The list returned from this code is empty. assume that formatUser is an async method that formats the user from the remote to a suitable format. why is that filterUsers list does'nt change when we map the other list?
Future<List<User>> fetchUsers() async{
final list<User> usersFromRemote = await getUserFromRemote();
final List<User> filterUsers = [];
usersFromRemote.map((user) async {
if(user.name != 'jim')
{
filterUsers.add(await formatUser(user));
}
});
return filterUsers;
}
You are using map wrongly. You need to use filter(aka where) and map for your use case.
Future<List<User>> fetchUsers() async {
final List<User> usersFromRemote = await getUserFromRemote();
final List<User> filterUsers = await Future.wait(
usersFromRemote.where((u) => u.name != 'jim').map(
(user) async {
return formatUser(user);
},
),
);
return filterUsers;
}
Or you can use forEach but which is not very functional.
Use forEach() instead of map(). According to the docs, unless the iterable returned by map() is iterated over, the transforming function will not be called.
Related
So I am trying to pass a list of String values from firestore table, but I am getting an exception type 'Future<dynamic>' is not a subtype of type 'List<dynamic>'
This is the function
getLectureList(String userId) async {
var collection = FirebaseFirestore.instance.collection('students');
var docSnapshot = await collection.doc(userId).get();
Map<String, dynamic>? data = docSnapshot.data();
List<String> _lectureList =
await data!['attendance']; //This line is kinda giving me trouble
userInfo = FirestoreWrapper()
.getStudentFromData(docId: currentUser(), rawData: data);
return _lectureList;
}
And this is the function where I am getting the exception thrown
#override
void initState() {
lectureList = getLectureList(currentUser()); // Getting an exception here
NearbyConn(context).searchDevices(devices: deviceList);
super.initState();
}
tried using await in the getLectureList() method but still getting the same problem
Why do you await your data? You already got it.
List<String> _lectureList = data!['attendance'];
Please note that I don't know what your data structure looks like, so I cannot tell you if this is correct, I can only tell you that it is more correct than before, because the await did not belong there.
You are getting an exception here lectureList = getLectureList(currentUser()); because the the parameter required by the getLectureList() method is the userId which is a string. I do not know what currentUser() return but I'm assuming it's the userId that you need when calling the getLectureList() method. Based on the error, it looks like currentUser() is an async method that returns a future after some time.
You're not awaiting that future. You shouldn't make the initState() method async so move the code block out of it into a separate method and then call it from initState().
Something like this,
#override
void initState() {
super.initState();
_getData();
}
void _getData() async {
lectureList =
getLectureList(await currentUser());
NearbyConn(context).searchDevices(devices: deviceList);
}
or
#override
void initState() {
super.initState();
_getData();
}
void _getData() async {
String _userID = await currentUser();
lectureList = getLectureList(_userID);
NearbyConn(context).searchDevices(devices: deviceList);
}
Which I recommend so you can see all the parts.
Making your method parameters required named parameters also help you to easily see what is needed to pass to a function/class/.
Eg.
getLectureList({required String userId}){
...
}
Your IDE will alert you on the type of object the function requires and it makes things clearer.
Ultimately, I think typing your classes makes it so much more easier to fetch data from fireStore Typing CollectionReference and DocumentReference
This way you can easily do this,
final moviesRef = FirebaseFirestore.instance.collection('movies').withConverter<Movie>(
fromFirestore: (snapshot, _) => Movie.fromJson(snapshot.data()!),
toFirestore: (movie, _) => movie.toJson(),
);
and get your data this way,
Future<void> main() async {
// Obtain science-fiction movies
List<QueryDocumentSnapshot<Movie>> movies = await moviesRef
.where('genre', isEqualTo: 'Sci-fi')
.get()
.then((snapshot) => snapshot.docs);
// Add a movie
await moviesRef.add(
Movie(
title: 'Star Wars: A New Hope (Episode IV)',
genre: 'Sci-fi'
),
);
// Get a movie with the id 42
Movie movie42 = await moviesRef.doc('42').get().then((snapshot) => snapshot.data()!);
}
Keeps everything dry and tidy.
< The data comes to list format thats why showing the exception of datatype >
List<String> lectureList = await getLectureList(currentUser()); // use
Future<List<String>> getLectureList(String userId) async {
- your code -
}
Instead of
List _lectureList =
await data!['attendance'];
Try this
_lectureList = await data![] As List
Still new to Flutter :(. Can anyone help...
I have a class that stores a bunch of project information. One part of this is a list of topics (for push notification), which it grabs from a JSON file.
I apply a getter for the list of topics, and when getting it it calls an async function which will return a List
Future<List<String>> _pntopics() async{
final _json = await http.get(Uri.parse(_topicsUrl));
if (_json.statusCode == 200) {
return (jsonDecode(_json.body));
}else {
return [""];
}
}
Future<List<String>> get topics => _pntopics();
In my main.dart file, it calls this value like so...
Future<List<String>> _topiclist = await projectvalues.topics;
The response is however empty, pressumably because it is a Future - so it is grabbing the empty value before it is filled.
But I can't remove the "Future" part from the async method, because asnc methods require a Future definition.
Then I decided to remove the declarations entirely:
_pntopics() async{
final _json = await http.get(Uri.parse(_topicsUrl));
if (_json.statusCode == 200) {
return (jsonDecode(_json.body));
}else {
return [""];
}
}
get topics => _pntopics();
and in main.dart, a general declaration...
var _topiclist = await projectvalues.topics;
...and this works!
So what declaration should I actually be using for this to work? I'm happy to not use declarations but we're always to declare everthing.
You should return back Future<List<String>> return types to the function and the getter but for _topicslist you must use var, final or List<String> declaration because:
(await Future<T>) == T
i.e.
var _topiclist = await projectvalues.topics; // The type of _topiclist is List<String>
final _topiclist = await projectvalues.topics; // The type of _topiclist is List<String>
UPDATE
Your code should be:
Future<List<String>> _pntopics() async{
final _json = await http.get(Uri.parse(_topicsUrl));
if (_json.statusCode == 200) {
return List<String>.from(jsonDecode(_json.body));
}else {
return <String>[""];
}
}
Doing this you force _pnptopics returns List<String> as jsonDecode returns List<dynamic>.
P.S. It is good practice do not use dynamic types where they can be changed to specified types.
I'm stuck with a problem and I wondered if you can help me.
I have a functions (in Flutter) that returns a List of Items. Now this List of Items should be Filled by an other function, which goes thought my Database and collect the right items. My Problem is, that my Function runs after the Return Statement... Here is some Code:
Future<List<MaterialItem>> getItems(String path, String fach) async {
// This is a empty List that I want to fill
List<MaterialItem> list = [];
// That's my Function, that fills the List
var result = await _db
.collection("$path/$fach/")
.get()
.then((QuerySnapshot querySnapshot) {
querySnapshot.docs.forEach((doc) {
// Here the List gets filled
list.add(MaterialItem.fromSnapshot(doc.data() as Map<String, dynamic>));
});
});
// Here the List should be returned, but after my Function fills it.
return list;
}
Hope you know what my problem is, and someone can help me.
I think you could solve this using a Completer. Your function should return the Future property of the Completer and the database call should then complete it.
Take a look at the API and the example:
https://api.dart.dev/stable/2.12.4/dart-async/Completer-class.html
For example: (pseudo code)
Future<List<MaterialItem>> getItems(String path, String fach) async {
// declare a completer
Completer<List<MaterialItem>> completer = Completer();
List<MaterialItem> list = [];
final result = await _db
.collection("$path/$fach/")
.get()
.then((QuerySnapshot querySnapshot) {
querySnapshot.docs.forEach((doc) {
list.add(MaterialItem.fromSnapshot(doc.data() as Map<String, dynamic>));
});
// let the database call complete the completer
completer.complete(list);
});
// return the future value of the completer
return completer.future;
}
I have two methods the first one for getting a map stored in Firestore
Future daysMap(FirebaseUser user, String packageCode, int totalDays) async {
await users.document(user.uid).collection('myPackages')
.document(packageCode)
.get().then((doc){
// print(doc.data['DaysMap']);
return doc.data['DaysMap'];
});
}
It works correctly and prints out the map. The second method is for setting the map from Firestore to a new map in order to loop on it.
currentWorkout(FirebaseUser user, String packageCode,totalDays) async {
Map<dynamic, dynamic> days = await daysMap(user, packageCode, totalDays);
print(days);
}
When i print our the new map 'days' it always prints null.
Try using the async/await style.
Future daysMap(FirebaseUser user, String packageCode, int totalDays) async {
DocumentSnapshot doc = await users.document(user.uid).collection('myPackages').document(packageCode).get();
return doc.data['DaysMap'];
}
I have a list List<Item> list and a function Future<bool> myFilter(Item).
Is there a way to filter my list using the Future returning function myFilter()?
The idea is to be able to do something like this:
final result = list.where((item) => myFilter(item)).toList();
But this is not possible since where expects bool and not Future<bool>
Since the iteration involves async operation, you need to use a Future to perform the iteration.
final result = <Item>[];
await Future.forEach(list, (Item item) async {
if (await myFilter(item)) {
result.add(item);
}
});
You can iterate over your collection and asynchronously map your value to the nullable version of itself. In asyncMap method of Stream class you can call async methods and get an unwrapped Future value downstream.
final filteredList = await Stream.fromIterable(list).asyncMap((item) async {
if (await myFilter(item)) {
return item;
} else {
return null;
}
}).where((item) => item != null).toList()
You can try bellow:
1, Convert List => Stream:
example:
Stream.fromIterable([12, 23, 45, 40])
2, Create Future List with this function
Future<List<int>> whereAsync(Stream<int> stream) async {
List<int> results = [];
await for (var data in stream) {
bool valid = await myFilter(data);
if (valid) {
results.add(data);
}
}
return results;
}
Here's a complete solution to create a whereAsync() extension function using ideas from the accepted answer above. No need to convert to streams.
extension IterableExtension<E> on Iterable<E> {
Future<Iterable<E>> whereAsync(Future<bool> Function(E element) test) async {
final result = <E>[];
await Future.forEach(this, (E item) async {
if (await test(item)) {
result.add(item);
}
});
return result;
}
}
You can now use it in fluent-style on any iterable type. (Assume the function validate() is an async function defined elsewhere):
final validItems = await [1, 2, 3]
.map((i) => 'Test $i')
.whereAsync((s) async => await validate(s));
Try this:
final result = turnOffTime.map((item) {
if(myFilter(item)) {
return item;
}
}).toList();