I have API communication service in my Flutter app with 10+ different services, and 100+ API calls that heed to parse data. In order to reuse code I've decided to create some common parsing code that is going to parse data from API:
ApiResponse handleObjectResponse({
#required http.Response serverResponse,
#required Function objectConstructor,
}) {
if (serverResponse.statusCode == 200) {
dynamic responseObject = objectConstructor(json.decode(serverResponse.body));
return ApiResponse(responseObject: responseObject);
} else {
ApiError error = responseHasError(serverResponse.body);
return ApiResponse(error: error);
}
}
This way I am able to parse JSON object from API in a reusable way no matter what the Object class is, just by passing constructor function to this method.
When I call this method in any of the Services I've created for fetching data like this:
handleObjectResponse(serverResponse: response, objectConstructor: ChartData.fromJson);
I get error: The getter 'fromJson' isn't defined for the class 'ChartData'.
Try importing the library that defines 'fromJson', correcting the name to the name of an existing getter, or defining a getter or field named 'fromJson'.
Where I think the problem is is in this model class and factory statement, but I don't know how to fix it:
class ChartData {
List<ChartDataPoint> points;
ChartData({
this.points,
});
factory ChartData.fromJson(Map<String, dynamic> json) {
List jsonPoints = json["data"];
return ChartData(
points: List.generate(jsonPoints.length,
(i) => ChartDataPoint.fromJsonArray(jsonPoints[i])));
}
}
You cannot pass constructors as functions. You need to create a function what will call the constructor instead:
(int a) => Foo(a);
Just a 2022 update: since 2.15 it's possible by Class.new, see the complete issue: https://github.com/dart-lang/language/issues/216.
class A {
final String a;
const A(this.a);
#override
String toString() => 'A($a)';
}
class B {
final String b;
const B(this.b);
#override
String toString() => 'B($b)';
}
void main() {
final List<Object Function(String)> constructors = [A.new, B.new];
for (final Object Function(String) constructor in constructors) {
final Object instance = constructor('My Constructor Parameter');
if (instance is A) {
print(instance.toString());
}
}
}
Note that if you're using named params, both class constructors must have the same param name, otherwise the constructor signatures won't match and then it will generate this static error:
The element type X can't be assigned to the list type Y.
Related
I have a Flutter class that uses Freezed to create a sealed union that represents either data or an error:
#freezed
class DataOrError<T, E> with _$DataOrError {
const factory DataOrError.loading() = Loading;
const factory DataOrError.data(T data) = DataOrE<T, E>;
const factory DataOrError.error(E error) = DOrError<T, E>;
static DataOrError<T, E> fromEither<T, E>(Either<E, T> val) {
final result = val.fold(
(l) => DataOrError<T, E>.error(l), (r) => DataOrError<T, E>.data(r));
return result;
}
}
I use riverpod so I have a riverpod StateNotifier that looks like:
class RolesNotifier
extends StateNotifier<DataOrError<List<Role>, RoleFailure>> {
final Ref _ref;
StreamSubscription? sub;
RolesNotifier(Ref ref)
: _ref = ref,
super(const DataOrError.loading());
/// Will run the fetch
void fetch() {
// fetch roles
state = const DataOrError.loading();
sub = _ref.read(firebaseRoleService).getRoles().listen((event) {
state = DataOrError.fromEither<List<Role>, RoleFailure>(event);
});
}
// ... this class has been shortened for simplicity.
}
final rolesProvider = StateNotifierProvider.autoDispose<RolesNotifier,
DataOrError<List<Role>, RoleFailure>>((ref) {
return RolesNotifier(ref);
});
When I consume this provider; however, the types for DataOrError are gone:
ref
.read(rolesProvider)
.when(loading: (){}, data: (d) {
// d is dynamic type not List<Role>
}, error: (e){});
For some reason both d and e are dynamic types and not List<Role> & RoleFailure respectively. Everything appears to be typed correctly so why is this not working? I'm not sure if the error is with Freezed or Riverpod. I would like to avoid type casting (i.e. d as List<Role>) because that defeats the purpose of the generics.
The mixin has to be defined with the same generic types Like
class DataOrError<T, E> with _$DataOrError<T,E>
Although if you did not explicitly define the mixin generics ,the runner will mimic the original class types but in that case there will be no relation between them. in our case it will be a different set of T and E that is why it will be dynamic , bottom line is that you have to tell the mixin that it is indeed the same set of types.
Your code seems to be setup correctly, my intuition is that the types are not being inferred because you are using ref.read. This may be intentional in riverpod for some reason.
Finally, using ref.read inside the build method is an anti-pattern and your UI will not change whenever the state notifier updates.
Replace ref.read(rolesProvider) with:
ref.watch(rolesProvider).when(
loading: (){},
data: (d) {}, // Types should be inferred correctly now
error: (e){},
);
I'm getting an error like
The expression doesn't evaluate to a function, so it can't be invoked.
Is there any workaround to achieve this I have different classes being returned from a map according to the key.
void main() {
Map<String, Type> map = {'user': User};
Type T = map['user']!;
User a = T(5);
print(a.id);
}
class User {
User(this.id);
final int id;
}
I gathered the following understanding for creating a singleton in dart with params
class Foo extends ChangeNotifier {
late String channel;
void instanceMemberFunction () {
print('Foo created with channel $channel')
}
static final Foo _instance = Foo._internal();
Foo._internal() {
instanceMemberFunction();
}
factory Foo({
required String channel
}) {
_instance.channel = channel;
return _instance;
}
}
and I am calling the instance like so
Foo({channel: "bar"})
Now I want to have some working that if I use
Foo({channel: "baz"})
Then a new instance is created and it's okay in that case to destroy the old one. How can I achieve this in dart?
It seems like you've copied some existing example for creating a singleton without fully understanding what it's doing and why. The core parts are:
The single instance is stored in a global or static variable.
The class has one or more public factory constructors that returns that global/static variable, initializing it if necessary.
All other constructors for the class are private to force consumers to go through the factory constructors.
Therefore, if you want your factory constructor to replace its singleton based on its argument, you need to:
Make your factory constructor check if the argument is appropriate for the existing instance. If it is, return the existing instance. If not (or if there is no existing instance), create and return a new instance.
Since you need to check if the existing instance is initialized, make it nullable. (You alternatively could initialize it to a non-null sentinel value, e.g. Foo._internal(channel: '').
Pass the argument along to the private constructor.
class Foo extends ChangeNotifier {
final String channel;
void instanceMemberFunction () {
print('Foo created with channel $channel');
}
static Foo? _instance;
Foo._internal({required this.channel}) {
instanceMemberFunction();
}
factory Foo({required String channel}) {
if (channel != _instance?.channel) {
_instance = Foo._internal(channel: channel);
}
return _instance!;
}
}
Note that this implementation will create a new object if the constructor argument changes, which isn't very singleton-like. Depending on what you want to do, you could:
Return a new object (which could allow multiple simultaneous instances).
Return the existing object.
Return the existing object, but mutate it with the constructor argument.
In java it would be something like this ...
public Request compose(LoginDevice login) {
JSONObject params = new JSONObject();
try {
if (login.granType != null)
params.put("grant_type", login.granType);
if (login.clientId != null)
params.put("client_id", login.clientId);
} catch (JSONException e) {
e.printStackTrace();
}
return (Request)new BaseRequest("oauth/v2/token", params.toString(), new HashSet());
}
And in Dart and tried something similar but it doesn't work... the parameter 'put' does not exist in JsonObjectLite...
Request compose(LoginDevice login)
{
JsonObjectLite params = new JsonObjectLite();
try {
if (login.granType != null) {
params.put("grant_type", login.granType);
}
if (login.clientId != null) {
params.put("client_id", login.clientId);
}
} on JsonObjectLiteException catch (e) {
print(e);
}
return new BaseRequest("oauth/v2/token", params.toString(), new HashSet());
}
How could I do it? Thank you
The class JsonObjectLite doesn't contain the method put.
How you can understand dart doesn't is Java, in this cases your the class JsonObjectLite has a method called putIfAbsent, the implementation is the following
/// If [isImmutable] is false, or the key already exists,
/// then allow the edit.
/// Throw [JsonObjectLiteException] if we're not allowed to add a new
/// key
#override
void putIfAbsent(dynamic key, Function() ifAbsent) {
if (isImmutable == false || containsKey(key)) {
_objectData.putIfAbsent(key, ifAbsent);
} else {
throw const JsonObjectLiteException('JsonObject is not extendable');
}
}
look also the Source code
So an example of code should be the following
import 'package:json_object_lite/json_object_lite.dart';
class AuthorAnswer {
var _username;
var _status;
AuthorAnswer(this._username, this._status);
String get username => _username;
String get status => _status;
}
int main() {
var author = '#vincenzopalazzo';
var sentences = 'Follow me on Github';
var authorObject = AuthorAnswer(author, sentences);
try{
JsonObjectLite params = new JsonObjectLite();
params.isImmutable = false;
params.putIfAbsent("author", () => authorObject.username);
params.putIfAbsent("sencence", () => authorObject.status);
print(params.toString());
} on JsonObjectLiteException catch (err){
print('--------- ERROR ----------');
print(err);
}
return 0;
}
You should be set the params.isImmutable = false and after you can add your propriety, with your logic.
In my opinion, I don't see any motivation to use this library, dart have 2 types of the module to implement the serialization, and I think should better use it because on the web exist the documentation, like this dart json, flutter json
Inside the flutter app, there are also the line guides, for the small application you can use dart:convert:json also for the other you can use the json_serializable
I want to add also an example of dart:convert
/**
*
*/
import "dart:core";
import "dart:convert";
class ClassToJsonOne {
var _propOne;
var _propTwo;
ClassToJsonOne(this._propOne, this._propTwo);
Map<String, dynamic> toJSon() => {
'prop_one': _propOne,
'prop_two': _propTwo
};
ClassToJsonOne.fromJson(Map<String, dynamic> json):
_propOne = json['prop_one'],
_propTwo = json['prop_two'];
#override
String toString() => 'First Class: $_propOne, $_propTwo';
}
class ClassToJsonTwo{
var _propOne;
var _propTwo;
ClassToJsonTwo(this._propOne, this._propTwo);
Map<String, dynamic> toJSon() => {
'prop_one': _propOne,
'prop_two': _propTwo
};
ClassToJsonTwo.fromJson(Map<String, dynamic> json):
_propOne = json['prop_one'],
_propTwo = json['prop_two'];
#override
String toString() => 'Second Class: $_propOne, $_propTwo';
}
main(List<String> args) {
print('------- Declare Objecr -------\n');
var objectToJsonOne = ClassToJsonOne('I am the fist object', 'empty');
var objectToJsonTwo = ClassToJsonTwo('I contains the first object', 'empty');
String jsonStringObjOne = jsonEncode(objectToJsonOne.toJSon());
print('\n---------- Object one JSON format ---------\n');
print(jsonStringObjOne);
String jsonStringObjTwo = jsonEncode(objectToJsonTwo.toJSon());
print('\n---------- Object one JSON format ---------\n');
print(jsonStringObjTwo);
print('\n---------- DECODE JSON to OBJECT ---------\n');
var fromJsonObjectOne = jsonDecode(jsonStringObjOne);
print(fromJsonObjectOne.toString());
var fromJsonObjectTwo = jsonDecode(jsonStringObjTwo);
print(fromJsonObjectTwo.toString());
}
Inside the classes, you can see the following methods
Map<String, dynamic> toJSon() => {
'prop_one': _propOne,
'prop_two': _propTwo
};
ClassToJsonTwo.fromJson(Map<String, dynamic> json):
_propOne = json['prop_one'],
_propTwo = json['prop_two'];
The result of the method toJSon, you should be pass to the method of the library jsonEncode and when you go to deserialize you can use the library method jsonDecode(youtStringJSOn) and the result you can pass to the method of your class fromJson.
In addition, you can configure the library json_serializable.
In conclusion, I want to fix my comment
I think the json_serializable worked how GSON, I can make an example for you, on this day.
On flutter, documentation has reported this text
Is there a GSON/Jackson/Moshi equivalent in Flutter?
The simple answer is no.
Such a library would require using runtime reflection, which is disabled in Flutter. Runtime reflection interferes with tree shaking, which Dart has supported for quite a long time. With tree shaking, you can “shake off” unused code from your release builds. This optimizes the app’s size significantly.
Since reflection makes all code implicitly used by default, it makes tree shaking difficult. The tools cannot know what parts are unused at runtime, so the redundant code is hard to strip away. App sizes cannot be easily optimized when using reflection.
Although you cannot use runtime reflection with Flutter, some libraries give you similarly easy-to-use APIs but are based on code generation instead. This approach is covered in more detail in the code generation libraries section.
you can found the source code inside this answer here
I am making a pokemon app and I have a question that I already had in other projects and I would like to know if anyone can help me with a solution.
I receive a variable called pokemonName from other screen, I want to pass the variable and concatenate with the class "Strings", it will be like Strings.+pokemonName.toLowerCase(), converting to lowercase to get the map from the class Strings, but I don't know how to achieve this to remove the switch and don't need to use a lot of cases for each pokemon.
class PokemonDetailScreen extends StatelessWidget {
final String pokemonName;
final String image;
Map<String, dynamic> pokemonMap = {};
PokemonDetailScreen(this.pokemonName, this.image, this.index){
getPokemonMap();
}
#override
Widget build(BuildContext context) {
return Container();
}
void getPokemonMap(){
switch(pokemonName){
case "Bulbasaur":
pokemonMap = Strings.bulbasaur;
break;
case "Charmander":
pokemonMap = Strings.charmander;
break;
}
}
}
**Class in another dart file:**
class Strings {
static Map bulbasaur = {};
}
What I needed is something like this:
void getPokemonMap(){
pokemonMap = Strings.$pokemonMap.toLowerCase();
}
What you could do is have a static map indexed by the name of your Pokemons and whose values are maps.
class Strings {
static Map<String, dynamic> map = {
'Bulbasor': {},
'Charmander': {},
// ...
};
}
And you’ll use it like this: pokemonMap = Strings.map[pokemonName].
~You can use JSON file to do all this things instead use a class.~
I recommend not use a static class to do that thing, instead you can just make a normal class and instantiate on another file, so when the class that you call your another class will be dispose when the parent was.
class PokemonStrings {
Map bulbasaur = {your map here};
}
To call that in another file you need just do
PokemonString _pokemonString = PokemonString();
And call whatever you need in the class that you instantiate
var bulbasaurMap = _pokemonString.bulbasaur;
But even so you need walk with static class. Just call the name of class followed by dot to access all the static attributes
var bulbasaurMap = PokemonString.bulbasaur;