Flutter StreamBuilder returns null from Firestore - firebase

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"];
}

Answer 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.