I just want to take from outside A function that like "Future<List?> Function(String userId)" then use it, but this error appear, this tell me I try to "equalize" (if this word don't fit please edit, and title is not fitted), No I dont want to equalize, I just want to run this function and take value. You can run this code on https://dartpad.dev/?
A value of type 'Future<List<String>?> Function(String)' can't be assigned to a variable of type 'List<String>?'.
Try changing the type of the variable, or casting the right-hand type to 'List<String>?'.
--
import 'package:flutter/material.dart';
typedef getAllProducts = Future<List<String>?> Function(String userId);
class AdanaView extends StatefulWidget {
const AdanaView({Key? key, required this.getProducts}) : super(key: key);
final getAllProducts getProducts;
#override
State<AdanaView> createState() => _AdanaViewState();
}
class _AdanaViewState extends State<AdanaView> {
List<String>? products;
#override
void initState() {
super.initState();
getAll();
}
Future<void> getAll() async {
products = await widget.getProducts;
setState(() {});
}
#override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: Text('Adana'),
),
);
}
}
Related
I would like to create a StatefulWidget which I'll use all over the app for listening streams of different types. Since I try to keep all the widgets Stateless I wanted to extract this functionality.
I've created this:
class StreamListener<T> extends StatefulWidget {
const StreamListener({
Key? key,
required this.stream,
required this.onNewData,
required this.child,
}) : super(key: key);
final Stream<T?> stream;
final void Function(T data) onNewData;
final Widget child;
#override
State<StreamListener> createState() => _StreamListenerState<T>();
}
class _StreamListenerState<T> extends State<StreamListener> {
late StreamSubscription<T?> streamSubscription;
#override
void initState() {
streamSubscription = (widget.stream as Stream<T?>).listen(
(T? data) {
if (data != null) {
widget.onNewData(data);
}
},
);
super.initState();
}
#override
Widget build(BuildContext context) {
return widget.child;
}
#override
void dispose() {
streamSubscription.cancel();
super.dispose();
}
}
Then somewhere in the Widgets tree I use:
return StreamListener<int>(
stream: context.read<MyCubit>().toastStream,
onNewData: (int data) {
print("Received: $data");
},
child: SomeStatelessWidget(),
}
Stream logic is added to the Cubit like that:
mixin ToastStreamForCubit<T> {
final StreamController<T> _toastStreamController = StreamController<T>();
get toastStream => _toastStreamController.stream;
void emitToastEvent(T event) {
_toastStreamController.add(event);
}
}
And when I call let's say emitToastEvent(1).
I receive type '(int) => void' is not a subtype of type '(dynamic) => void'.
on line widget.onNewData(data);.
I'm not sure what is going on. I thought I've mapped all the functions and classes to a particular generic type (T), but it still says something about dynamic.
You are missing T while extending State<StreamListener>. It should be
class _StreamListenerState<T> extends State<StreamListener<T>>
I hope you could help me!
Error saying 'tables' has not been initiliazed. But when I set tables = [] instead of
widget.data.then((result) {tables = result.tables;})
it works. I think the problem comes from my app state data which is a Future.
My simplified code:
class NavBar extends StatefulWidget {
final Future<Metadata> data;
const NavBar({Key? key, required this.data}) : super(key: key);
#override
State<NavBar> createState() => _NavBarState();
}
class _NavBarState extends State<NavBar> {
late List<MyTable> tables;
#override
void initState() {
widget.data.then((result) {
tables = result.tables;
});
super.initState();
}
#override
Widget build(BuildContext context) {
return Scaffold(
body: SafeArea(
child: buildPages(page.p)
)
);
}
Widget buildPages(index){
switch (index) {
case 0:
return ShowTablesNew(tables: tables);
case 1:
return const Details();
case 2:
return const ShowTables();
default:
return const ShowTables();
}
}
}
Future doesn't contain any data. It's an asynchronous computation that will provide data "later". The initialization error happens because the variable 'tables' is marked as late init but is accessed before the future is completed, when in fact it's not initialized yet.
Check this codelab for async programming with dart.
For your code you can use async/await in the initState method doing something like this
String user = '';
#override
void initState() {
asyncInitState();
super.initState();
}
void asyncInitState() async {
final result = await fetchUser();
setState(() {
user = result;
});
}
but since you're using a list of custom objects the most straightforward way is probably to use a FutureBuilder widget
I have a app_bar_base.dart file where i have an AppBar.
class AppBarBase extends StatelessWidget implements PreferredSizeWidget {
late double appBarHeight = LoadAppStyle().loadAppStyle();
AppBarBase({Key? key}) : super(key: key);
#override
Widget build(BuildContext context) {
return AppBar();
}
#override
Size get preferredSize => Size.fromHeight(appBarHeight);
}
I am calling the method LoadAppStyle().loadAppStyle() from the file load_app_style:
class LoadAppStyle {
loadAppStyle() async {
String jsonData =
await rootBundle.loadString('assets/config/app_style.json');
Map<String, dynamic> data = jsonDecode(jsonData);
var getHeight = double.parse(data["app_bar"]["app_bar_height"]);
return getHeight;
}
}
In the load_app_style.dart file i grab the value of app_bar_heigt from the app_style.json
in app_style.json i have key app_bar_height where i want to change the value manually to change the height of the App bar
{
"app_bar":
{
"app_bar_height": 78
},
}
But for some reason i get the error : type 'Future<dynamic>' is not a subtype of type 'double'
You can add the type to your loadAppStyle method. Since your method is async it returns a Future.
Future<double> loadAppStyle() async {
...
return getHeight;
}
Now your error should be
type 'Future<double>' is not a subtype of type 'double'
Since your method returns a Future you have to use await to get the value.
loadAppStyle() // Future<double>
await loadAppStyle() // double
If you want to use a value of a Future inside a Widget, have a look at FutureBuilder.
For your case you could e.g. use the FutureBuilder to retrieve the height and then pass it to AppBarBase
FutureBuilder<double>(
future: loadAppStyle(),
builder: (context, snapshot) {
if(snapshot.hasData) {
return AppBarBase(height: snapshot.data);
} else {
return const Center(child: CirclularProgressIndicator));
}
}
)
And change your AppBarBase to the following.
class AppBarBase extends StatelessWidget implements PreferredSizeWidget {
AppBarBase({
Key? key,
required this.height,
}) : super(key: key);
final double height;
#override
Widget build(BuildContext context) {
return AppBar();
}
#override
Size get preferredSize => Size.fromHeight(height);
}
In your example, loadAppStyle() has no defined return type (dynamic) and it is marked as async (Future), hence the return type of this function is Future<dynamic>. Size.fromHeight function requires the double value, hence you get this error - the expected type is double, but Future<dynamic> was found here.
To resolve the type differences, you should set the return type of a function:
class LoadAppStyle {
Future<double> loadAppStyle() async {
String jsonData =
await rootBundle.loadString('assets/config/app_style.json');
Map<String, dynamic> data = jsonDecode(jsonData);
var getHeight = double.parse(data["app_bar"]["app_bar_height"]);
return getHeight;
}
}
Now, since your function is async, you must wait for your Future to finish and only then you could retrieve the double value. It would look something like this:
class AppBarBase extends StatelessWidget implements PreferredSizeWidget {
late double appBarHeight = await LoadAppStyle().loadAppStyle(); // Throws error
AppBarBase({Key? key}) : super(key: key);
#override
Widget build(BuildContext context) {
return AppBar();
}
#override
Size get preferredSize => Size.fromHeight(appBarHeight);
}
However, this throws an error since you cannot use the asynchronous code when initialising a value this way. What could be a better way to do this is to wait for this value somewhere outside of your widget and pass the result via the constructor:
class AppBarBase extends StatelessWidget implements PreferredSizeWidget {
final double appBarHeight;
AppBarBase({
required this.appBarHeight,
Key? key,
}) : super(key: key);
#override
Widget build(BuildContext context) {
return AppBar();
}
#override
Size get preferredSize => Size.fromHeight(appBarHeight);
}
This way, you separate your UI code from the widget. Anyway, the way of keeping this UI-specific configuration inside the JSON file sounds way overengineered - consider just passing this value via constructor directly, like: AppBarBase(appBarHeight: 78).
I am trying to assign an async function to an instance variable.
I have tried:
class TextBox extends StatefulWidget {
late String message;
late bool isPass;
late Function(String?) callback;
TextBox(String message, bool isPass, Future<dynamic> Function(String?) async func) {
this.message = message;
this.isPass = isPass;
this.callback = func;
}
}
But get the following exception:
Expected to find ')'
I know why I get the error. I just dont know the proper syntax to do this in dart.
You can use this line of code:
final Future<void> callback;
You can change the void type to any data type you want.
You do not need to use the keyword async because making the function return a Future is enough.
Also, you can write a constructor without a body.
class TextBox extends StatefulWidget {
final String message;
final bool isPass;
final Future<dynamic> Function(String?) callback;
TextBox(this.message, this.isPass, this.callback);
...
import 'package:flutter/material.dart';
class MyHomePage extends StatefulWidget {
late String message;
late bool isPass;
late Future<String> data;
MyHomePage(this.message, this.isPass, this.data);
#override
State<MyHomePage> createState() => _MyHomePageState();
}
class _MyHomePageState extends State<MyHomePage> {
String getData = '';
#override
MyHomePage get widget => super.widget;
#override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: Text('$getData Test Demo'),
),
body: Container(),
);
}
getFutureData() async {
getData = await widget.data;
setState(() {});
}
}
Achieve like this have used String you can use your custom class
I've a question:
In my Widget build(BuildContext context), I want to store a certain value,
final userName = book.owner
(book is the reference to the certain value from Firestore)
But it's done not in the right way to my lack of knowledge. I'd appreciate if someone could guide through that.
Thank you in advance!
Snippet of my code
class BookView extends StatefulWidget {
final Book book;
BookView({Key key, #required this.book}) : super(key: key);
DatabaseMethods databaseMethods = new DatabaseMethods();
var userName;
#override
_BookViewState createState() => _BookViewState(book);
}
class _BookViewState extends State<BookView> {
Book book;
_BookViewState(this.book);
String userName;
#override
void initState() {
userName = book.owner;
super.initState();
}
// final Book book;
createChatroomAndStartConversation({var userName}) {
if (userName != Constants.myName) {
String roomId = getChatRoomId(userName, Constants.myName);
List<String> users = [userName, Constants.myName];
Map<String, dynamic> chatRoomMap = {
"Users": users,
"roomId": roomId,
};
DatabaseMethods().createChatRoom(roomId, chatRoomMap);
Navigator.push(
context,
MaterialPageRoute(
builder: (context) => ConversationScreen(roomId, userName)),
);
} else {
print("You cannot send msg to your self");
}
}
#override
Widget build(BuildContext context) {
//widget.book;
return Scaffold(
resizeToAvoidBottomInset: false,
appBar: AppBar(
...
FlatButton(
child: Text(
"Get contact with",
style: TextStyle(color: Colors.white),
),
color: Colors.blue,
onPressed: () {
createChatroomAndStartConversation(
userName: userName);
...
}
Snippet of Value not in range: 1
getChatRoomId(String a, String b) {
if (a.substring(0, 1).codeUnitAt(0) > b.substring(0, 1).codeUnitAt(0)) {
return "$b\_$a";
} else {
return "$a\_$b";
}
}
It's not a good practice to store any data in build() method, because this method is invoked too many times to do the such kind of move. Consider using StatefulWidget to store any state you have in the widget, for the very beginning. When you use this widget, you can define this all in such way:
class YourWidget extends StatefulWidget {
#override
_YourWidgetState createState() => _YourWidgetState();
}
class _YourWidgetState extends State<YourWidget> {
String userName;
#override
void initState() {
userName = book.owner;
super.initState()
}
#override
Widget build(BuildContext context) {
return Container(child: Text(userName),);
}
}
Here, in initState() you can retrieve value from book and set it to userName. But for more complex and bigger applications, consider using StateManagement solutions and some kind of architectural patterns i.e. Riverpod, Provider, MobX, BLoC.. Because changing the state via setState() method will cause rebuilding whole child widget tree, which could freeze whole UI in complex app.
UPD to 'Snippet of my code':
According to your code, if you are using a 'book' from Widget, not its state - use widget.book, in such way you have access to widget members, because of this you don't need a constructor of state. So, due to these changes, your code might looks like:
class BookView extends StatefulWidget {
final Book book;
BookView({Key key, #required this.book}) : super(key: key);
// You DON'T need this here, because you are retrieving these methods
// inside your state via DatabaseMethods constructor
DatabaseMethods databaseMethods = DatabaseMethods();
#override
_BookViewState createState() => _BookViewState(book);
}
class _BookViewState extends State<BookView> {
String userName;
#override
void initState() {
// Using widget.book to retrieve Book object from state's widget
userName = widget.book.owner;
super.initState();
}
createChatroomAndStartConversation({var userName}) {
if (userName != Constants.myName) {
String roomId = getChatRoomId(userName, Constants.myName);
// Also, it's just a recommendation, try to omit local variables types
// because they are already known with List type (String). Also, this
// all is about chatRoomMap
var users = <String>[userName, Constants.myName];
final chatRoomMap = <String, dynamic>{
"Users": users,
"roomId": roomId,
};
DatabaseMethods().createChatRoom(roomId, chatRoomMap);
Navigator.push(
context,
MaterialPageRoute(
builder: (context) => ConversationScreen(roomId, userName)),
);
} else {
print("You cannot send msg to your self");
}
}
#override
Widget build(BuildContext context) {
// your widgets here
}
}
UPD 2:
Second trouble and issue with 'Snippet of Value not in range: 1'. I could to reproduce it with given value of 'a' as empty string. So, your function invocation is like getChatRoomId('', 'user123'), because of empty 'userName', substring function can't take values from range [0, 1), so exception is raised.