on method

EventListener on(String event, Object context, EventCallback? callback)

API to register for notification. It is mandatory to pass event name and callback parameters.

Implementation

EventListener on(String event, Object context, EventCallback? callback) {
  if (null == callback) {
    throw ArgumentError.notNull("callback");
  }

  // Check if the particular listener is there in the listeners collection
  // Return the listener instance, if already registered.
  EventListener? listener;

  Set<EventListener> subs =
      this._listeners.putIfAbsent(event, () => new Set<EventListener>());

  if (null == listener) {
    // Create new element.
    listener = new EventListener(event, context, callback, () {
      subs.remove(listener);
      if (subs.length == 0) {
        this._listeners.remove(listener?.eventName);
      }
    });
    subs.add(listener);
  }

  return listener;
}