void main() {
Login('John');
}
class User {
String name;
User(this.name);
}
class Login {
User user;
Login(this.user);
print(user.name);
}
Error: The argument type 'String' can't be assigned to the parameter type 'User'.
How to fix this error or use other use case;
Your Login constructor takes a User as first argument, not a String. You need to construct a User object and provide that to your Login constructor.
void main() {
Login(User('John'));
}
Another issue in your code is the print(user.name); line directly in your Login class definition. That is not allowed. If you want to execute code as part of calling the constructor, you can do that like:
Login(this.user) {
print(user.name);
}
So the full solution is going to look like this:
void main() {
Login(User('John'));
}
class User {
String name;
User(this.name);
}
class Login {
User user;
Login(this.user) {
print(user.name);
}
}
Related
I would like to make a generic class which only accepts Lists as a type parameter. But I also want the type parameter of the List. Something like this:
class MyClass<L extends List<T>> {
T foo() {
// ....
}
}
The problem is that that does not work. T is not found. But this does:
class MyClass<L extends List<T>, T> {
T foo() {
// ....
}
}
My only issue with this is that I have to always pass in the extra parameter T which should be inferred from the List type.
var instance = MyClass<List<int>>();
var instance = MyClass<List<int>, int>(); // Extra int kind of redundant
Is there any workaround to this?
The solution is similar to the one provided in this question (the same problem, but in Java): basically, you can't do that in Dart. What you can do is
create a new subclass:
class MyClass2<T> extends MyClass<List<T>, T> { ... }
or
create a factory method:
class MyClass<L extends List<T>, T> {
static MyClass<List<T>, T> fromList<T>(List<T> list) {
return MyClass(...);
}
}
How can I abstract that a methods has optional parameters?
abstract class CopyWith<T>{
T copyWith({}); // Error : Expected an identifier.
}
If I add an identifier like {test} it works and subclasses can have additional arguments
What I want to achieve?
I have a complex state manager, I make some abstraction , the following code is a minimal code, show my problem
import 'dart:collection';
abstract class CopyWith<T> {
T copyWith(OPTIONAL_NAMED_ARGUMENTS);
}
abstract class Manager<K, V extends CopyWith> {
final _map = HashMap<K, V>();
add(K key,V value){
_map[key] = value;
}
void copyWith(K key,OPTIONAL_NAMED_ARGUMENTS) {
assert(key != null);
if (_map.containsKey(key)) {
_map[key].copyWith(OPTIONAL_NAMED_ARGUMENTS);
}
}
}
class User implements CopyWith {
final int id;
final String name;
User({this.id, this.name});
User copyWith({int id, String name}) {
return User(
id: id ?? this.id,
name: name ?? this.name,
);
}
}
class UserManager extends Manager<int, User> {}
void main() {
final userManager = UserManager();
userManager.add(1,User(1,'test'));
userManager.copyWith(1,{test:'test2'})
}
As some one who has faced this issue in my library, I would say the only way is to not put a copyWith in your base class.
Why? Because you should only make a function polymorphic when there IS actually a shared calling convention and behavior. In your example, The way that these two classes perform copyWith is just different. It is, and should be, an error to send a name to Manager.copyWith, because Manager does not have a name to begin with. If you encounter a name inside a Manager.copyWith, that means there is some serious error in your code.
Also, if you actually try to invoke copyWith, as a responsible programmer, you will probably check if you are allowed to pass a name, which is,
if (someObj is User) {
someObj.copyWith(key, name: name);
} else if (someObj is Manager) {
throw IllegalStateError('You should not pass a name to a Manager! What am I supposed to do with the name now?');
}
There, you have already done type checking, so no need to make copyWith polymorphic.
However, some common behaviors can be made polymorphic, like updateKey. You can make Keyable as an interface, and Keyable updateKey(Key key) as an abstract method, and delegate to a non-polymorphic copyWith inside each subclasses.
I am trying to create a base class for my models but I am struggling with the error The name 'cls' isn't a type so it can't be used as a type argument.. So, how can I pass the object's constructor to the Hive.box method?
import 'package:hive/hive.dart';
class AppModel {
#HiveField(0)
int id;
#HiveField(1)
DateTime createdAt;
#HiveField(2)
DateTime updatedAt;
save() async {
final Type cls = this.runtimeType;
// The name 'cls' isn't a type so it can't be used as a type argument.
final Box box = await Hive.openBox<cls>(cls.toString());
await box.put(this.id, this);
return this;
}
}
#HiveType(typeId: 0)
class UserModel extends AppModel {
#HiveField(3)
String email;
#HiveField(4)
String displayName;
}
void main() {
final UserModel user = UserModel()
..email = 'user#domain.com'
..displayName = 'john doe';
user.save().then(() {
print('saved');
});
}
Dart does not have a way to refer to the dynamic type of this (a "self type").
The way such things are often handled is to have a self-type as type argument, so:
class AppModel<T extends AppModel> {
save() async {
final Box box = await Hive.openBox<T>(T.toString());
await box.put(this.id, this as T);
return this;
}
...
and then ensure that each subclass tells the superclass what type it is:
class UserModel extends AppModel<UserModel> {
...
}
(or, if you expect to subclass UserModel eventually:
class UserModel<T extends UserModel> extends AppModel<T> {
...
}
so that a subclass can still pass its type through).
You are also talking about constructors, and for that there is no easy solution.
Dart's type parameters are types, not classes. You cannot access static members or constructors from a type variable, and there is also no other way to pass a class around.
The only way you can have something call a constructor that it doesn't refer to statically, is to wrap the constructor call in a function and pass that function.
(I can't see how you need the constructor here).
Consider the following generic class:
class Foo<T extends Bar<dynamic>> {}
How can I obtain the generic argument of Bar at compile time?
I would expect the following code to print int, but it prints dynamic:
class Bar<T> {}
class Foo<T extends Bar<dynamic>> {
Foo(this.value) {
_debugBarType(value);
}
final T value;
void _debugBarType<A>(Bar<A> value) {
print(A);
}
}
void main() {
Foo<Bar<int>>(Bar());
}
I know I could do:
class Foo<A, Bar<A>> {}
But I would like to define the Foo class using a single generic parameter instead of two.
This is currently not possible to do on any class.
There's currently a pending feature request on Dart to support such use-case: https://github.com/dart-lang/language/issues/620
It's worth noting that there's a workaround if you can add a method on the class concerned.
For example, say you have a generic class Model<T>:
class Model<T> {
T value;
}
Then you can add a "capture" method as such:
class Model<T> {
T value;
R capture<T>(void cb<P>(P value)) => cb<T>(value);
}
This then allows you to write:
void someFunction<T extends Model<dynamic>>(T model) {
model.capture(<P>(value) {
// `P` will be the captured type instead of `dynamic`
});
}
I'm get null returned when attempting to access a subclass static variable through a overridden subclass accessor:
library resource;
abstract class Resource
{
String name;
String description;
Resource(this.name, this.description);
Resource.map(Map data)
{
...
_getDb()[this] = data;
}
abstract Map _getDb();
}
class Skill extends Resource
{
static Map _skills = {}
Skill.map(Map data) : super.map(data);
Map_getDb()
{
return _skills;
}
}
import 'resource.dart'
void main() {
useVMConfiguration();
test('constructor', () {
Skill skill = new Skill.map({
'name': 'foo'
});
}
}
Here I'm trying to call _getDb() on the (hopefully) now constructed subclass in the super constructor. Despite _skills being instantiated, _getDb() returns null.
Is this possible?
EDIT:
_skills is not present when inspecting this at _getDb():
this Skill [id=0]
description "bar" [id=19]
name "foo" [id=18]
Your example has several flaws as DartEditor shows.
Map_getDb() is missing a space between Map and _getDb().
Is this only in your question or in the code you run too?
abstract Map _getDb(); is also a syntax error.
In Dart a method is made abstract when you don't provide an implementation (; instead of {})
After this fixes the code works fine.