Function set_close_handler 

pub fn set_close_handler(
    prefix: &str,
    handler: impl Fn(&str, CloseReason) + Send + Sync + 'static,
)

Call handler with the tag and reason of every notification whose tag starts with prefix that the platform closes, on whatever thread reports it. Replaces an earlier handler for prefix; where prefixes nest, the longest one wins.

Only some platforms report this (Feature::Close), and only for notifications they closed themselves: a notification the app withdrew is the app’s own doing and is not reported. A dismissal (CloseReason::Dismissed) is the person’s answer; CloseReason::Expired and CloseReason::Closed say nothing about them.