listen_it

Releases

1. ▸▾6.0.019 Sept 2026

   Operator chains (map, select, where, debounce, async, combineLatest*, mergeWith and Listenable.debounce) now detach from their source when their last

   Breaking behavior change: chains detach when unobserved

   Operator chains (map, select, where, debounce, async, combineLatest*, mergeWith and Listenable.debounce) now detach from their source when their last listener is removed and re-attach on the next listener.

   • Chains still subscribe eagerly on creation (or on the first listener with lazy: true) - nothing changes for chains that never had a listener or that always have at least one.
   • Once the last listener is removed, the chain removes its handler from the source. In a longer chain this cascades down to the original source, so a discarded chain no longer keeps a listener registered on a long-lived ValueNotifier.
   • The next addListener re-attaches the chain (cascading up again) and resyncs its value from the source before the new listener is registered, so the resync never notifies that listener. Notifications resume on the next change.
   • .value is always correct while detached: it is derived from the current source value on read (map calls convert, select calls selector, where returns the source value if it passes the filter and its last stored value otherwise, debounce/async return the source value immediately, combineLatest* call the combiner). This also applies to lazy: true chains before their first listener.
     • Consequence: transformation functions may be invoked on read while a chain is unobserved, so they must be pure and cheap.
     • Exception: mergeWith can't know which of its sources changed last while detached and keeps the last value it received while it had listeners.
   • dispose() still works on attached and detached chains; debounce now also cancels a pending timer on dispose/detach.

   Why: a chain created per widget instance (e.g. inline in a watch_it selector like watchValue((M m) => m.counter.map(...))) used to stay subscribed to m.counter forever after the widget was disposed, accumulating one listener - and one live chain doing work - per widget instance ever built.

   History: functional_listener (the predecessor of listen_it) detached on the last listener until v4.0.0 (Sept 2024), where that was removed because of functional_listener#13: after addListener + removeListener, mapped.value no longer followed source.value. This release re-introduces the detach, but thanks to derive-on-read the scenario from #13 now works:

   final mapped = source.map((v) => v + 1);
   mapped.addListener(l);
   mapped.removeListener(l);
   source.value = 123;
   print(mapped.value); // 124 - derived from the source, chain is detached
   

   Thanks to @Feichtmeier for the detach mechanism (#2).

   Internal

   • New ChainLifecycle mixin holds the shared attach/detach/resync logic used by FunctionalValueNotifier, the new CombiningValueNotifierBase (base of CombiningValueNotifier..CombiningValueNotifier6), MergingValueNotifiers and FunctionalChangeNotifier.

   • pubspec.lock is no longer committed, following the Dart guidelines for library packages.
   • Shortened package description and fixed analyzer infos reported by pub.dev.