I have a problem in which I have to map some elements of a list (here types) into some other elements of the same type but with an async operation.
Future<String> getData() async {
return await "hi";
}
void main() {
List<String> types = ["", "", ""];
List<String> otherTypes = types.map((e) async {
return await getData();
}).toList();
}
But the code will not compile with the following error.
Error compiling to JavaScript:
main.dart:9:6:
Error: A value of type 'List<Future<String>>' can't be assigned to a variable of type 'List<String>'.
- 'List' is from 'dart:core'.
- 'Future' is from 'dart:async'.
}).toList();
^
Error: Compilation failed.
PS: I tried above code already on dartpad.dev.
List<Future> cannot be converted to Future<List> without using another for loop and alternatively you can create a list and use conventional for each loop. example is follows
Future<String> getData() async {
return await "hi";
}
void main() async{
List<String> types = ["", "", ""];
List<String> otherTypes = List();
for(var s in types)
otherTypes.add(await getData());
}
P.S. future value can be accessed only with async function, or else you have to use (future value).then() call back.
Related
Objective is to convert a String to List using map and return the value to a function call.
I am using SharedPreferences to save a list of object called where in I save the data at a point and get the data when it is to be put on view.
The below block is the function where the error is occurring.
void getData() async {
final prefs = await SharedPreferences.getInstance();
final String taskString = prefs.getString('task_data').toString();
List<Task> tasksData = Task.decode(taskString);
_tasks = tasksData;
notifyListeners();
}
decode() looks basically does the conversion.
static List<Task> decode(String tasks) {
return (jsonDecode(tasks) as List<dynamic>).map<Task>((task) {
return Task.fromJson(task);
}).toList();
It advises to check for null condition in type cast of decode(). But on performing the check, it gives the same error.
your response might be not a proper map so it cannot decode that data using the jsonDecode function so it returns Null, so you can use your function like this might be helpful for you :
static List<Task> decode(String tasks) {
var data = (jsonDecode(tasks) as List<dynamic>?);
if(data != null){
return (jsonDecode(tasks) as List<dynamic>?)!.map<Task>((task) {
return Task.fromJson(task);
}).toList();
} else {
return <Task>[];
}
}
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 have a problem with the two lines
after adding async and await
a new two errors appear in the await lines
"This expression has a type of 'void' so its value can't be used."
this is the code with the problem
void onTextFieldSubmitted(String input) async {
await fetchSearch(input);
await fetchLocation();
}
you can see the source code here
https://github.com/Akhele/Flutter-Weather-App/blob/master/lib/main.dart
Change your methods to 'Future':
Future<void> fetchSearch(String input) async {
and
Future<void> fetchLocation() async {
I'm trying to pass as parameter of a method, a function that returns Future<Response>.
I tried to do
Future<String> _execute(Function<Future<Response>>() function) async { }
but it does not even compile.
What's the correct syntax?
You can do it like this,
Future<String> _myFunction(Future<Response> Function() function) {
...
}
You just need to specify that your parameter is a Function:
Future<bool> kappa() async{
await Future.delayed(Duration(seconds: 1));
return true;
}
Future<bool> foo(Function f) async{
var k = await f();
return k;
}
void main() async{
print(await foo(kappa));
}
This will print true. In your case, your function parameter can be:
Future<String> _execute(Function function) async { }