Struct LayoutObservers 

pub struct LayoutObservers<'a> { /* private fields */ }

Two-channel observer the layout engine threads through every render entry point per ADR-0026. warn is required (the engine uses it for segment-render error diagnostics); on_decision is optional and receives a typed LayoutDecision per emit site.

The TUI live preview attaches on_decision to collect per-frame status; the production stdout path doesn’t attach one, so disabled-path cost is one Option::is_none() check per emit site (struct construction is deferred behind Self::emit_with’s closure, so even Cow::Owned user-config ids don’t allocate when no observer is attached).

Single lifetime 'a because Rust unifies independent borrows to their shortest common lifetime — a second lifetime parameter would buy no flexibility for &mut dyn FnMut borrows.

Implementations

impl<'a> LayoutObservers<'a>

pub fn new(warn: &'a mut dyn FnMut(&str)) -> Self

Construct with the required warn channel; no observer attached.

pub fn with_decision( self, on_decision: &'a mut dyn FnMut(&LayoutDecision), ) -> Self

Attach an on_decision callback so the engine routes typed LayoutDecision events through it at every emit site. Calling twice replaces the previous callback (last-write-wins).