Flutter StreamBuilder returns null from Firestore

Question

The idea is to display a string from a random document within a collection in Firebase. A simple function getRandom() retrieves the total number of documents and generates a random integer r that is fed into the Firebase instance.

The output in the app is always null.

StreamBuilder(
        initialData: Words(),
          stream: getWords(),
          builder: (context, snapshot){

            if(!snapshot.hasData){
              return Center(child: Text("NO DATA"));
            }else {
              var r = snapshot.data;
              return Center(child: Text("${r.english}"));
            }
      })

Stream<Words> getWords() async* {
  int r = await getRandom();

  print("RANDOM NO: " + "$r");

  Firestore.instance.document("vocabs/foods/words/$r")
      .get()
      .then((snapshot){
    try {
      return Words().english;

    } catch(e){
      print("ERROR");
      return null;
    }
  });
}

class Words{

  Words(): super();

  String english;

  Words.fromSnapshot(DocumentSnapshot snapshot)
  : english = snapshot.data["english"];
}

For async generators, isn't the keyword yield or yield* instead of return ? Also, seems like the return type should be Stream<String>, since you are yielding the english string of your Word. — Daniel V.
– Daniel V., Commented May 14, 2019 at 13:21
@DanielV. Stream<String indeed makes more sense. yield is not possible within the then - callback. It has to be return So far it still returns null, though. — Marc Köhler
– Marc Köhler, Commented May 14, 2019 at 13:48

Daniel V. · Accepted Answer · 2019-05-14 18:20:10Z

1

I've constructed a this piece of sample code for you to give you some options to achieve what you'd like to do:

import 'dart:async';

class Word {    
  final String english;
  const Word(this.english);
}

Future<Iterable<Word>> get firebaseSnapshot async => [ Word('aWord'), Word('bWord'), Word('cWord') ];

Stream<String> getEnglishWords() async* {
  yield* await firebaseSnapshot.then((words) => Stream.fromIterable(words.map((w) => w.english)));
}

Stream<String> getEnglishWords2() async* {    
  final words = await firebaseSnapshot.then((words) => words.map((w) => w.english));
  yield* Stream.fromIterable(words);
}

Stream<String> getEnglishWords3() async* {    
  final snapshot = await firebaseSnapshot;
  for(final word in snapshot) {
    yield word.english;
  }  
}

main() async {
  await for(final englishWord in getEnglishWords()) {
    print(englishWord);
  }

  await for(final englishWord in getEnglishWords2()) {
    print(englishWord);
  }

  await for(final englishWord in getEnglishWords3()) {
    print(englishWord);
  }
}

Option No. 2 is the one I'd use. There is some significant performance consideration around it. I am scraping the back of my mind for the lecture around it... Nope, can't recall... If I find it, I'll update ya.

answered May 14, 2019 at 18:20

Daniel V.

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4 Comments

Marc Köhler Over a year ago

First of all, thank you very much for taking the time. I am not as experienced, so I struggle a little bit with understanding how to implement your sample code. As I can see, there is no Firestore.instance that determines the path within my Firestore database. How do I add it to the getEnglishWords function? Also, I am unable to add the async tag to the main function. I assume, in the build Widget, there still needs to be a StreamBuilder but there is nowhere for me to add async. Again, thanks very much!

Daniel V. Over a year ago

My sample code was to mimic how Firestore works. There is no instance in it. I recommend skipping the async generator and simplify your code for now. Another starter: gist.github.com/daniel-v/1fbe7697ae970cb98ab12eb1bed999b8 FutureBuilder class docs is really good. docs.flutter.io/flutter/widgets/FutureBuilder-class.html

Marc Köhler Over a year ago

I had to add my Words.fromSnapshot line in the Words class, so that I was able to use .fromSnapshot in the getWords function. However, it still returns a Words instead of a Future<String> hence why I'm getting an error in that function.

Marc Köhler Over a year ago

I checked it again, and I realised I just needed to return word.toString(). My mistake, it finally works now. I have been working on this for days! Thank you, Daniel. Appreciate your help!!

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