unsubscribe method
Removes a previously registered listener.
Throws an AssertionError if the Pingora has already been disposed or if the listener was never subscribed.
Implementation
void unsubscribe(VoidCallback listener) {
assert(
!_disposed,
AssertionError('Can not unsubscribe from a disposed Pingora'),
);
final removed = _listeners.remove(listener);
assert(removed, 'Tried to remove a listener that was not subscribed');
developer.log(
'Pingora<$runtimeType> unsubscribed (listeners: ${_listeners.length})',
name: 'pingora',
);
}