Wait for callback to complete in Dart / Flutter

Question

Currently writing a flutter app using the flutter_tts library.

I have a list of sentences to read out, but currently having trouble waiting for the setCompletionHandler() to complete.

How can I wait for the setCompletionHandler() callback to complete before moving on to the next string? Currently, the TTS.speak() function finishes immediately with the while loop incrementing right away so it only reads out the last sentence in the list.

// code shortened for brevity

FlutterTts TTS;
TtsState ttsState = TtsState.stopped;

get isPlaying => ttsState == TtsState.playing;
get isStopped => ttsState == TtsState.stopped;

List<String> sentences = ['Hello, World', 'How are you?', 'The quick brown fox jumps over the lazy dog'];

@override
void initState() {
    super.initState();
    TTS = FlutterTts();
}

void readOutSentences(sentences) async {
    int i = 0;
    bool readCompleted = false;

    while (i < sentences.length) {
        readCompleted = await runSpeak(sentences[i].toString());

        if (readCompleted)
          i++;
    }
}

Future<bool> runSpeak(String currentSentence) async {
    TTS.setStartHandler(() {
      setState(() {
        ttsState = TtsState.playing;
      });
    });

    TTS.setCompletionHandler(() {
      setState(() {
        ttsState = TtsState.stopped;
      });
    });

    await TTS.speak(currentSentence);

    return true;
}

readOutSentences(sentences);

Solution

Forgive about the setCompletionHandler)

You can use such async functions:

  Future<void> _speak(String _text) async {
    if (_text != null && _text.isNotEmpty) {
      await flutterTts.awaitSpeakCompletion(true);
      await flutterTts.speak(_text);
    }
  }

  readAllSentencesList(List<String> allSentences) async {
    for (int i=0; i<allSentences.length; i++){
      await _speak(allSentences[i]);
    }
  }

Don't forget to use last flutter_tts library!

Comments

[Gabriel S]

Which function do you want to wait for?

[srbrussell]

TTS.setCompletionHandler() runs after TTS.speak() is completed. Need to wait/listen to the TTS.setCompletionHandler() function before incrementing the while loop.

[Gabriel S]

Shouldn't you declare the completion and start handlers outside of runSpeak and just calling them inside that function? I think that because you just declare the completion and start handlers in the function, Dart just calls TTS.speak(currentSentence) because the other two are just declarations.