void registerRemoveListener(GlobalKey key, GlobalKeyRemoveListener listener)

Calls listener whenever a widget with the given global key is removed from the tree.

Listeners can be removed with unregisterRemoveListener.

Source 

static void registerRemoveListener(GlobalKey key, GlobalKeyRemoveListener listener) {
  assert(key != null);
  Set<GlobalKeyRemoveListener> listeners =
      _removeListeners.putIfAbsent(key, () => new HashSet<GlobalKeyRemoveListener>());
  bool added = listeners.add(listener);
  assert(added);
}