var listener;

    if (eventName === 'change') {
      listener = RCTDeviceEventEmitter.addListener(
        VOICE_OVER_EVENT,
        handler
      );
    }
Add an event handler. Supported events:

- `change`: Fires when the state of the screen reader changes. The argument
  to the event handler is a boolean. The boolean is `true` when a screen
  reader is enabled and `false` otherwise.
- `announcementFinished`: iOS-only event. Fires when the screen reader has
  finished making an announcement. The argument to the event handler is a
  dictionary with these keys:
    - `announcement`: The string announced by the screen reader.
    - `success`: A boolean indicating whether the announcement was
      successfully made.

See http://facebook.github.io/react-native/docs/accessibilityinfo.html#addeventlistener