Function on

src/js/utils/events.js:266–337

(elem, type, fn)

 *        Event listener.
 */
export function on(elem, type, fn) {
  if (Array.isArray(type)) {
    return _handleMultipleEvents(on, elem, type, fn);
  }

  if (!DomData.has(elem)) {
    DomData.set(elem, {});
  }

  const data = DomData.get(elem);

  // We need a place to store all our handler data
  if (!data.handlers) {
    data.handlers = {};
  }

  if (!data.handlers[type]) {
    data.handlers[type] = [];
  }

  if (!fn.guid) {
    fn.guid = Guid.newGUID();
  }

  data.handlers[type].push(fn);

  if (!data.dispatcher) {
    data.disabled = false;

    data.dispatcher = function(event, hash) {

      if (data.disabled) {
        return;
      }

      event = fixEvent(event);

      const handlers = data.handlers[event.type];

      if (handlers) {
        // Copy handlers so if handlers are added/removed during the process it doesn't throw everything off.
        const handlersCopy = handlers.slice(0);

        for (let m = 0, n = handlersCopy.length; m < n; m++) {
          if (event.isImmediatePropagationStopped()) {
            break;
          } else {
            try {
              handlersCopy[m].call(elem, event, hash);
            } catch (e) {
              log.error(e);
            }
          }
        }
      }
    };
  }