How to use the evt.emitWhenListener function in evt

//      switch to the latest account which user just added.
            var account = showLatest ? acctsSlice.items.slice(-1)[0] :
                                       acctsSlice.defaultAccount;

            this.changeAccount(account, callback);
          } else if (callback) {
            callback();
          }

          this.inited = true;
          this._callEmit('acctsSlice');

          // Once the API/worker has started up and we have received account
          // data, consider the app fully loaded: we have verified full flow
          // of data from front to back.
          evt.emitWhenListener('metrics:apiDone');
        }).bind(this);

// is only known to the front end, so publish event about it.
      // Need a CPU lock since otherwise the app can be paused
      // mid-function, which could lead to unexpected behavior, and the
      // sync should be completed as quick as possible to then close
      // down the app.
      // TODO: removed wifi wake lock due to network complications, to
      // be addressed in a separate changset.
      if (navigator.requestWakeLock) {
        var locks = [
          navigator.requestWakeLock('cpu')
        ];

        debug('wake locks acquired: ' + locks +
              ' for account IDs: ' + data.accountIds);

        evt.emitWhenListener('cronSyncWakeLocks',
                             makeAccountKey(data.accountIds), locks);
      }

      debug('alarm dispatch started at ' + (new Date()));

      dispatcher._sendMessage('alarm',
                              [data.accountIds, data.interval]);
    });
  }