How to get list of document ids? - firebase

Question

I am trying to retrieve a list of document Ids from firestore.
My firestore functions are stored as static methods in a FirebaseAPI class I created.
This is the function that should return the list of documentIds:
static List<String> avaiableDocuments() {
List<String> ids = [];
final Future<QuerySnapshot> snapshot = Firestore.instance.collection('admin').getDocuments();
snapshot.then((value) {
value.documents.forEach((element) {
ids.add(element.documentID);
});
});
return ids;
}
When I restart the app, the list of document ids do not appear.
However, they do appear when the is then hot reloaded.
My understanding is that when the app restarts, the list of strings is still empty.
This is because the page renders before the future returns.
Then when hot reload calls the build method again, the list is already populated so it displays it.
I tried adding "await" before the getDocuments call.
This turns the Future to just QuerySnapshot. Fair Enough.
Now "async" must be added to the function.
Then the function is now requiring a future return instead of a list.
Can't I just get a list from this function?
I tried to go with it and ended up with this:
static Future<List<String>> avaiableDocuments() async{
List<String> ids = [];
final QuerySnapshot snapshot = await Firestore.instance.collection('admin').getDocuments();
snapshot.documents.forEach((element) {
ids.add(element.documentID);
});
return ids;
}
How is it that I'm allowed to return a list from a function that says it's returning a future?
When I tried to consume the future in the screen class, same thing happens. List only appears on hot reload after restart.
Note: I feel it's a simple issue, but I pretty much a noob when it comes to asynchronous voodoo.
Class that should display the list of document Ids
import 'package:flutter/material.dart';
import 'package:hisab/database/firestore_api.dart';
class PreviousCarts extends StatefulWidget {
const PreviousCarts({Key key}) : super(key: key);
#override
_PreviousCartsState createState() => _PreviousCartsState();
}
class _PreviousCartsState extends State<PreviousCarts> {
List<String> documentIds = [];
#override
void initState() {
super.initState();
Future<List<String>> f = FirestoreAPI.avaiableDocuments();
f.then((value) {
value.forEach((element) {
documentIds.add(element);
});
});
}
#override
Widget build(BuildContext context) {
print(documentIds);
return Material(
child: Padding(
padding: const EdgeInsets.only(left: 8.0, top: 8.0),
child: Column(
crossAxisAlignment: CrossAxisAlignment.start,
children: documentIds.map((e) {
return Card(child: ListTile(title: Text(e)));
}).toList(),
),
),
);
}
}

Answer 1

Can't I just get a list from this function?
No. Since the database query is asynchronous, everything that involves it must also be async. It's a bad idea to try to turn an async method in to an immediate return method, as that would cause your app to freeze if a query is slow or the app is offline.
I suggest reading this for some context. https://medium.com/firebase-developers/why-are-the-firebase-apis-asynchronous-e037a6654a93
How is it that I'm allowed to return a list from a function that says it's returning a future?
Because the dart compiler is making sure that the caller receives a Future. It's handling the hard stuff behind the scenes with respect to async/await to make it easier for you to write code, so that you don't have to be explicit about futures whenever they appear.
This style of asynchronous programming is very common, and comes up in other languages such as JavaScript. If you want to be effective with dart, and async programming in general, then I suggest taking time to become familiar with these techniques. It takes practice.