Read data

Read in realtime with collection().snapshots.

The snapshots method

The collection().snapshots() method returns a stream that watches the documents in a collection in real time. When data is added, changed, or removed, the stream emits a new QuerySnapshot. This lets the UI update automatically without re-fetching the data.

db.collection('messages').snapshots().listen((snap) {
  for (final doc in snap.docs) {
    print(doc.data());
  }
});

QuerySnapshot and docs

QuerySnapshot holds all documents in the query result. You access the list of documents with snap.docs; each document yields its fields as a Map via doc.data(). doc.id is the document id.

snap.docs.forEach((doc) {
  print(doc.id);
  print(doc.data());
});

Exercise Listen to documents and count them

Get db. Listen to the snapshots stream of the 'messages' collection. Print the document count on each snapshot. Wait 10 milliseconds after starting to listen.

Show example solution

class QueryDocumentSnapshot {
  final Map<String, dynamic> data;
  QueryDocumentSnapshot(this.data);
}

class QuerySnapshot {
  final List<QueryDocumentSnapshot> docs;
  QuerySnapshot(this.docs);
}

class CollectionReference {
  final String name;
  CollectionReference(this.name);
  Stream<QuerySnapshot> snapshots() async* {
    yield QuerySnapshot([
      QueryDocumentSnapshot({'text': 'Merhaba'}),
    ]);
  }
}

class FirebaseFirestore {
  static final FirebaseFirestore instance = FirebaseFirestore._();
  FirebaseFirestore._();
  CollectionReference collection(String name) => CollectionReference(name);
}

void main() async {
  final db = FirebaseFirestore.instance;
  db.collection('messages').snapshots().listen((snap) {
    print(snap.docs.length);
  });
  await Future.delayed(Duration(milliseconds: 10));
}

Test yourself

What does the snapshots method return?
- Stream<QuerySnapshot>
- Future<QuerySnapshot>
- Future<List>
- DocumentReference
Show answer

Stream<QuerySnapshot> — snapshots returns a real-time Stream<QuerySnapshot>; a new snapshot is emitted on every data change.

Listen to the documents

db.collection('messages').____().listen((snap) {
  print(snap.docs.length);
});

Show answer

db.collection('messages').snapshots().listen((snap) {
  print(snap.docs.length);
});

Reading parts

snapshots

Real-time stream
QuerySnapshot

Query result
docs

Document list
doc.data()

Document fields

Next lesson: Query and where

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