This is my Notifier:
class Counter extends Notifier<int> {
final int initial;
Counter(this.initial);
#override
int build() => initial;
}
I need to pass initial value to it, but I'm unable to do that using the family modifier anymore.
// Error
final counterProvider = NotifierProvider.family<Counter, int, int>((initial) {
// How to get the initial value to pass here?
return Counter(initial);
});
The syntax for using family/autoDispose using Notifier/AsyncNotifier is different. You're supposed to change the inherited type
So instead of:
final provider = NotifierProvider(MyNotifier.new);
class MyNotifier extends Notifier<Value> {
With family you should do:
final provider = NotifierProvider.family(MyNotifier.new);
class MyNotifier extends FamilyNotifier<Value, Param> {
And the same reasoning applies with autoDispose.
Related
When using Flutter and Riverpod, how do I update its values from my business logic?
I understand that I can get and set values from the UI side.
class XxxNotifier extends StateNotifier<String> {
XxxNotifier() : super("");
}
final xxxProvider = StateNotifierProvider<XxxNotifier, int>((ref) {
return XxxNotifier();
});
class MyApp extends HookConsumerWidget {
#override
Widget build(BuildContext context, WidgetRef ref) {
// getValue
final String value = ref.watch(xxxProvider);
// setValue
context.read(xxxProvider).state = "val";
return Container();
}
}
This method requires a context or ref.
How do I get or set these states from the business logic side?
Passing a context or ref from the UI side to the business logic side might do that, but I saw no point in separating the UI and business logic. Perhaps another method exists.
Perhaps I am mistaken about something. You can point it out to me.
You can pass ref in your XxxNotifier class:
class XxxNotifier extends StateNotifier<String> {
XxxNotifier(this._ref) : super("");
final Ref _ref;
void setNewState() {
state = 'to setting';
// use `_ref.read` to read state other provider
}
}
final xxxProvider = StateNotifierProvider<XxxNotifier, int>((ref) {
return XxxNotifier(ref);
});
// or using tear-off
final xxxProvider = StateNotifierProvider<XxxNotifier, int>(XxxNotifier.new);
You can create methods in your XxxNotifier class to modify the state of your provider.
For example, your notifier class can look like this.
class TodosNotifier extends StateNotifier <List<Todo>> {
TodosNotifier(): super([]);
void addTodo(Todo todo) {
state = [...state, todo];
}
}
You can then read the provider in a callback.
ref.read(xxxProvider.notifier).addTodo(todo);
I was reading a flutter code as below:
import 'package:flutter/material.dart';
import 'package:get/get.dart';
import 'package:test_get_app/user_controller.dart';
class AuthMiddleware extends GetMiddleware {
final authService = UserController.findOrInitialize; // Here is error, this line can't find UserController
#override
int? get priority => 1;
bool isAuthenticated = false;
#override
RouteSettings? redirect(String? route) {
isAuthenticated = true;
if (isAuthenticated == false) {
return const RouteSettings(name: '/login');
}
return null;
}
}
When I reached to the following line, I couldn't understand it's syntax and how does it work?
int? get priority => 1;
int? Means it is an int but the int can be null
=> 1 Means () {return 1;}
This is a so-called getter. Getters can be used to provide read access to class properties.
They can also return values directly, like in your case.
They are accessed like properties of the class they are declared in:
final middleWare = AuthMiddleware();
final priority = middleWare.priority;
In your case the getter probably must or can be implemented (see the #override annotation), since all implementations of a middleware must declare their priority, I guess. Since the declared type is int? it may also return null instead of an integer.
Getters can be declared using an expression. Like in your case. Using a block body does also work:
int? get priority {
return 1;
}
I am sure there is a way to return a dynamic or generic instance of a class from a riverpod FutureProvider?
I can fetch different tracker data from an API. It can be for Physical Activity or Weight data for instance. So far, I tried this but does not work, not even compile:
final trackerDataProvider = FutureProvider.family<AbstractTrackerData, TrackerType>((ref, trackerType) async {
final repository = ref.read(trackerRepositoryProvider);
AbstractTrackerData trackerData =
await repository.getTrackerData(trackerType);
return trackerData;
});
and in my Widget
class PhysicalActivityLandingTrackerScreen extends HookWidget {
#override
Widget build(BuildContext context) {
final trackerData = useProvider(trackerDataProvider(TrackerType.physicalactivity));
...
}
}
and my tracker data classes
class SmokeFree extends AbstractTrackerData {
...
}
class PhysicalActivity extends AbstractTrackerData {
...
}
class Weight extends AbstractTrackerData {
...
}
and the enum
enum TrackerType {
physicalactivity,
exercise,
weeklyweight,
bloodpressure,
saltyfoodsubstitution,
goodfatsubstitution,
medicationuse,
smokefree
}
Yes, because Generic methods have a type parameter (the diamond operator enclosing the type) before the return type of the method declaration. ... Generic methods can have different type parameters separated by commas in the method signature. Method body for a generic method is just like a normal method.
Hence we can say that it is returned by provider.
class LevelUp extends GetxController {
Map<String, String> params = Get.arguments;
var myTest = params.[comLevel];
}
Error report--"The instance member 'params' can't be accessed in an initializer." I am new to programming and this is being called directly from a widget. I checked the LevelUp map and it has contents. The error occurs where I am trying to assign the param value to myTest. It doesn't matter if I put the key in quotes or provide an integer. Any advice would be greatly appreciated.
You can't access params before you've initialized the object. To fix your example, move your myTest initialization into a constructor.
Also, I don't believe you should have a period before [comLevel].
class LevelUp extends GetxController {
Map<String, String> params = Get.arguments;
String myTest;
LevelUp() {
myTest = params[comLevel];
}
}
Null safety update:
Use late keyword: Dart 2.12 comes with late keyword which helps you do the lazy initialization which means until the field bar is used it would remain uninitialized.
class Test {
int foo = 0;
late int bar = foo; // No error
}
Although this question has been answered for the OP's case, I want to offer a solution to those receiving this error in a StatefulWidget scenario.
Consider a situation where you would want to have a list of selectable items that dictate which category to display. In this case, the constructor might look something like this:
CategoryScrollView({
this.categories,
this.defaultSelection = 0,
});
final List<String> categories;
final int defaultSelection;
Note the property defaultSelection is responsible for specifying which category should be selected by default. You would probably also want to keep track of which category is selected after initialization, so I will create selectedCategory. I want to assign selectedCategory to defaultSelection so that the default selection takes effect. In _CategoryScrollViewState, you cannot do the following:
class _CategoryScrollViewState extends State<CategoryScrollView> {
int selectedCategory = widget.defaultSelection; // ERROR
#override
Widget build(BuildContext context) {
...
}
}
The line, int selectedCategory = widget.defaultSelection; will not work because defaultSelection it hasn't been initialized yet (mentioned in other answer). Therefore, the error is raised:
The instance member 'widget' can't be accessed in an initializer.
The solution is to assign selectedCategory to defaultSelection inside of the initState method, initializing it outside of the method:
class _CategoryScrollView extends State<CategoryScrollView> {
int selectedCategory;
void initState() {
selectedCategory = widget.defaultSelection;
super.initState();
}
A simple example, where it shows how we can resolve the above issue,
Example: Create an instance of class B, and pass an instance of class A in the parameter of it
WRONG(Compile time error of initializer):
final A _a = A();
final B _b = B(_a);
shows error: The instance member '_a' can't be accessed in an initializer.
Right:
final A _a = A();
late final B _b;
AppointmentRepository() {
_b = B(_a);
}
#100% working solution
:
Juts place var myTest = params.[comLevel];
below your Build method.
eg.
class LevelUp extends GetxController {
Map<String, String> params = Get.arguments;
Widget build(BuildContext context) {
var myTest = params.[comLevel];
}
}
For me it happened Because i was trying to access a Property of a class instance (Lets Say Class A ) And Use this property to initialize Another Class (Class B) , The Property Was Integer Number and Was Defined
However , Since i didn't Make an Object from "Class A" I can access those propertied Belong to it !
I tried to use this property inside the "Build" Method so that an object is "Created/Built" And it Worked !
I also got the similar error.
And I found the solution as follows.
My first code:
final BuildContext mycontext = GlobalContextClass.navigatorKey.currentContext;
final PsValueHolder psValueHolder = Provider.of<PsValueHolder>(mycontext, listen: false);
Next is the code where the error is fixed:
final PsValueHolder psValueHolder = Provider.of<PsValueHolder>(GlobalContextClass.navigatorKey.currentContext, listen: false);
Instead of defining 2 variables in a row, I placed the first variable directly in the place of the 2nd variable.
Another solution is making your variable, a GetX parameter.
int count_myProducts = cartItems.length; //The instance member 'cartItems' can't be accessed in an initializer. (Documentation)
int get count_myProducts => cartItems.length;
see this video at 27:34
GetX State Management tutorial with Flutter
https://www.youtube.com/watch?v=ZnevdXDH25Q&ab_channel=CodeX
Just carry
var myTest = params.[comLevel];
into Widget build{} below.
like this :
class LevelUp extends GetxController {
Map<String, String> params = Get.arguments;
Widget build(BuildContext context) {
var myTest = params.[comLevel];
}
}
I am passing variables from one activity to another in flutter but getting the error "Instance member ‘latitude’ can’t be accessed using static access" I need it converted in that block so I can assign it to a static URL.
class Xsecond extends StatefulWidget {
final double latitude;
final double longitude;
Xsecond(this.latitude, this.longitude, {Key key}): super(key: key);
#override
_Xsecond createState() => _Xsecond();
}
class _Xsecond extends State<Xsecond> {
static String lat = Xsecond.latitude.toString(); // Error: Instance member ‘latitude’ can’t be accessed using static access
...
followed by
...
String url = 'https://maps.googleapis.com/maps/api/place/nearbysearch/json?location=${lat},$lng&radius=$radius&type=restaurant&key=$api';
...
In your code both latitude and longitude are defined as non-static i.e. are instance variables. Which means they can only be called using a class instance.
class _Xsecond extends State<Xsecond> {
final xsecond = Xsecond();
static String lat = xsecond.latitude.toString();
...
Please read the basics of any Object Oriented Programming language e.g. Dart, java, C++
However, in your context the first class is your StatefullWidget. So you can access that by the widget field of your state class.
FIX:
class _Xsecond extends State<Xsecond> {
static String lat = widget.latitude.toString();
...
This error occurs if you use non-static variable like static. Let's take a look at this example:
class MyPage extends StatefulWidget {
final foo = Foo();
// ...
}
class _MyPageState extends State<MyPage> {
final newFoo = MyPage.foo; // Error
// ...
}
MyPage.foo isn't a static member but you are using if it was.
To solve this issue, you can either make the variable static
static final foo = Foo();
or
Use widget variable to get hold of the underlying variable.
class _MyPageState extends State<MyPage> {
#override
void initState() {
super.initState();
final newFoo = widget.foo; // No Error
}
// ...
}