class ChromeTts extends ChromeApi {
 static final JsObject _tts = chrome['tts'];

 ChromeTts._();

 bool get available => _tts != null;

 /**
  * Speaks text using a text-to-speech engine.
  * 
  * [utterance] The text to speak, either plain text or a complete, well-formed
  * SSML document. Speech engines that do not support SSML will strip away the
  * tags and speak the text. The maximum length of the text is 32,768
  * characters.
  * 
  * [options] The speech options.
  */
 Future speak(String utterance, [TtsSpeakParams options]) {
   if (_tts == null) _throwNotAvailable();

   var completer = new ChromeCompleter.noArgs();
   _tts.callMethod('speak', [utterance, jsify(options), completer.callback]);
   return completer.future;
 }

 /**
  * Stops any current speech and flushes the queue of any pending utterances.
  * In addition, if speech was paused, it will now be un-paused for the next
  * call to speak.
  */
 void stop() {
   if (_tts == null) _throwNotAvailable();

   _tts.callMethod('stop');
 }

 /**
  * Pauses speech synthesis, potentially in the middle of an utterance. A call
  * to resume or stop will un-pause speech.
  */
 void pause() {
   if (_tts == null) _throwNotAvailable();

   _tts.callMethod('pause');
 }

 /**
  * If speech was paused, resumes speaking where it left off.
  */
 void resume() {
   if (_tts == null) _throwNotAvailable();

   _tts.callMethod('resume');
 }

 /**
  * Checks whether the engine is currently speaking. On Mac OS X, the result is
  * true whenever the system speech engine is speaking, even if the speech
  * wasn't initiated by Chrome.
  * 
  * Returns:
  * True if speaking, false otherwise.
  */
 Future<bool> isSpeaking() {
   if (_tts == null) _throwNotAvailable();

   var completer = new ChromeCompleter<bool>.oneArg();
   _tts.callMethod('isSpeaking', [completer.callback]);
   return completer.future;
 }

 /**
  * Gets an array of all available voices.
  * 
  * Returns:
  * Array of [TtsVoice] objects representing the available voices for speech
  * synthesis.
  */
 Future<List<TtsVoice>> getVoices() {
   if (_tts == null) _throwNotAvailable();

   var completer = new ChromeCompleter<List<TtsVoice>>.oneArg((e) => listify(e, _createTtsVoice));
   _tts.callMethod('getVoices', [completer.callback]);
   return completer.future;
 }

 /**
  * Used to pass events back to the function calling speak().
  */
 Stream<TtsEvent> get onEvent => _onEvent.stream;

 final ChromeStreamController<TtsEvent> _onEvent =
     new ChromeStreamController<TtsEvent>.oneArg(_tts, 'onEvent', _createTtsEvent);

 void _throwNotAvailable() {
   throw new UnsupportedError("'chrome.tts' is not available");
 }
}