I have a class with name "RecognisedText" in one of my packages which is required in my Flutter application. This class is:
class RecognisedText {
RecognisedText._(this.text, this.blocks);
factory RecognisedText.fromMap(Map<dynamic, dynamic> map) {
var resText = map["text"];
var textBlocks = <TextBlock>[];
for (var block in map["blocks"]) {
var textBlock = TextBlock.fromMap(block);
textBlocks.add(textBlock);
}
return RecognisedText._(resText, textBlocks);
}
///String containing all the text identified in a image.
final String text;
///All the blocks of text present in image.
final List<TextBlock> blocks;
}
I want to define a variable from this class:
RecognisedText myobject = RecognisedText(mytext,myblocks);
but it raise this error:
The class 'RecognisedText' doesn't have a default constructor
I tried to define myobject in different way:
RecognisedText myobject;
myobject.text = mytext;
myobject.blocks = myblocks;
but it raise this error:
'text' can't be used as a setter because it's final.
How i can define my new variable from RecognisedText class and set properties? I can't change any part of class RecognisedText because it is a remote class (added from pubspec.yaml)
Edit:
I finally solved my problem with this code:
RecognisedText myobject = RecognisedText.fromMap({"text": mytext, "blocks": myblocks});
Your Class can't create its instance so try using YourClassName.fromMap constructor.
Class RecognisedText is designed in the way that you can not create its instance other than using its fromMap constructor, which accepts map.
So the only way to create it is to create map with values you need and pass it to RecognisedText.fromMap constructor.
If you already have your block and your text as variables, you can modify you class like this to accept a default constructor :
class RecognisedText {
//----Replace this line ----
//RecognisedText._(this.text, this.blocks);
//----By this line ----
RecognisedText.(this.text, this.blocks);
factory RecognisedText.fromMap(Map<dynamic, dynamic> map) {
var resText = map["text"];
var textBlocks = <TextBlock>[];
for (var block in map["blocks"]) {
var textBlock = TextBlock.fromMap(block);
textBlocks.add(textBlock);
}
return RecognisedText._(resText, textBlocks);
}
///String containing all the text identified in a image.
final String text;
///All the blocks of text present in image.
final List<TextBlock> blocks;
}
To complete Alex's answer, the constructor fromMap takes a map as a parameter. It should look like :
RecogniserdText.fromMap({'text': myText, 'blocks': myblock1, 'blocks': myblock2, 'blocks': myblock3});
Related
In this case I have class. Where I took a variable. Also I have a Json map. So I want to change Json map object replace with variables. Here is my code example....
So how can I achieve that
I want replace Json object with dart variable
class Data {
late String slug;
Map<String, String> singleProductVariable = {"slug": "$slug"};
}
Firstly, there is no JSON in your code sample.
I assume that you would like to set the value of the corresponding key in your Map when setting the variable.
If so, you might want to use a setter in a next way:
class Data {
String _slug;
late Map<String, String> v = {"slug": _slug};
Data(String slug) : _slug = slug;
set slug(String str) => v['slug'] = str;
}
void main() {
final d = Data("slug");
print(d.v);
d.slug = "newSlug";
print(d.v);
}
The output of the code above will be:
{slug: val}
{slug: newVal}
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 $channel');
}
static Foo? _instance;
Foo._internal({required this.channel}) {
instanceMemberFunction();
}
factory Foo({required String channel}) {
if (channel != _instance?.channel) {
_instance = Foo._internal(channel: channel);
}
return _instance!;
}
}
Note that this implementation will create a new object if the constructor argument changes, which isn't very singleton-like. Depending on what you want to do, you could:
Return a new object (which could allow multiple simultaneous instances).
Return the existing object.
Return the existing object, but mutate it with the constructor argument.
I am an Android Developer and new to Flutter. I really like the way Mobx works, because it remembers me Android's ViewModel. By the way, when I create a ViewModel, I like to create it passing the repository as a parameter, so I can test it with different Data Sources (ie. local or cloud).
So, this is my class right now.
import 'package:mobx/mobx.dart';
part 'create_bill_controller.g.dart';
class CreateBillController = _CreateBillControllerBase
with _$CreateBillController;
abstract class _CreateBillControllerBase with Store {
final appBarTitle = 'Criar Conta';
final criarConta = 'Criar conta';
final nomeDaConta = 'Nome da conta';
final seuNome = 'Seu nome';
#action
createBill(String billname, String userName) {
// here, dataSource should be given in a constructor
datasource.createBill(billName, userName);
}
}
How can I pass a DataSource (repository) as a parameter to this class?
What you need is to declare constructor for CreateBillController instead of _CreateBillControllerBase, because constructor is not inherited by child class in Dart. The simplest way is to assign the passed in datasource to the corresponding property in parent class in the constructor, as you can see in the below snippet. You can also implement a constructor for _CreateBillControllerBase as well and call super(datasource) in CreateBillController's constructor.
import 'package:mobx/mobx.dart';
part 'create_bill_controller.g.dart';
class CreateBillController extends _CreateBillControllerBase with _$CreateBillController {
// HERE! Implement constructor for CreateBillController
// Do this if you have a constructor for _CreateBillControllerBase
//
// CreateBillController(DataSource datasource) : super(datasource)
//
CreateBillController(DataSource datasource) {
super.datasource = datasource;
}
}
abstract class _CreateBillControllerBase with Store {
final appBarTitle = 'Criar Conta';
final criarConta = 'Criar conta';
final nomeDaConta = 'Nome da conta';
final seuNome = 'Seu nome';
// HERE! Declare datasource
DataSource datasource;
#action
createBill(String billname, String userName) {
datasource.createBill(billName, userName);
}
}
So after dart made new keyword optional,
we can initialize an object with exact same syntax but different internal implementation.
class Color {
int r = 0, g = 0, b = 0;
Color({this.r, this.b, this.g});
//Named constructors
Color.red() //Implementation
Color.cyan() //Implementation
// Static Initializers
static Color red() => //Initialze with parameter
static Color cyan() => //Initialze with parameter
}
We can use them like this regardless of being it a named constructor or static method:
Color red = Color.red();
Color cyan = Color.cyan();
What is the place to use each of them?
In practice there is little difference between a factory constructor and a static method.
For a generic class, it changes where you can (and must) write a type parameter:
class Box<T> {
T value;
Box._(this.value);
factory Box.withValue(this.value) => Box<T>._(value);
static Box<T> fromValue<T>(T value) => Box<T>._(value);
}
...
var box1 = Box<int>.withValue(1);
var box2 = Box.fromValue<int>(2);
So, for generic classes, factory constructors are often what you want. They have the most pleasant syntax.
For non-generic classes, there is very little difference, so it's mainly about signaling intent. And deciding which category the name goes into in the DartDoc.
If the main objective of the function is to create a new object, make it a constructor.
If the main objective is to do some computation and eventually return an object (even if it's a new object), make it a static function.
That's why parse methods are generally static functions.
In short, do what feels right for your API.
Constructors and static functions are different. You usually create a named constructor that returns an instance of an object with some predefined values. For example, you have a class called Person which stores Name and Job. You can create this named constructor Person.doctor(name) which you will return a Person object with Job = 'doctor'
class Person{
final name;
final job;
Person(this.name, this.job);
Person.doctor(this.name, {this.job = "doctor"});
}
Static functions or variable persists on all the instance of a class. Let us say, Person has a static variable called count. You increment the count variable whenever an instance of Person is created. You can call Person.count anywhere later in your code to get the value of count (Number of instances of Person)
class Person{
final name;
final job;
static int count;
Person(this.name, this.job){
count++;
}
Person.doctor(this.name, {this.job = "doctor"});
}
Another very useful feature of static class methods is that you can make them asynchronous, i.e. wait for full initialisation in case this depends on some asynchronous operation:
Future<double> getCurrentPrice(String ticker) async {
double price;
// for example, fetch current price from API
price = 582.18;
return price;
}
class Stock {
String ticker;
double currentPrice=0.0;
Stock._(this.ticker);
static Future<Stock> stockWithCurrentPrice(String ticker) async {
Stock stock = Stock._(ticker);
stock.currentPrice = await getCurrentPrice (ticker);
return stock;
}
}
void main() async {
Stock stock = await Stock.stockWithCurrentPrice('AAPL');
print ('${stock.ticker}: ${stock.currentPrice}');
}
Another benefit of the distinction between named constructor and static function is that in the documentation generated the function will be either filed in the construction section or the methods section, which further makes it's intentions clearer to the reader.
A person looking for a constructor in the constructor section of the documentation will easily discover the named constructors as opposed to having to also dig through the static functions section too.
I am making a pokemon app and I have a question that I already had in other projects and I would like to know if anyone can help me with a solution.
I receive a variable called pokemonName from other screen, I want to pass the variable and concatenate with the class "Strings", it will be like Strings.+pokemonName.toLowerCase(), converting to lowercase to get the map from the class Strings, but I don't know how to achieve this to remove the switch and don't need to use a lot of cases for each pokemon.
class PokemonDetailScreen extends StatelessWidget {
final String pokemonName;
final String image;
Map<String, dynamic> pokemonMap = {};
PokemonDetailScreen(this.pokemonName, this.image, this.index){
getPokemonMap();
}
#override
Widget build(BuildContext context) {
return Container();
}
void getPokemonMap(){
switch(pokemonName){
case "Bulbasaur":
pokemonMap = Strings.bulbasaur;
break;
case "Charmander":
pokemonMap = Strings.charmander;
break;
}
}
}
**Class in another dart file:**
class Strings {
static Map bulbasaur = {};
}
What I needed is something like this:
void getPokemonMap(){
pokemonMap = Strings.$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;