How can I resolve a future within an async* stream in dart? - asynchronous

Question

I have a stream of posts that works fine but I want to check whether the current user likes a particular post. To implement the latter, I have to get info from Firestore which is a future. I have tried using yield* but it doesn't work. My code is as below;
CollectionReference posts = FirebaseFirestore.instance.collection('posts');
String userId = FirebaseAuth.instance.currentUser?.uid;
Stream<List<Post>> get allPosts async* {
yield* posts.snapshots().map((snapShot) => snapShot.docs.map((document) {
DocumentSnapshot isLikedByCurrentUser = await FirebaseFirestore //The await here is erroneous and doesnt work
.instance
.collection('likes')
.doc(this.userId + '_' + document.id)
.get();
return Post.fromJson(
document.id, document.data(), isLikedByCurrentUser.exists);
}).toList());
}

Answer 1

This is more complicated than it needs to be. I'd unwrap the map calls to for loops:
Stream<List<Post>> get allPosts async* {
for (var snapshot in posts.snapshots()) {
var element = <Post>[];
for (var document in shapshot.docs) {
DocumentSnapshot isLikedByCurrentUser = await FirebaseFirestore
.instance
.collection('likes')
.doc(this.userId + '_' + document.id)
.get();
element.add(Post.fromJson(
document.id, document.data(), isLikedByCurrentUser.exists));
}
yield element;
}
}
You can even use a list literal, if you don't mind inlining the isLikedByCurrentUser variable:
Stream<List<Post>> get allPosts async* {
for (var snapshot in posts.snapshots()) {
yield <Post>[
for (var document in shapshot.docs)
Post.fromJson(
document.id,
document.data(),
(await FirebaseFirestore.instance
.collection('likes')
.doc(this.userId + '_' + document.id)
.get())
.exists)
];
}
}

Answer 2

This is how I have re-written it based on this answer by jamesdlin.
Future<List<Post>> _postFromSnapshot(QuerySnapshot snapshot) async {
var futures = snapshot.docs.map((doc) async {
DocumentSnapshot isLikedByCurrentUser = await FirebaseFirestore.instance
.collection('likes')
.doc(this.userId + '_' + doc.id)
.get();
return Post.fromJson(
doc.id, doc.data(), isLikedByCurrentUser.exists);
}).toList();
return await Future.wait(futures);
}
Stream<List<Post>> get allPosts {
return posts.snapshots().asyncMap(_postFromSnapshot);
}