How to use the evt.on function in evt

if (document.hidden) {
        appSelf.latest('self', function(app) {
          app.launch();
        });
      }

      this.emitWhenListener('notification', data);
    }
  });

  if ('mozSetMessageHandler' in navigator) {
    navigator.mozSetMessageHandler(
        'activity', appMessages.onActivityRequest.bind(appMessages));
    navigator.mozSetMessageHandler(
        'notification', appMessages.onNotification.bind(appMessages));
    evt.on(
        'notification', appMessages.onNotification.bind(appMessages));

    // Do not listen for navigator.mozSetMessageHandler('request-sync')
    // type, that is only done in the back end's cronsync for now.
  } else {
    console.warn('Activity support disabled!');
  }

  return appMessages;
});

showFinalCardState(function() {
    resetCards('message_list', args);
  });
}

// Handles visibility changes: if the app becomes visible
// being hidden via a cronsync startup, trigger UI creation.
document.addEventListener('visibilitychange', function onVisibilityChange() {
  if (startedInBackground && finalCardStateCallback && !document.hidden) {
    finalCardStateCallback();
    finalCardStateCallback = null;
  }
}, false);

// The add account UI flow is requested.
evt.on('addAccount', function() {
  cards.removeAllCards();

  // Show the first setup card again.
  pushStartCard('setup_account_info', {
    allowBack: true
  });
});

function resetApp() {
  // Clear any existing local state and reset UI/model state.
  waitForAppMessage = false;
  waitingForCreateAccountPrompt = false;
  activityCallback = null;

  cards.removeAllCards();
  model.init();

messageSuid: data.suid
          });

          sendNotification(
            BACKGROUND_SEND_NOTIFICATION_ID,
            mozL10n.get('background-send-error-title'),
            data.localizedDescription,
            iconUrl + '#' + dataString
          );
        });
      }
    };

    // When inbox is viewed, be sure to clear out any possible notification
    // for that account.
    evt.on('inboxShown', function(accountId) {
      fetchNotificationsData('sync').then(function(notificationsData) {
        if (notificationsData.hasOwnProperty(accountId)) {
          notificationsData[accountId].notification.close();
        }
      });
    });
  });
});

return true;
  }
  return false;
}

// An account was deleted. Burn it all to the ground and
// rise like a phoenix. Prefer a UI event vs. a slice
// listen to give flexibility about UI construction:
// an acctsSlice splice change may not warrant removing
// all the cards.
evt.on('accountDeleted', resetApp);
evt.on('resetApp', resetApp);

// A request to show the latest account in the UI.
// Usually triggered after an account has been added.
evt.on('showLatestAccount', function() {
  cards.removeAllCards();

  model.latestOnce('acctsSlice', function(acctsSlice) {
    var account = acctsSlice.items[acctsSlice.items.length - 1];

    model.changeAccount(account, function() {
      pushStartCard('message_list', {
        // If waiting to complete an activity, do so after pushing the
        // message list card.
        onPushed: activityContinued
      });
    });
  });
});

model.on('acctsSlice', function() {

if (activityCallback) {
    var activityCb = activityCallback;
    activityCallback = null;
    activityCb();
    return true;
  }
  return false;
}

// An account was deleted. Burn it all to the ground and
// rise like a phoenix. Prefer a UI event vs. a slice
// listen to give flexibility about UI construction:
// an acctsSlice splice change may not warrant removing
// all the cards.
evt.on('accountDeleted', resetApp);
evt.on('resetApp', resetApp);

// A request to show the latest account in the UI.
// Usually triggered after an account has been added.
evt.on('showLatestAccount', function() {
  cards.removeAllCards();

  model.latestOnce('acctsSlice', function(acctsSlice) {
    var account = acctsSlice.items[acctsSlice.items.length - 1];

    model.changeAccount(account, function() {
      pushStartCard('message_list', {
        // If waiting to complete an activity, do so after pushing the
        // message list card.
        onPushed: activityContinued
      });
    });

function activityContinued() {
  if (activityCallback) {
    var activityCb = activityCallback;
    activityCallback = null;
    activityCb();
    return true;
  }
  return false;
}

// An account was deleted. Burn it all to the ground and
// rise like a phoenix. Prefer a UI event vs. a slice
// listen to give flexibility about UI construction:
// an acctsSlice splice change may not warrant removing
// all the cards.
evt.on('accountDeleted', resetApp);
evt.on('resetApp', resetApp);

// A request to show the latest account in the UI.
// Usually triggered after an account has been added.
evt.on('showLatestAccount', function() {
  cards.removeAllCards();

  model.latestOnce('acctsSlice', function(acctsSlice) {
    var account = acctsSlice.items[acctsSlice.items.length - 1];

    model.changeAccount(account, function() {
      pushStartCard('message_list', {
        // If waiting to complete an activity, do so after pushing the
        // message list card.
        onPushed: activityContinued
      });

this.msgVScroll.vScroll.on('inited', this._hideSearchBoxByScrolling);
      this.msgVScroll.vScroll.on('dataChanged', this._hideSearchBoxByScrolling);
      this.msgVScroll.vScroll.on('recalculated', function(calledFromTop) {
        if (calledFromTop) {
          this._hideSearchBoxByScrolling();
        }
      }.bind(this));

      this._topBar = new MessageListTopBar(
        this.querySelector('.message-list-topbar')
      );
      this._topBar.bindToElements(this.scrollContainer,
                                  this.msgVScroll.vScroll);

      this.onFolderPickerClosing = this.onFolderPickerClosing.bind(this);
      evt.on('folderPickerClosing', this.onFolderPickerClosing);
    },