I'm new to flutter, and i bumped into a problem.
I have a Feed model in my app that looks like this:
import 'package:uuid/uuid.dart';
class Feed {
// Static Members
var uuid = new Uuid();
// Members
String id;
bool isScheduled;
DateTime createdTime;
DateTime feedingTime;
String deviceId;
// Constructors
Feed({this.feedingTime, this.deviceId, this.isScheduled}) {
id = uuid.v4();
createdTime = DateTime.now();
}
Feed.fromDevice(deviceId) {
Feed(deviceId: deviceId, feedingTime: DateTime.now(), isScheduled: false);
}
}
Now i have my AddFeedForm that i'm trying to initialize with default values, in the InitState:
class _AddFeedFormState extends State<AddFeedForm> {
// Final Members
final _formKey = GlobalKey<FormState>();
final List<Machine> _devices = machinesFromServer;
// Members
Feed _feed;
#override
void initState() {
_feed = Feed.fromDevice(_devices.first.id);
super.initState();
}
But somehow after the initState the _feed parameter stays null!
Any ideas?
But somehow after the initState the _feed parameter stays null!
Are you sure this is the case, and not that you're getting a Feed instance that has null fields?
It looks like your named constructor is incorrect:
Feed.fromDevice(deviceId) {
Feed(deviceId: deviceId, feedingTime: DateTime.now(), isScheduled: false);
}
Here you're calling the default Feed constructor inside a named constructor, but not doing anything with the result - this is creating another Feed and then throwing it away. The one returned by the named constructor has not been initialised.
What you probably wanted was this:
Feed.fromDevice(deviceId):
this(deviceId: deviceId, feedingTime: DateTime.now(), isScheduled: false);
This makes the fromDevice constructor call the default constructor for initialisation of the instance, rather than creating another copy that goes unused.
Another option would be to make it a static method:
static fromDevice(deviceId) {
return Feed(deviceId: deviceId, feedingTime: DateTime.now(), isScheduled: false);
}
There wouldn't be much difference in this case.. Constructors seem nicer, but sometimes you might find that you want to a) make initialisation async (static methods can return a Future<Feed> but constructors cannot or b) do more processing of the arguments before they're passed to the real constructor that might not fit nicely in the initialiser call.
Related
I am making first steps with Riverpod and just want to check if my understanding of handling changes of some data class properties using Riverpod is correct.
Imagine, I have a data class like that:
class MyClass {
final String name;
final int id;
const MyClass(this.name, this.id);
}
Then I create a StateNotifier:
class MyClassStateNotifier extends StateNotifier<MyClass> {
MyClassStateNotifier(MyClass state) : super(state);
void setName(String name) {
state.name = name;
}
}
And this won't work - UI will not be rebuilt after calling setName this way.
So I need to modify classes in the following way:
class MyClass {
final String name;
final int id;
const MyClass(this.name, this.id);
MyClass copyWith({name, id}) {
return MyClass(name ?? this.name, id ?? this.id);
}
}
and the StateNotifier as following:
class MyClassStateNotifier extends StateNotifier<MyClass> {
MyClassStateNotifier(MyClass state) : super(state);
void setName(String name) {
state = state.copyWith(name: name);
}
}
This pair will work and the UI will be rebuilt.
So, my question: does one always need to reinstantiate the object in this way?..
From my perspective, this is a bit strange (simple datatypes like String / int do not require this) and the boilerplate for copyWith method might become pretty huge if I have a dozen of object's properties.
Is there any better solution available for Riverpod or is it the only one and correct?..
Thanks in advance! :)
To trigger a state change you have to use the state setter. The implementation looks like this:
#protected
set state(T value) {
assert(_debugIsMounted(), '');
final previousState = _state;
_state = value;
/// only notify listeners when should
if (!updateShouldNotify(previousState, value)) {
return;
}
_controller?.add(value);
// ...
The internal StreamController<T> _controller needs to be triggered (add), to notify listeners (in this case riverpod) about updates.
By using state.name = something you're not informing the StateNotifier about a new state (not calling the state setter). Only your object holds the new value but nobody was notified.
Your state is mutable and that very often leads to such misbehavior. By using an immutable object you can prevent such errors in the first place. Write it yourself or use freezed.
Learn more about immutability in my talk
e.g. I have class ProfileModel with bunch of fields
many of them don't have default values unless they're initialising when I get user info from backend
with riverpod I need to write something like
final profileProvider = StateNotifierProvider((ref) => ProfileState());
class ProfileState extends StateNotifier<ProfileModel> {
ProfileState() : super(null);
}
I understand I need to pass something like ProfileState.empty() into super() method instead passing null
but in this case I have to invent default values for every ProfileModels fields
this sounds weird for me, I don't want to break my head to care about empty or default state of EVERY model in project
in my example there are no default values for user name, age etc
this is pure immutable class
what I'm doing wrong or missing?
or I can declare model as nullable extends StateNotifier<ProfileModel?>
but I'm not sure is this a good way
It is fine to use the StateNotifier with a nullable model. If you semantically want to indicate the value can be actually absent, I would say that that having null is alright.
However, what I usually do and what I think is better, is create a state model that contains the model, but also properties that relate to the different states the app could be in.
For example, while fetching the data for the model from an API, you might want to have a loading state to show a spinner in the UI while waiting for the data to be fetched. I wrote an article about the architecture that I apply using Riverpod.
A simple example of the state model would be:
class ProfileState {
final ProfileModel? profileData;
final bool isLoading;
ProfileState({
this.profileData,
this.isLoading = false,
});
factory ProfileState.loading() => ProfileState(isLoading: true);
ProfileState copyWith({
ProfileModel? profileData,
bool? isLoading,
}) {
return ProfileState(
profileData: profileData ?? this.profileData,
isLoading: isLoading ?? this.isLoading,
);
}
#override
bool operator ==(Object other) {
if (identical(this, other)) return true;
return other is ProfileState &&
other.profileData == profileData &&
other.isLoading == isLoading;
}
#override
int get hashCode => profileData.hashCode ^ isLoading.hashCode;
}
Lets say a User in my App can note his weight. Now I want to use this weight in many other Widgets all over my App for example to calculate some data, which depends on the user weight.
Is it a good practise to use a static variable like this:
class UserManager {
static double weight;
}
So now I have access to the User weight in every Class and can make calculations for example:
double value = UserManager.weight * 0.4;
Is this a good practise or are there some better solutions?
You can use GetStorage()
final appData = GetStorage();
appData.writeIfNull("data", false);
bool yourVar = appData.read("data");
or SharedPreferences
var yourData;
final Future<SharedPreferences> _prefs = SharedPreferences.getInstance();
void _getSomeTh() async {
final prefs = await _prefs;
final result = prefs.getBool("data");
yourData = result;
}
It would be better not to work with static variables, but to have an object that you distribute to all widgets in the widget tree. You can achieve this, for example, with an InheritedWidget or with packages such as BLoC or Provider. Making such data static seems to me to be an anti-pattern.
With the BLoC pattern:
class User {
final double weight;
User({required this.weight});
}
class UserManager extends Cubit<User>{
...
}
This way you have a state managing system with which you could very easily rebuild all the widgets concerned when the weight changes.
For a simple requirement as this, the static variable approach might be fine. However, for a more complex object that you wish to share in a hierarchy of widgets, you should use packages like provider
For example, you have a User object that you wish to share app-wide, then
class User with ChangeNotifier {
late String uuid;
late String name;
// Our one and only one static instance on which we operate
static late User? instance;
// There is a good chance that the init functions like below are async too
static void initUser(String uuid, String name) {
instance = User._init(uuid, name);
}
// It is usually the 'instance' that calls the below function
void changeFunction(/*params*/){
// Change the user instance, say, assign it to null if a user logs out
// More importanly, notify the listeners about the change
notifyListeners();
}
// Note this constructor is not exposed to the public.
User._init(this.uuid,this.name);
}
Then propagate the User object using a ChangeNotifierProvider using the guidelines mentioned here
I need to create a class called GeneralAppAndDeviceDetails which has the following fields:
import 'package:flutter/material.dart';
class GeneralAppAndDeviceDetails {
final bool isDarkModeEnabled;
final double deviceWidth;
final double deviceHeight;
final Color primaryColor;
final Color secondaryColor;
}
It basically stores general device and app information at the start of the program so that I don't need to create a new instance of Theme or MediaQuery class whenever I want to access these details.
The problem I'm facing is that how can I write this class so that after the fields' values are assigned, They will be unmodifiable? (so that nothing can change the field values)
(I tried to create a singleton class but I need to pass the values to the constructor and by using factory and private constructor, A user can create new classes with different parameters passed to the factory.)
The thing I need is to have static fields that can receive a value once and become unmodifiable after that. How can I achieve something similar?
Thank you
Update:
I wrote the class as below:
import 'package:flutter/material.dart';
class GeneralAppAndDeviceDetails {
final bool isDarkModeEnabled;
final double deviceWidth;
final double deviceHeight;
final Color primaryColor;
final Color secondaryColor;
static bool _isAlreadyCreated = false;
static GeneralAppAndDeviceDetails _instance;
factory GeneralAppAndDeviceDetails(bool isDarkModeEnabled, double deviceWidth,
double deviceHeight, Color primaryColor, Color secondaryColor) {
if (_isAlreadyCreated == false) {
_isAlreadyCreated = true;
_instance = GeneralAppAndDeviceDetails._(isDarkModeEnabled, deviceWidth,
deviceHeight, primaryColor, secondaryColor);
}
return _instance;
}
const GeneralAppAndDeviceDetails._(this.isDarkModeEnabled, this.deviceWidth,
this.deviceHeight, this.primaryColor, this.secondaryColor);
}
I use a flag to check if an instance was created before or not in here and with this code, a similar instance will be returned every time but is it the best way to achieve this?
This is your singleton class
class Test{
final String str;
static Test _singleton;
Test._internal({this.str});
factory Test(String str) {
return _singleton ??= Test._internal(
str: str
);
}
}
example code for you to try and test
void main() {
Test obj = Test('ABC');
print(obj.str);
Test obj1 = Test('XYZ');
print(obj1.str);
}
class Test{
final String str;
static Test _singleton;
Test._internal({this.str});
factory Test(String str) {
return _singleton ??= Test._internal(
str: str
);
}
}
try running this in dartpad for better understanding
you can make the class as singleton and then make these fields as private, accessible only through getters and setters, inside the setter you can check and discard the new value if there is already some value assigned to the field.
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];
}
}