Time Field serverTimestamp() in cloud Firestore returns null on the first snapshot

问题

Im reading some data(messages) from Cloud Firestore and in a document i have a timestamp field for time. I have a Stream:

Stream<QuerySnapshot> get chats {
    return chatCollection.document(roomid).collection("messages").snapshots();
  }

for getting the messages if a an update occurs in my database (ex New message).

So when i start the app it reads all the data(messages) from the DB and i print them in my app. Here is what my console printing the messages every time i get a new snapshot:

D/ViewRootImpl@a0e5145[MainActivity]( 3045): MSG_RESIZED: frame=Rect(0, 0 - 1440, 2560) ci=Rect(0, 96 - 0, 1164) vi=Rect(0, 96 - 0, 1164) or=1
D/ViewRootImpl@a0e5145[MainActivity]( 3045): Relayout returned: old=[0,0][1440,2560] new=[0,0][1440,2560] result=0x1 surface={valid=true 492514541568} changed=false
I/flutter ( 3045): DEBUG: TEST-MESSAGE what??? 2020-10-09 22:30:12.249
I/flutter ( 3045): DEBUG: TEST-MESSAGE λολ 2020-10-09 21:59:58.212
I/flutter ( 3045): DEBUG: TEST-MESSAGE gamieste 2020-10-09 22:33:10.902
I/flutter ( 3045): DEBUG: TEST-MESSAGE holly 2020-10-09 22:26:39.672
I/flutter ( 3045): DEBUG: TEST-MESSAGE γφ 2020-10-09 22:08:47.617
I/flutter ( 3045): DEBUG: TEST-MESSAGE ελα 2020-10-09 22:13:38.167
I/flutter ( 3045): DEBUG: TEST-MESSAGE see re 2020-10-09 22:29:14.277
I/flutter ( 3045): DEBUG: TEST-MESSAGE ιξι 2020-10-09 22:10:07.442
I/flutter ( 3045): DEBUG: TEST-MESSAGE ολα καλα ρε 2020-10-09 22:05:00.703
I/flutter ( 3045): DEBUG: TEST-MESSAGE what??? 2020-10-09 22:30:12.249
I/flutter ( 3045): DEBUG: TEST-MESSAGE λολ 2020-10-09 21:59:58.212
I/flutter ( 3045): DEBUG: TEST-MESSAGE gamieste 2020-10-09 22:33:10.902
I/flutter ( 3045): DEBUG: TEST-MESSAGE holly fuck 2020-10-09 22:26:39.672
I/flutter ( 3045): DEBUG: TEST-MESSAGE γφ 2020-10-09 22:08:47.617
I/flutter ( 3045): DEBUG: TEST-MESSAGE ελα μου 2020-10-09 22:13:38.167

So no problem on that. The problem occurs when i add a new document(message) in my DB from my code, then immediately it gets the update from the DB and i have the new snapshot with the new list of messages. But for about 0.5 sec i get an error on the screen oh my android and then it gets normal and has loaded all messages correctly. The error is a null pointer for a specific field of the document. TIME. Time is timestamp field in my Cloud Firestore DB.

So here is the error:

The following NoSuchMethodError was thrown building:
The method 'toDate' was called on null.
Receiver: null
Tried calling: toDate()

When the exception was thrown, this was the stack:
#0      Object.noSuchMethod (dart:core-patch/object_patch.dart:51:5)
#1      _ChatScreenState._buildMessage (package:flutter_firebase_chat_app/screens/chat/chat_screen.dart:48:43)
#2      _ChatScreenState.build.<anonymous closure>.<anonymous closure> (package:flutter_firebase_chat_app/screens/chat/chat_screen.dart:226:40)
#3      SliverChildBuilderDelegate.build (package:flutter/src/widgets/sliver.dart:448:22)
#4      SliverMultiBoxAdaptorElement._build.<anonymous closure> (package:flutter/src/widgets/sliver.dart:1136:67)
#5      _HashMap.putIfAbsent (dart:collection-patch/collection_patch.dart:140:29)
#6      SliverMultiBoxAdaptorElement._build (package:flutter/src/widgets/sliver.dart:1136:26)
#7      SliverMultiBoxAdaptorElement.performRebuild.processElement (package:flutter/src/widgets/sliver.dart:1082:66)

You will see that its just a null exception. The thing is why im getting a null for a Time field only when i upload a message. And the thing is that after that the message comes again from another snapshot with no-null value!! If you look on the error code segment i have printed the whole object and in the top right corner you will see that time field has null value. I made an if statement to print the whole object whenever time field is null.

So at last the thing i think is causing this is that time field is a timestamp and when i upload the data i give it a value of ServerTimestamp(). Code here:

Future sendMessage(Message message) async {
    if (message.senderId != AuthService.myUid) return null;
    return await chatCollection.document(roomid).collection("messages").add({
      "message": message.text,
      "sender": message.senderId,
      "receiver": message.receiverId,
      "time": FieldValue.serverTimestamp(),
      "isLiked": message.isLiked,
      "unread": message.unread,
    });
  }

As you can see the Time field is the only one that takes that FieldValue.serverTimestamp() , so i think that somehow i get snapshot of the document before the Timestamp value to field time has been assigned. And after i get it all (with the Time field being no-Null)

Any ideas?

回答1:

Instead of using the FieldValue.serverTimestamp(), you can try it by adding using the DateTime module which is provided in Dart.

You can use DateTime.now() will get the current date as well as time and then while fetching the data from DB you can format it as you wish. I think this will solve your problem.

https://api.dart.dev/stable/2.10.1/dart-core/DateTime-class.html

来源：https://stackoverflow.com/questions/64287252/time-field-servertimestamp-in-cloud-firestore-returns-null-on-the-first-snapsh

标签

firebase

Flutter

google-cloud-firestore

timestamp