Flutter Dart: Future.wait() pass value by reference? - dart

Question

In a scenario like the following, how to pass a variable List<Document> itemList by reference ?
Future.wait([futDocs]).then((dataRet){
dataRet.forEach((doco) {
var docList = doco.documents;
docList.forEach((doc){
var docTitle = doc['title'];
print("data is $docTitle");
itemList.add(docTitle); <--- change not reflected outside this function
The itemList does not change when itemList.add(docTitle) is performed in the above Future.wait(). I believe the reason is because itemList is not passed by reference. If I cannot pass itemList by reference, how can I make this work? Can I return dataRet as a list and use it outside Future.wait()?
Adding more info...
Above call is performed within the constructor of class ListBuilder as shown in the following code:
class ListBuilder {
List<Document> itemList = new List<Document>();
ListBuilder(){
var futDocs = Firestore.instance.collection('Data').orderBy('time').limit(10).getDocuments();
Future.wait([futDocs]).then((dataRet) {
dataRet.forEach((doco) {
var docList = doco.documents;
docList.forEach((doc){
var docTitle = doc['title'];
print("data is $docTitle");
itemList.add(docTitle); <--- change not reflected outside this function
[...]
While waiting for an answer, i have tried custom setters and getters as well as the use of this.itemList.add(...) but without success.
Any help, it is much appreciated.
Thanks in advance!

Answer 1

All objects in Dart are passed by reference.
The problem is likely [itemList] isn't updated immediately because Future.wait is an async function. Any code dealing with itemList has to run inside the callback function ( .then((dataret) { ... }); ).
class ListBuilder {
List<Document> itemList = new List<Document>();
ListBuilder() {
var Docs =
Firestore.instance.collection('Data').orderBy('time').limit(10).getDocuments();
Future.wait([futDocs]).then((dataRet) {
dataRet.forEach((doco) {
var docList = doco.documents;
docList.forEach((doc) {
var docTitle = doc['title'];
itemList.add(docTitle);
});
});
/*
itemList has been updated here because we are inside a callback function that
runs _after_ Future.wait has completed.
*/
});
/*
itemList hasn't been updated here _yet_ because this code runs immediately,
regardless how long the Future.wait call takes to complete.
*/
}
}
If you want you can use async/await logic instead of a callback function, which is often easier to read and maintain. More info here: https://www.dartlang.org/tutorials/language/futures

Answer 2

More code would be helpful, but the first things that come to mind are these:
It will probably work to call await before your Future.wait so that the asynchronous code is for sure run
This seems less likely, but you may have to call setState when adding your to your list
Edit:
I would suggest code like this:
class ListBuilder {
List<Document> itemList = new List<Document>();
Future<void> myFunc(var futDocs) async {
await Future.wait([futDocs]).then((dataRet){
[...] *Rest of your Future code here*
}
ListBuilder() {
var futDocs = Firestore.instance.collection('Data').orderBy('time').limit(10).getDocuments();
myFunc(futDocs)
[...]

Answer 3

I have solved the problem however it was not related to pass value by reference. Thank you for your answers, they actually got me thinking and pointed me in the right direction.
The following was the solution to a problem which is not the one raised above :D LOL
Tl;dr; answer:
Change the widget to use streamBuilder
Long answer
The problem was due to the fact that changes to a list were not reflected outside the .then() function. Well, the issue itself is not related to how the list was passed to the function but to the use of async/await logic. When the app is executed, it will populate the itemList in Widget with the current items in the list - this is empty when it starts. In the meanwhile, with an async call, firebase fetch the data and list is updated. However, the UI is not changed. Now, setState gave me an idea but was not the solution. In my case, I don't need a stateful Widget but rather the use of StreamBuilder within the creation of the widget. This bind the list to the async call with the use of stream
new StreamBuilder(
stream: Firestore.instance.collection('data').orderBy('time').limit(10).snapshots(),
builder: (context, snapshot) {
return itemList...
Thanks again!