registry method
Implementation
VoidCallback registry(K key, V value) {
assert(key != null, 'key must not be null.');
assert(value != null, 'value must not be null.');
_maps[key] ??= <V>{};
_maps[key]?.add(value);
return () {
_maps[key]?.remove(value);
if (_maps[key]?.isEmpty ?? true) {
_maps.remove(key);
}
};
}