at runtime. Also adds
`History::observer()` and `into_observer()`, so a non-shared observer's
state can be inspected in place or reclaimed after `converge` without
needing interior mutability at all.
`tests/observer.rs` is simplified to the shared spelling, so the
recommended pattern is the one demonstrated rather than the workaround.
Closes#40
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011hcFjNDmHXZF8URGLku5zZ
logaritmiskandClaude Opus 5
2fff745c3b