Method onKeyboardStateChanged

src/input.js:153–158

* Registers an event handle for changes in the keyboard input. * @param {function(boolean)} handler * @param {boolean=} opt_fireImmediately * @return {!UnlistenDef}

(handler, opt_fireImmediately)

   * @return {!UnlistenDef}
   */
  onKeyboardStateChanged(handler, opt_fireImmediately) {
    if (opt_fireImmediately) {
      handler(this.isKeyboardActive());
    }
    return this.keyboardStateObservable_.add(handler);
  }

  /**
   * @param {!./service/ampdoc-impl.AmpDoc} ampdoc