import {actionCreators as ac, actionTypes as at} from "common/Actions.jsm";
import {perfService as perfSvc} from "common/PerfService.jsm";

const VISIBLE = "visible";
const VISIBILITY_CHANGE_EVENT = "visibilitychange";

export class DetectUserSessionStart {
  constructor(store, options = {}) {
    this._store = store;
    // Overrides for testing
    this.document = options.document || global.document;
    this._perfService = options.perfService || perfSvc;
    this._onVisibilityChange = this._onVisibilityChange.bind(this);
  }

  /**
   * sendEventOrAddListener - Notify immediately if the page is already visible,
   *                    or else set up a listener for when visibility changes.
   *                    This is needed for accurate session tracking for telemetry,
   *                    because tabs are pre-loaded.
   */
  sendEventOrAddListener() {
    if (this.document.visibilityState === VISIBLE) {
      // If the document is already visible, to the user, send a notification
      // immediately that a session has started.
      this._sendEvent();
    } else {
      // If the document is not visible, listen for when it does become visible.
      this.document.addEventListener(VISIBILITY_CHANGE_EVENT, this._onVisibilityChange);
    }
  }

  /**
   * _sendEvent - Sends a message to the main process to indicate the current
   *              tab is now visible to the user, includes the
   *              visibility_event_rcvd_ts time in ms from the UNIX epoch.
   */
  _sendEvent() {
    this._perfService.mark("visibility_event_rcvd_ts");

    try {
      let visibility_event_rcvd_ts = this._perfService
        .getMostRecentAbsMarkStartByName("visibility_event_rcvd_ts");

      this._store.dispatch(ac.AlsoToMain({
        type: at.SAVE_SESSION_PERF_DATA,
        data: {visibility_event_rcvd_ts},
      }));
    } catch (ex) {
      // If this failed, it's likely because the `privacy.resistFingerprinting`
      // pref is true.  We should at least not blow up.
    }
  }

  /**
   * _onVisibilityChange - If the visibility has changed to visible, sends a notification
   *                      and removes the event listener. This should only be called once per tab.
   */
  _onVisibilityChange() {
    if (this.document.visibilityState === VISIBLE) {
      this._sendEvent();
      this.document.removeEventListener(VISIBILITY_CHANGE_EVENT, this._onVisibilityChange);
    }
  }
}
