Firestore Transactions do not work - firebase

Question

I found some issue thread with people also not being able to complete transactions, but it does not even provide solutions.
Problem
Suddenly, my transactions crashes when using transaction.get.
runTransaction(
(Transaction transaction) async {
await transaction.get(documentReference); // "Timed out waiting for Task"
}
The PlatformException does also not really help me because it crashes in platform_channel...
E/flutter (16297): [ERROR:topaz/lib/tonic/logging/dart_error.cc(16)] Unhandled exception:
E/flutter (16297): PlatformException(Error performing transaction, Timed out waiting for Task, null)
E/flutter (16297): #0 StandardMethodCodec.decodeEnvelope (package:flutter/src/services/message_codecs.dart:547:7)
E/flutter (16297): #1 MethodChannel.invokeMethod (package:flutter/src/services/platform_channel.dart:279:18)
E/flutter (16297): <asynchronous suspension>
E/flutter (16297): #2 Firestore.runTransaction (file:///G:/flutter/.pub-cache/hosted/pub.dartlang.org/cloud_firestore-0.7.3/lib/src/firestore.dart:115:10)
// here comes another <asynchronous suspension> followed by my code
This comes from Android.

Answer 1

This Error comes sometimes because:
don't debug the transaction while it's running, so put the breakpoint in a proper line before or after.
Another errors come because you don't :
do all get calls first, then your write or update comes.
as Firestore document said:
"Read operations must come before write operations."
https://firebase.google.com/docs/firestore/manage-data/transactions#transactions

Answer 2

The issue I was having has since been fixed by the cloud_firestore team.
If you are still experiencing a similar issue, you should either ask a question here on StackOverflow or create an issue.

Answer 3

Note: You can also make atomic changes to data using transactions.
While this is a bit heavy-handed for incrementing a vote total, it is
the right approach for more complex changes. Here is what a
transaction that updated the votes count might look like.
onTap: () => Firestore.instance.runTransaction((transaction) async {
final freshSnapshot = await transaction.get(record.reference);
final fresh = Record.fromSnapshot(freshSnapshot);
await transaction
.update(record.reference, {'votes': fresh.votes + 1});
}),
How does this work? By wrapping the read and write operations in one
transaction, you're telling Cloud Firestore to only commit a change if
there was no external change to the underlying data while the
transaction was running. If two users aren't concurrently voting on
that particular name, the transaction runs exactly once. But if the
number of votes changes between the transaction.get(...) and the
transaction.update(...) calls, the current run isn't committed, and
the transaction is retried. After 5 failed retries, the transaction
fails.
Source
https://codelabs.developers.google.com/codelabs/flutter-firebase/index.html#10

Answer 4

This works without any problems:
import 'dart:async';
import 'package:flutter/material.dart';
import 'package:firebase_core/firebase_core.dart';
import 'package:cloud_firestore/cloud_firestore.dart';
Future<void> main() async {
final FirebaseApp app = await FirebaseApp.configure(
name: 'yourappname',
options: const FirebaseOptions(
googleAppID: 'yourgoogleid',
gcmSenderID: 'yourgmssenderid',
apiKey: 'yourapikey',
projectID: 'yourprojectid',
),
);
final Firestore firestore = Firestore(app: app);
await firestore.settings(
timestampsInSnapshotsEnabled: true,
persistenceEnabled: true,
sslEnabled: true
);
runApp(MaterialApp(
title: 'Firestore Example', home: MyHomePage(firestore: firestore)));
}
class MessageList extends StatelessWidget {
MessageList({this.firestore});
final Firestore firestore;
#override
Widget build(BuildContext context) {
return StreamBuilder<QuerySnapshot>(
stream: firestore.collection('messages').snapshots(),
builder: (BuildContext context, AsyncSnapshot<QuerySnapshot> snapshot) {
if (!snapshot.hasData) return const Text('Loading...');
final int messageCount = snapshot.data.documents.length;
return ListView.builder(
itemCount: messageCount,
itemBuilder: (_, int index) {
final DocumentSnapshot document = snapshot.data.documents[index];
return ListTile(
title: Text(document['message'] ?? '<No message retrieved>'),
subtitle: Text('Message ${index + 1} of $messageCount'),
);
},
);
},
);
}
}
class MyHomePage extends StatelessWidget {
MyHomePage({this.firestore});
final Firestore firestore;
CollectionReference get messages => firestore.collection('messages');
Future<void> _addMessage() async {
final DocumentReference postRef = firestore.document('posts/123');
firestore.runTransaction((Transaction tx) async {
DocumentSnapshot postSnapshot = await tx.get(postRef);
if (postSnapshot.exists) {
await tx.update(postRef, <String, dynamic>{'likesCount': postSnapshot.data['likesCount'] + 1});
}
});
}
#override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: const Text('Firestore Example'),
),
body: MessageList(firestore: firestore),
floatingActionButton: FloatingActionButton(
onPressed: _addMessage,
tooltip: 'Increment',
child: const Icon(Icons.add),
),
);
}
}