shared-event-listener.html

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<script>
  (() => {
    'use strict';
    /**
     * SharedEventListener allows to register callback for an event on
     * document ensuring that the last registered callback is the one called.
     * e.g. `node1`, `node2`, then `node3` listen for `click` event -> `node3`
     * is the one to handle it.
     */
    window.SharedEventListener = class SharedEventListener {
      constructor() {
        this._events = {};
        this._handleEvent = this._handleEvent.bind(this);
      }
      /**
       * Expects a callback that returns true when it handles the event, false
       * to allow the event to be handled by the next listener.
       */
      listen(eventName, listener, callback) {
        if (!this._events[eventName]) {
          this._events[eventName] = [];
          document.addEventListener(eventName, this._handleEvent, {
            capture: true
          });
        }
        // It's fine if we have duplicates, as the last one counts.
        this._events[eventName].push({
          listener: listener,
          callback: callback
        });
      }
      unlisten(eventName, listener) {
        if (!this._events[eventName]) {
          return;
        }
        // Remove all matching listeners.
        const listeners = this._events[eventName].filter(obj => obj.listener !== listener);
        if (listeners.length) {
          this._events[eventName] = listeners;
        } else {
          this._events[eventName] = null;
          document.removeEventListener(eventName, this._handleEvent, {
            capture: true
          });
        }
      }
      _handleEvent(event) {
        const handlers = this._events[event.type];
        let i = handlers.length - 1;
        let handler = null;
        while ((handler = handlers[i--]) && !handler.listener[handler.callback](event)) {
          // Keep looping until someone handles the event.
        }
      }
    };
  })();
</script>