I'm in Flutter, and I know something about React Native. In React Native I create optional props like this:
foo?: boolean;
How do I do the same thing in Flutter?
I looked here and didn't find it.
The same way to you reproduce optional props in Flutter is something like this:
class MyClass {
final String myProperty;
final List<int> anotherProp;
final String? nullSafetyProp; //can be optional
late final String lateProp; // shold be initialized in Future. Before read the value
MyClass({
required this.myProperty,
required this.anotherProp,
this.nullSafetyProp, // optional property
required this.lateProp
});
}
about keyword final:
https://dart.dev/guides/language/language-tour#final-and-
about
nullsafety: https://dart.dev/null-safety
late variables: https://dart.dev/null-safety/understanding-null-safety#late-variables
late final variables: https://dart.dev/null-safety/understanding-null-safety#late-final-variables
Related
I'm new to Flutter development and trying to learn.
I want to create a model with a constructor, one of which contains a field of type DateTime which is optional.
I tried by making it like this:
import 'package:equatable/equatable.dart';
class Customer extends Equatable {
final int indexs;
final DateTime apply_date;
Customer({
required this.indexs,
this.apply_date,
});
#override
List<Object?> get props => throw UnimplementedError();
}
But an error message appears like this
The parameter 'apply_date' can't have a value of 'null' because of its
type, but the implicit default value is 'null'. Try adding either an
explicit non-'null' default value or the 'required' modifier.
I've tried to learn from this and this reference, and what I understand there are 3 ways:
Include required modifiers
Set initial value
Nulllabel parameter / Fill it with (?) => I don't understand this
So how to do this properly?
I don't want to make this field required, because it's optional.
I also don't know what to fill if I want to fill it with an initialvalue.
Thank you!
Making the attribute nullable is the same as making it an optional attribute.
You can do that by adding ? behind the attribute's type.
class Customer extends Equatable {
final int indexs;
final DateTime? apply_date;
Customer({
required this.indexs,
this.apply_date,
});
#override
List<Object?> get props => throw UnimplementedError();
}
please help me solve the following problem.
This part of my code:
class MeditationCard extends StatelessWidget {
const MeditationCard({
required this.title,
required this.image,
this.route});
final String title;
final String image;
final String route;
...
I need the route variable as optional, but when I remove the flag, I get an error and ask me to make the variable mandatory.
Tried different approaches but didn't work for me
This alert dialog
The parameter 'route' can't have a value of 'null' because of its
type, but the implicit default value is 'null'.
This happened because of null safety check.In order to set nullable variable, you should use ?, try this:
final String? route;
You can make it nullable (?) or define default value
Example for nullable
class MeditationCard extends StatelessWidget {
const MeditationCard({
required this.title,
required this.image,
this.route});
final String title;
final String image;
final String? route;
I know you got the proper answer to your question. But I wanted to mention another method to solve this issue that might help you in the future, which is to provide a default value. This way you avoid weird null errors and null checking.
class MeditationCard extends StatelessWidget {
const MeditationCard({
required this.title,
required this.image,
required this.route});
final String title;
final String image;
final String route = 'default route';
}
This way you ensure that the property 'route' has a value and is never null. And you can override it when you need to.
I'm trying to add a positional parameter with default value with GetX in Flutter but I got an error.
import 'package:get/get.dart';
class MainParentModel {
int id;
String name;
RxBool isSelected;
MainParentModel(this.id, this.name, {this.isSelected = false.obs});
}
Error: The default value of an optional parameter must be constant.dartnon_constant_default_value
How can I add this default value?
Thank you
.obs is a getter that returns an Rx<T> of whatever type you add it to. But it's not a constant value so that won't work. You'll notice that if you just use a regular bool and remove .obs the error goes away.
If it has to be type RxBool, then I'm pretty sure your only option is making it required and passing in false.obs whenever you create a new MainParentModel.
MainParentModel(this.id, this.name, {required this.isSelected});
Then a new instance would look like this.
final model = MainParentModel(1, 'Bob', isSelected: false.obs);
Another approach is creating reactive objects where you'll need, I never use classes with reactive variables, for example:
class MainParentModel {
int id;
String name;
bool isSelected;
MainParentModel(this.id, this.name, {this.isSelected = false});
}
In the controller:
Rx<MainParentModel> mainParentModel = MainParentModel().obs;
if you need to rebuild the page when changing isSelected variable:
mainParentModel.update((model){
model.isSelected = true;
});
or
mainParentModel.refresh();
This will trigger rebuild all Obx that you use mainParentModel.value.?
I have a class - Setting. This class looks as follows:
abstract class BaseSetting {
final dynamic value;
final String key;
final bool secure;
final String description;
//...
I also have another class for other kinds of Settings - like numbers, ranges, text, etc. I want the BaseSetting to have a value ofc, (I think), so I tried BaseSetting<T>and the value not being dynamic - but T.
I am not sure if my approach will work but the idea is that EVERY setting has a value, but every child-setting has a fixed type for that value.
(This is my approach to Settings (refresh / minute, language, metric / imperial etc.) by the way - if what I want to achieve is possible but it is a bad way to do so, please comment so I can look at a better approach.)
try to do like this:
abstract class BaseSetting<T> {
final T value;
final String key;
final bool secure;
final String description;
BaseSetting(
this.value,
this.key,
this.secure,
this.description
);
}
I'm learning flutter coming from a react background. I want to use my model in another class.
This is my model
class User {
final String id;
final String userName;
User({
this.id,
this.userName,
});
}
On my Widget i want to use the properties of that model, so i can get some type safe.
class _SignUpScreenState extends State<SignUpScreen> {
#override
Widget build(BuildContext context) {
final User _user; // I get an error on this line.
final GlobalKey<FormState> _formKey = GlobalKey<FormState>();
I get teh following error.
The value of the local variable '_user' isn't used.
Try removing the variable, or using it.dartunused_local_variable
The final variable '_user' must be initialized.
Try initializing the variable.
I'm a little bit confused how do you properly infer a model on class widget?
Think about the model as a template - the way you are initializing the model - means there is nothing in it, and using the final variable means it will never change.
This causes an error, because the object is effectively null, and always will be.
If you initialize a variable with final you need give it data:
final User _user = User(id: 1, userName: test);
print(_user);
//Prints: Instance of User
Otherwise, don't use the final variable, and you can assign data to _user later"
User _user;
_user = User(id: 1, userName: test);
print(_user);
//Prints: Instance of User
I will explain what your error means one by one.
Flutter will have a warning that you are creating a value that you aren't using.
The value of the local variable '_user' isn't used.
This is what they suggested:
Try removing the variable, or using it.dartunused_local_variable
These two error code is saying that your variable has to be instantiated:
The final variable '_user' must be initialized.
Try initializing the variable.
The solution to your problem is to instansiate User:
final User _user=User(id:"myId",userName:"myUserName");
Note that the final keyword is means that you are declaring a variable that will never have its value change.