Timestamp from firestore gets converted to a Map when using cloud function

Question

So I have a Timestamp in cloud firestore. I am using cloud function to retrieve data from firestore to flutter. But JSON formats timestamp to map due to which I am not able to use it as timestamp. How to convert it again into timestamp?
This is how I ulpoaded timestamp to firestore.

var reference = Firestore.instance.collection('posts');
      reference.add({
        'postTitle': this.title,
        'timestamp': DateTime.now(),
        'likes': {},
        'ownerId': userId,
      })

To retrieve data this is the code:

 factory Post.fromJSON(Map data){
    return Post(
      timestamp: data['timestamp'],
    );
  }

List<Post> _generateFeed(List<Map<String, dynamic>> feedData) {
    List<Post> listOfPosts = [];

    for (var postData in feedData) {
      listOfPosts.add(Post.fromJSON(postData));
    }

    return listOfPosts;
  }

but this returns an error.

I/flutter (17271): The following assertion was thrown building FutureBuilder<DocumentSnapshot>(dirty, state:
I/flutter (17271): _FutureBuilderState<DocumentSnapshot>#1536b):
I/flutter (17271): type '_InternalLinkedHashMap<String, dynamic>' is not a subtype of type 'Timestamp'

This is my cloud function.
getFeed.ts

import * as functions from 'firebase-functions';
import * as admin from 'firebase-admin';

export const getFeedModule = function(req, res){
    const uid = String(req.query.uid);

    async function compileFeedPost(){
        const following = await getFollowing(uid, res)as any;

        let listOfPosts = await getAllPosts(following, res);

        listOfPosts = [].concat.apply([], listOfPosts);

        res.send(listOfPosts);
    }

    compileFeedPost().then().catch();
}

async function getAllPosts(following, res) {
    let listOfPosts = [];

    for (let user in following){
        listOfPosts.push( await getUserPosts(following[user], res));
    }
    return listOfPosts;
}

function getUserPosts(userId, res){
    const posts = admin.firestore().collection("posts").where("ownerId", "==", userId).orderBy("timestamp")

    return posts.get()
    .then(function(querySnapshot){
        let listOfPosts = [];

        querySnapshot.forEach(function(doc){
            listOfPosts.push(doc.data());
        });

        return listOfPosts;
    })
}

function getFollowing(uid, res){
    const doc = admin.firestore().doc(`user/${uid}`)
    return doc.get().then(snapshot => {
        const followings = snapshot.data().followings;

        let following_list = [];

        for (const following in followings){
            if (followings[following] === true){
                following_list.push(following);
            }
        }
        return following_list;
    }).catch(error => {
        res.status(500).send(error)
    })
}

cloud function index.ts

import * as functions from 'firebase-functions';
import * as admin from 'firebase-admin';
import { getFeedModule } from "./getFeed"
admin.initializeApp();

export const getFeed = functions.https.onRequest((req, res) => {
    getFeedModule(req, res);
})

invoked by this

_getFeed() async {
    print("Starting getFeed");
    FirebaseUser user = await FirebaseAuth.instance.currentUser();

    SharedPreferences prefs = await SharedPreferences.getInstance();

    String userId = user.uid;
    var url =
        'https://us-central1-jaluk-quiz.cloudfunctions.net/getFeed?uid=' + userId;
    var httpClient = HttpClient();

    List<QuizViewer>listOfPosts;
    String result;
    try {
      var request = await httpClient.getUrl(Uri.parse(url));
      var response = await request.close(); 
      if (response.statusCode == HttpStatus.ok) {
        String json = await response.transform(utf8.decoder).join();
        prefs.setString("feed", json);
        List<Map<String, dynamic>> data =
            jsonDecode(json).cast<Map<String, dynamic>>();
        listOfPosts = _generateFeed(data);
        result = "Success in http request for feed";
      } else {
        result =
            'Error getting a feed: Http status ${response.statusCode} | userId $userId';
      }
    } catch (exception) {
      result = 'Failed invoking the getFeed function. Exception: $exception';
    }
    print(result);

    setState(() {
      feedData = listOfPosts;
    });
  }

Answer 1

If you are dealing with a Timestamp that's been serialized as an object with seconds and nanoseconds components, you can use those components to create a new Timestamp object with new Timestamp(seconds, nanoseconds).

Answer 2

Indeed, timestamps gets returned as plain Map when using Cloud functions. But if you use Firebase SDK it returns Timestamp object. I use the following function to handle both cases:

/// https://stackoverflow.com/a/57865272/1321917
DateTime dateTimeFromTimestamp(dynamic val) {
  Timestamp timestamp;
  if (val is Timestamp) {
    timestamp = val;
  } else if (val is Map) {
    timestamp = Timestamp(val['_seconds'], val['_nanoseconds']);
  }
  if (timestamp != null) {
    return timestamp.toDate();
  } else {
    print('Unable to parse Timestamp from $val');
    return null;
  }
}

Works perfectly with json_annotation lib:

  @JsonKey(
      fromJson: dateTimeFromTimestamp,
      toJson: dateTimeToTimestamp,
      nullable: true)
  final DateTime subscriptionExpiryDate;