Consider this class that is used as a data model in a Model-View-Controller scenario (I'm using TypeScript 3.5):
export class ViewSource {
private viewName : string;
private viewStruct : IViewStruct;
private rows : any[];
private rowIndex : number|null;
constructor(viewName : string) {
// Same as this.setViewName(viewName);
this.viewName = viewName;
this.viewStruct = api.meta.get_view_struct(viewName);
if (!this.viewStruct) {
throw new Error("Clould not load structure for view, name=" + (viewName));
}
this.rows = [];
this.rowIndex = null;
}
public setViewName = (viewName: string) => {
this.viewName = viewName;
this.viewStruct = api.meta.get_view_struct(viewName);
if (!this.viewStruct) {
throw new Error("Clould not load structure for view, name=" + (viewName));
}
this.rows = [];
this.rowIndex = null;
}
public getViewStruct = ():IViewStruct => { return this.viewStruct; }
public getCellValue = (rowIndex: number, columnName: string) : any => {
const row = this.rows[rowIndex] as any;
return row[columnName];
}
}
This is not a complete class, I only included a few methods to demonstrate the problem. ViewSource is a mutable object. It can be referenced from multiple parts of the application. (Please note that being a mutable object is a fact. This question is not about choosing a different data model that uses immutable objects.)
Whenever I want to change the state of a ViewSource object, I call its setViewName method. It does work, but it is also very clumsy. Every line of code in the constructor is repeated in the setViewName method.
Of course, it is not possible to use this constructor:
constructor(viewName : string) {
this.setViewName(viewName);
}
because that results in TS2564 error:
Property 'viewStruct' has no initializer and is not definitely assigned in the constructor.ts(2564)
I do not want to ignore TS2564 errors in general. But I also do not want to repeat all attribute initializations. I have some other classes with even more properties (>10), and the corresponding code duplication looks ugly, and it is error prone. (I might forget that some things have to bee modified in two methods...)
So how can I avoid duplicating many lines of code?
I think the best method to avoid code duplication in this case would be to create a function that contains the initialization code, but instead of setting the value, it retunrs the value that need to be set.
Something like the following:
export class ViewSource {
private viewName : string;
private viewStruct : IViewStruct;
private rows : any[];
private rowIndex : number|null;
constructor(viewName : string) {
const {newViewName, newViewStruct, newRows, newRowIndex} = this.getNewValues(viewName);
this.viewName = newViewName;
this.newViewStruct = newViewStruct;
// Rest of initialization goes here
}
public setViewName = (viewName: string) => {
const {newViewName, newViewStruct, newRows, newRowIndex} = this.getNewValues(viewName);
// Rest of initialization goes here
}
privat getNewValues = (viewName) => {
const newViewName = viewName;
const newViewStruct = api.meta.get_view_struct(viewName);
if (!newViewStruct) {
throw new Error("Clould not load structure for view, name=" + (viewName));
}
const newRows = [];
const newRowIndex = null;
return {newViewName, newViewStruct, newRows, newRowIndex};
}
}
This way the only thing you duplicate is setting the values, not calculating them, and if the values calculations will get more complicated you can simply expand the returned value.
A less complex approach than the accepted answer is to use the //#ts-ignore[1] comment above each member that is initialized elsewhere.
Consider this contrived example
class Foo {
// #ts-ignore TS2564 - initialized in the init method
a: number;
// #ts-ignore TS2564 - initialized in the init method
b: string;
// #ts-ignore TS2564 - initialized in the init method
c: number;
constructor(a: number, b: string) {
if(a === 0) {
this.init(a,b,100);
} else {
this.init(a,b,4912);
}
}
private init(a: number, b: string, c: number): void {
this.a = a;
this.b = b;
this.c = c;
}
}
Since TypeScript 3.9 there exists the //#ts-expect-error[2] comment, but I think #ts-ignore is suitable.
[1] Suppress errors in .ts files
[2] TS expect errors comment
Since TypeScript 2.7 you can use the definite assignment assertion modifier which means adding an exclamation mark between the variable name and the colon:
private viewName!: string
This has the same effect as adding a // #ts-ignore TS2564 comment above it as #RamblinRose suggested.
Is there a native (language supported) lazy evaluation syntax? Something like lazy val in Scala.
I've gone through the docs, and could not find anything. There is only a chapter about "lazily loading a library", but it's not what I am asking.
Based on this research I incline to believe (please correct me if I'm wrong) that currently there is no such thing. But maybe you know of any plans or feature requests which will provide the functionality? Or maybe it was considered and rejected by the Dart team?
If indeed there is no native support for this, then what is the best practice (best syntax) for implementing lazy evaluation? An example would be appreciated.
Edit:
The benefits of the feature that I am looking for are mostly the same as in implementation in other languages: Scala's lazy val or C#'s Lazy<T> or Hack's __Memorize attribute:
concise syntax
delayed computation until the value is needed
cache the result (the by-need laziness)
don't break pure functional paradigm (explanation below)
A simple example:
class Fibonacci {
final int n;
int _res = null;
int get result {
if (null == _res) {
_res = _compute(this.n);
}
return _res;
}
Fibonacci(this.n);
int _compute(n) {
// ...
}
}
main(List<String> args) async {
print(new Fibonacci(5).result);
print(new Fibonacci(9).result);
}
The getter is very verbose and has a repetitive code. Moreover I can't make the constructor const because the caching variable _res has to be computed on demand. I imagine that if I had a Scala-like lazy feature then I would also have language support for having a constant constructor. That's thanks to the fact, that the lazy evaluated _res is referentially transparent, and would not be in the way.
class Fibonacci {
final int n;
int lazy result => _compute(this.n);
const Fibonacci(this.n); // notice the `const`
int _compute(n) {
// ...
}
}
main(List<String> args) async {
// now these makes more sense:
print(const Fibonacci(5).result);
print(const Fibonacci(9).result);
}
Update 2021
Lazy initialization is now part of dart from the release 2.12.
Simply add late modifier to the variable declaration
late MyClass obj = MyClass();
And this object will be initialized only when it is first used.
From the docs:
Dart 2.12 added the late modifier, which has two use cases:
Declaring a non-nullable variable that’s initialized after its
declaration.
Lazily initializing a variable.
Checkout the example here:
https://dartpad.dev/?id=50f143391193a2d0b8dc74a5b85e79e3&null_safety=true
class A {
String text = "Hello";
A() {
print("Lazily initialized");
}
sayHello() {
print(text);
}
}
class Runner {
late A a = A();
run() async {
await Future.delayed(Duration(seconds: 3));
print("First message");
a.sayHello();
}
}
Here class A will be initialized only after "First message" has been displayed.
update2
From #lrn s comment - using an Expando for caching makes it work with const:
class Lazy<T> {
static final _cache = new Expando();
final Function _func;
const Lazy(this._func);
T call() {
var result = _cache[this];
if (identical(this, result)) return null;
if (result != null) return result;
result = _func();
_cache[this] = (result == null) ? this : result;
return result;
}
}
defaultFunc() {
print("Default Function Called");
return 42;
}
main([args, function = const Lazy(defaultFunc)]) {
print(function());
print(function());
}
Try it in DartPad
update
A reusable Lazy<T> could look like below in Dart but that also doesn't work with const and can't be used in field initializers if the calculation needs to refer instance members (this.xxx).
void main() {
var sc = new SomeClass();
print('new');
print(sc.v);
}
class SomeClass {
var _v = new Lazy<int>(() {
print('x');
return 10;
});
int get v => _v();
}
class Lazy<T> {
final Function _func;
bool _isEvaluated = false;
Lazy(this._func);
T _value;
T call() {
if(!_isEvaluated) {
if(_func != null) {
_value = _func();
}
_isEvaluated = true;
}
return _value;
}
}
Try it in DartPad
original
Dart version of http://matt.might.net/articles/implementing-laziness/ using a closure to lazy evaluate:
void main() {
var x = () {
print ("foo");
return 10;
}();
print("bar");
print(x);
// will print foo, then bar then 10.
print('===');
// But, the following Scala program:
x = () {
print("foo");
return 10;
};
print ("bar");
print (x());
// will print bar, then foo, then 10, since it delays the computation of x until it’s actually needed.
}
Try it in DartPad
Update
int _val;
int get val => _val ??= 9;
Thanks #Nightscape
Old
I think this little snippet might help you...
int _val;
int get val => _val ?? _val = 9;
I have an Enum and a function to create it from a String because i couldn't find a built in way to do it
enum Visibility{VISIBLE,COLLAPSED,HIDDEN}
Visibility visibilityFromString(String value){
return Visibility.values.firstWhere((e)=>
e.toString().split('.')[1].toUpperCase()==value.toUpperCase());
}
//used as
Visibility x = visibilityFromString('COLLAPSED');
but it seems like i have to rewrite this function for every Enum i have, is there a way to write the same function where it takes the Enum type as parameter? i tried to but i figured out that i can't cast to Enum.
//is something with the following signiture actually possible?
dynamic enumFromString(Type enumType,String value){
}
Mirrors aren't always available, but fortunately you don't need them. This is reasonably compact and should do what you want.
enum Fruit { apple, banana }
// Convert to string
String str = Fruit.banana.toString();
// Convert to enum
Fruit f = Fruit.values.firstWhere((e) => e.toString() == 'Fruit.' + str);
assert(f == Fruit.banana); // it worked
Thanks to #frostymarvelous for correcting the answer
As from Dart version 2.15, you can lookup an enum value by name a lot more conveniently, using .values.byName or using .values.asNameMap():
enum Visibility {
visible, collapsed, hidden
}
void main() {
// Both calls output `true`
print(Visibility.values.byName('visible') == Visibility.visible);
print(Visibility.values.asNameMap()['visible'] == Visibility.visible);
}
You can read more about other enum improvements in the official Dart 2.15 announcement blog post.
My solution is identical to Rob C's solution but without string interpolation:
T enumFromString<T>(Iterable<T> values, String value) {
return values.firstWhere((type) => type.toString().split(".").last == value,
orElse: () => null);
}
Null safe example using firstWhereOrNull() from the collection package
static T? enumFromString<T>(Iterable<T> values, String value) {
return values.firstWhereOrNull((type) => type.toString().split(".").last == value);
}
Update:
void main() {
Day monday = Day.values.byName('monday'); // This is all you need
}
enum Day {
monday,
tuesday,
}
Old solution:
Your enum
enum Day {
monday,
tuesday,
}
Add this extension (need a import 'package:flutter/foundation.dart';)
extension EnumEx on String {
Day toEnum() => Day.values.firstWhere((d) => describeEnum(d) == toLowerCase());
}
Usage:
void main() {
String s = 'monday'; // String
Day monday = s.toEnum(); // Converted to enum
}
This is all so complicated I made a simple library that gets the job done:
https://pub.dev/packages/enum_to_string
import 'package:enum_to_string:enum_to_string.dart';
enum TestEnum { testValue1 };
convert(){
String result = EnumToString.parse(TestEnum.testValue1);
//result = 'testValue1'
String resultCamelCase = EnumToString.parseCamelCase(TestEnum.testValue1);
//result = 'Test Value 1'
final result = EnumToString.fromString(TestEnum.values, "testValue1");
// TestEnum.testValue1
}
Update: 2022/02/10
Dart v2.15 has implemented some additional enum methods that may solve your problems.
From here: https://medium.com/dartlang/dart-2-15-7e7a598e508a
Improved enums in the dart:core library
We’ve made a number of convenience additions to the enum APIs in the dart:core library (language issue #1511). You can now get the String value for each enum value using .name:
enum MyEnum {
one, two, three
}
void main() {
print(MyEnum.one.name); // Prints "one".
}
You can also look up an enum value by name:
print(MyEnum.values.byName('two') == MyEnum.two); // Prints "true".
Finally, you can get a map of all name-value pairs:
final map = MyEnum.values.asNameMap();
print(map['three'] == MyEnum.three); // Prints "true".
Using mirrors you could force some behaviour. I had two ideas in mind. Unfortunately Dart does not support typed functions:
import 'dart:mirrors';
enum Visibility {VISIBLE, COLLAPSED, HIDDEN}
class EnumFromString<T> {
T get(String value) {
return (reflectType(T) as ClassMirror).getField(#values).reflectee.firstWhere((e)=>e.toString().split('.')[1].toUpperCase()==value.toUpperCase());
}
}
dynamic enumFromString(String value, t) {
return (reflectType(t) as ClassMirror).getField(#values).reflectee.firstWhere((e)=>e.toString().split('.')[1].toUpperCase()==value.toUpperCase());
}
void main() {
var converter = new EnumFromString<Visibility>();
Visibility x = converter.get('COLLAPSED');
print(x);
Visibility y = enumFromString('HIDDEN', Visibility);
print(y);
}
Outputs:
Visibility.COLLAPSED
Visibility.HIDDEN
Collin Jackson's solution didn't work for me because Dart stringifies enums into EnumName.value rather than just value (for instance, Fruit.apple), and I was trying to convert the string value like apple rather than converting Fruit.apple from the get-go.
With that in mind, this is my solution for the enum from string problem
enum Fruit {apple, banana}
Fruit getFruitFromString(String fruit) {
fruit = 'Fruit.$fruit';
return Fruit.values.firstWhere((f)=> f.toString() == fruit, orElse: () => null);
}
Here is an alternative way to #mbartn's approach using extensions, extending the enum itself instead of String.
Faster, but more tedious
// We're adding a 'from' entry just to avoid having to use Fruit.apple['banana'],
// which looks confusing.
enum Fruit { from, apple, banana }
extension FruitIndex on Fruit {
// Overload the [] getter to get the name of the fruit.
operator[](String key) => (name){
switch(name) {
case 'banana': return Fruit.banana;
case 'apple': return Fruit.apple;
default: throw RangeError("enum Fruit contains no value '$name'");
}
}(key);
}
void main() {
Fruit f = Fruit.from["banana"];
print("$f is ${f.runtimeType}"); // Outputs: Fruit.banana is Fruit
}
Less tedius, but slower
If O(n) performance is acceptable you could also incorporate #Collin Jackson's answer:
// We're adding a 'from' entry just to avoid having to use Fruit.apple['banana']
// which looks confusing.
enum Fruit { from, apple, banana }
extension FruitIndex on Fruit {
// Overload the [] getter to get the name of the fruit.
operator[](String key) =>
Fruit.values.firstWhere((e) => e.toString() == 'Fruit.' + key);
}
void main() {
Fruit f = Fruit.from["banana"];
print("$f is ${f.runtimeType}"); // Outputs: Fruit.banana is Fruit
}
I use this function, I think it's simple and doesn't need any kind of 'hack':
T enumFromString<T>(List<T> values, String value) {
return values.firstWhere((v) => v.toString().split('.')[1] == value,
orElse: () => null);
}
You can use it like this:
enum Test {
value1,
value2,
}
var test = enumFromString(Test.value, 'value2') // Test.value2
With Dart 2.15 we can now do this which is much cleaner
// Convert to string
String fruitName = Fruit.banana.name;
// Convert back to enum
Fruit fruit = Fruit.values.byName(fruitName);
print(fruit); // Fruit.banana
assert(fruit == Fruit.banana);
Since Dart 2.17 you can solve this elegantly with Enhanced Enums.
(see https://stackoverflow.com/a/71412047/15760132 )
Just add a static method to your enum of choice, like this:
enum Example {
example1,
example2,
example3;
static Example? fromName(String name) {
for (Example enumVariant in Example.values) {
if (enumVariant.name == name) return enumVariant;
}
return null;
}
}
Then you can look for the enum like this:
Example? test = Example.fromName("example1");
print(test); // returns Example.example1
I improved Collin Jackson's answer using Dart 2.7 Extension Methods to make it more elegant.
enum Fruit { apple, banana }
extension EnumParser on String {
Fruit toFruit() {
return Fruit.values.firstWhere(
(e) => e.toString().toLowerCase() == 'fruit.$this'.toLowerCase(),
orElse: () => null); //return null if not found
}
}
main() {
Fruit apple = 'apple'.toFruit();
assert(apple == Fruit.apple); //true
}
I had the same problem with building objects from JSON. In JSON values are strings, but I wanted enum to validate if the value is correct. I wrote this helper which works with any enum, not a specified one:
class _EnumHelper {
var cache = {};
dynamic str2enum(e, s) {
var o = {};
if (!cache.containsKey(e)){
for (dynamic i in e) {
o[i.toString().split(".").last] = i;
}
cache[e] = o;
} else {
o = cache[e];
}
return o[s];
}
}
_EnumHelper enumHelper = _EnumHelper();
Usage:
enumHelper.str2enum(Category.values, json['category']);
PS. I did not use types on purpose here. enum is not type in Dart and treating it as one makes things complicated. Class is used solely for caching purposes.
Generalising #CopsOnRoad's solution to work for any enum type,
enum Language { en, ar }
extension StringExtension on String {
T toEnum<T>(List<T> list) => list.firstWhere((d) => d.toString() == this);
}
String langCode = Language.en.toString();
langCode.toEnum(Language.values);
Simplified version:
import 'package:flutter/foundation.dart';
static Fruit? valueOf(String value) {
return Fruit.values.where((e) => describeEnum(e) == value).first;
}
Using the method describeEnum helps you to avoid the usage of the split to get the name of the element.
You can write getEnum like below, getEnum will go through enum values and returns the first enum that is equal to the desired string.
Sample getEnum(String name) => Sample.values.firstWhere(
(v) => v.name.toLowerCase() == name.toLowerCase(),
orElse: () => throw Exception('Enum value not found.'),
);
enum SampleEnum { first, second, third }
UPDATE
also, you can use this:
final SampleEnum nameEnum = SampleEnum.values.byName('second'); // SampleEnum.second
Usage:
void main() {
print(getEnum('first'));
}
In the latest version of Dart, enum can support custom fields and methods. So the most modern way to do this, is to write a custom field for name/label, and a static parser function.
For example:
enum Foo {
a('FIRST'),
b('SECOND'),
c('THIRD'),
unknown('UNKNOWN'); // make sure the last element ends in `;`
final String label; // define a private field
const Foo(this.label); // constructor
static Foo fromString(String label) { // static parser method
return values.firstWhere(
(v) => v.label == label,
orElse: () => Foo.unknown,
);
}
}
Sample Usage:
final foo = Foo.fromString('FIRST'); // will get `Foo.a`
There are a couple of enums packages which allowed me to get just the enum string rather than the type.value string (Apple, not Fruit.Apple).
https://pub.dartlang.org/packages/built_value (this is more up to date)
https://pub.dartlang.org/packages/enums
void main() {
print(MyEnum.nr1.index); // prints 0
print(MyEnum.nr1.toString()); // prints nr1
print(MyEnum.valueOf("nr1").index); // prints 0
print(MyEnum.values[1].toString()) // prints nr2
print(MyEnum.values.last.index) // prints 2
print(MyEnum.values.last.myValue); // prints 15
}
Here is the function that converts given string to enum type:
EnumType enumTypeFromString(String typeString) => EnumType.values
.firstWhere((type) => type.toString() == "EnumType." + typeString);
And here is how you convert given enum type to string:
String enumTypeToString(EnumType type) => type.toString().split(".")[1];
Generalizing on #Pedro Sousa's excellent solution, and using the built-in describeEnum function:
extension StringExtension on String {
T toEnum<T extends Object>(List<T> values) {
return values.singleWhere((v) => this.equalsIgnoreCase(describeEnum(v)));
}
}
Usage:
enum TestEnum { none, test1, test2 }
final testEnum = "test1".toEnum(TestEnum.values);
expect(testEnum, TestEnum.test1);
import 'package:collection/collection.dart';
enum Page {
login,
profile,
contact,
}
Widget page(String key){
Page? link = Page.values.firstWhereOrNull((e) => e.toString().split('.').last == key);
switch (link) {
case Page.login:
return LoginView();
case Page.profile:
return const ProfileView();
case Page.contact:
return const ContactView();
default:
return const Empty();
}
}
#Collin Jackson has a very good answer IMO. I had used a for-in loop to achieve a similar result prior to finding this question. I am definitely switching to using the firstWhere method.
Expanding on his answer this is what I did to deal with removing the type from the value strings:
enum Fruit { apple, banana }
class EnumUtil {
static T fromStringEnum<T>(Iterable<T> values, String stringType) {
return values.firstWhere(
(f)=> "${f.toString().substring(f.toString().indexOf('.')+1)}".toString()
== stringType, orElse: () => null);
}
}
main() {
Fruit result = EnumUtil.fromStringEnum(Fruit.values, "apple");
assert(result == Fruit.apple);
}
Maybe someone will find this useful...
I had the same problem in one of my projects and existing solutions were not very clean and it didn't support advanced features like json serialization/deserialization.
Flutter natively doesn't currently support enum with values, however, I managed to develop a helper package Vnum using class and reflectors implementation to overcome this issue.
Address to the repository:
https://github.com/AmirKamali/Flutter_Vnum
To answer your problem using Vnum, you could implement your code as below:
#VnumDefinition
class Visibility extends Vnum<String> {
static const VISIBLE = const Visibility.define("VISIBLE");
static const COLLAPSED = const Visibility.define("COLLAPSED");
static const HIDDEN = const Visibility.define("HIDDEN");
const Visibility.define(String fromValue) : super.define(fromValue);
factory Visibility(String value) => Vnum.fromValue(value,Visibility);
}
You can use it like :
var visibility = Visibility('COLLAPSED');
print(visibility.value);
There's more documentation in the github repo, hope it helps you out.
When migrating to null-safety, the Iterable.firstWhere method no longer accepts orElse: () => null. Here is the implementation considering the null-safety:
import 'package:collection/collection.dart';
String enumToString(Object o) => o.toString().split('.').last;
T? enumFromString<T>(String key, List<T> values) => values.firstWhereOrNull((v) => key == enumToString(v!));
enum Fruit { orange, apple }
// Waiting for Dart static extensions
// Issue https://github.com/dart-lang/language/issues/723
// So we will be able to Fruit.parse(...)
extension Fruits on Fruit {
static Fruit? parse(String raw) {
return Fruit.values
.firstWhere((v) => v.asString() == raw, orElse: null);
}
String asString() {
return this.toString().split(".").last;
}
}
...
final fruit = Fruits.parse("orange"); // To enum
final value = fruit.asString(); // To string
I think my approach is slightly different, but might be more convenient in some cases. Finally, we have parse and tryParse for enum types:
import 'dart:mirrors';
class Enum {
static T parse<T>(String value) {
final T result = (reflectType(T) as ClassMirror).getField(#values)
.reflectee.firstWhere((v)=>v.toString().split('.').last.toLowerCase() == value.toLowerCase()) as T;
return result;
}
static T tryParse<T>(String value, { T defaultValue }) {
T result = defaultValue;
try {
result = parse<T>(value);
} catch(e){
print(e);
}
return result;
}
}
EDIT: this approach is NOT working in the Flutter applications, by default mirrors are blocked in the Flutter because it causes the generated packages to be very large.
enum in Dart just has too many limitations. The extension method could add methods to the instances, but not static methods.
I really wanted to be able to do something like MyType.parse(myString), so eventually resolved to use manually defined classes instead of enums. With some wiring, it is almost functionally equivalent to enum but could be modified more easily.
class OrderType {
final String string;
const OrderType._(this.string);
static const delivery = OrderType._('delivery');
static const pickup = OrderType._('pickup');
static const values = [delivery, pickup];
static OrderType parse(String value) {
switch (value) {
case 'delivery':
return OrderType.delivery;
break;
case 'pickup':
return OrderType.pickup;
break;
default:
print('got error, invalid order type $value');
return null;
}
}
#override
String toString() {
return 'OrderType.$string';
}
}
// parse from string
final OrderType type = OrderType.parse('delivery');
assert(type == OrderType.delivery);
assert(type.string == 'delivery');
another variant, how it might be solved:
enum MyEnum {
value1,
value2,
}
extension MyEnumX on MyEnum {
String get asString {
switch (this) {
case MyEnum.value1:
return _keyValue1;
case MyEnum.value2:
return _keyValue2;
}
throw Exception("unsupported type");
}
MyEnum fromString(String string) {
switch (string) {
case _keyValue1:
return MyEnum.value1;
case _keyValue2:
return MyEnum.value2;
}
throw Exception("unsupported type");
}
}
const String _keyValue1 = "value1";
const String _keyValue2 = "value2";
void main() {
String string = MyEnum.value1.asString;
MyEnum myEnum = MyEnum.value1.fromString(string);
}
enum HttpMethod { Connect, Delete, Get, Head, Options, Patch, Post, Put, Trace }
HttpMethod httpMethodFromString({#required String httpMethodName}) {
assert(httpMethodName != null);
if (httpMethodName is! String || httpMethodName.isEmpty) {
return null;
}
return HttpMethod.values.firstWhere(
(e) => e.toString() == httpMethodName,
orElse: () => null,
);
}
You can do something like this:
extension LanguagePreferenceForString on String {
LanguagePreferenceEntity toLanguagePrerence() {
switch (this) {
case "english":
return LanguagePreferenceEntity.english;
case "turkish":
return LanguagePreferenceEntity.turkish;
default:
return LanguagePreferenceEntity.english;
}
}
}
I was wondering if is possible to create an instance of a generic type in Dart. In other languages like Java you could work around this using reflection, but I'm not sure if this is possible in Dart.
I have this class:
class GenericController <T extends RequestHandler> {
void processRequest() {
T t = new T(); // ERROR
}
}
I tried mezonis approach with the Activator and it works. But it is an expensive approach as it uses mirrors, which requires you to use "mirrorsUsed" if you don't want to have a 2-4MB js file.
This morning I had the idea to use a generic typedef as generator and thus get rid of reflection:
You define a method type like this: (Add params if necessary)
typedef S ItemCreator<S>();
or even better:
typedef ItemCreator<S> = S Function();
Then in the class that needs to create the new instances:
class PagedListData<T>{
...
ItemCreator<T> creator;
PagedListData(ItemCreator<T> this.creator) {
}
void performMagic() {
T item = creator();
...
}
}
Then you can instantiate the PagedList like this:
PagedListData<UserListItem> users
= new PagedListData<UserListItem>(()=> new UserListItem());
You don't lose the advantage of using generic because at declaration time you need to provide the target class anyway, so defining the creator method doesn't hurt.
You can use similar code:
import "dart:mirrors";
void main() {
var controller = new GenericController<Foo>();
controller.processRequest();
}
class GenericController<T extends RequestHandler> {
void processRequest() {
//T t = new T();
T t = Activator.createInstance(T);
t.tellAboutHimself();
}
}
class Foo extends RequestHandler {
void tellAboutHimself() {
print("Hello, I am 'Foo'");
}
}
abstract class RequestHandler {
void tellAboutHimself();
}
class Activator {
static createInstance(Type type, [Symbol constructor, List
arguments, Map<Symbol, dynamic> namedArguments]) {
if (type == null) {
throw new ArgumentError("type: $type");
}
if (constructor == null) {
constructor = const Symbol("");
}
if (arguments == null) {
arguments = const [];
}
var typeMirror = reflectType(type);
if (typeMirror is ClassMirror) {
return typeMirror.newInstance(constructor, arguments,
namedArguments).reflectee;
} else {
throw new ArgumentError("Cannot create the instance of the type '$type'.");
}
}
}
I don't know if this is still useful to anyone. But I have found an easy workaround. In the function you want to initialize the type T, pass an extra argument of type T Function(). This function should return an instance of T. Now whenever you want to create object of T, call the function.
class foo<T> {
void foo(T Function() creator) {
final t = creator();
// use t
}
}
P.S. inspired by Patrick's answer
2022 answer
Just came across this problem and found out that although instantiating using T() is still not possible, you can get the constructor of an object easier with SomeClass.new in dart>=2.15.
So what you could do is:
class MyClass<T> {
final T Function() creator;
MyClass(this.creator);
T getGenericInstance() {
return creator();
}
}
and when using it:
final myClass = MyClass<SomeOtherClass>(SomeOtherClass.new)
Nothing different but looks cleaner imo.
Here's my work around for this sad limitation
class RequestHandler {
static final _constructors = {
RequestHandler: () => RequestHandler(),
RequestHandler2: () => RequestHandler2(),
};
static RequestHandler create(Type type) {
return _constructors[type]();
}
}
class RequestHandler2 extends RequestHandler {}
class GenericController<T extends RequestHandler> {
void processRequest() {
//T t = new T(); // ERROR
T t = RequestHandler.create(T);
}
}
test() {
final controller = GenericController<RequestHandler2>();
controller.processRequest();
}
Sorry but as far as I know, a type parameter cannot be used to name a constructor in an instance creation expression in Dart.
Working with FLutter
typedef S ItemCreator<S>();
mixin SharedExtension<T> {
T getSPData(ItemCreator<T> creator) async {
return creator();
}
}
Abc a = sharedObj.getSPData(()=> Abc());
P.S. inspired by Patrick
simple like that.
import 'dart:mirrors';
void main(List<String> args) {
final a = A<B>();
final b1 = a.getInstance();
final b2 = a.getInstance();
print('${b1.value}|${b1.text}|${b1.hashCode}');
print('${b2.value}|${b2.text}|${b2.hashCode}');
}
class A<T extends B> {
static int count = 0;
T getInstance() {
return reflectClass(T).newInstance(
Symbol(''),
['Text ${++count}'],
{Symbol('value'): count},
).reflectee;
}
}
class B {
final int value;
final String text;
B(this.text, {required this.value});
}
Inspired by Patrick's answer, this is the factory I ended up with.
class ServiceFactory<T> {
static final Map<Type, dynamic> _cache = <String, dynamic>{};
static T getInstance<T>(T Function() creator) {
String typeName = T.toString();
return _cache.putIfAbsent(typeName, () => creator());
}
}
Then I would use it like this.
final authClient = ServiceFactory.getInstance<AuthenticationClient>(() => AuthenticationClient());
Warning: Erik made a very good point in the comment below that the same type name can exist in multiple packages and that will cause issues. As much as I dislike to force the user to pass in a string key (that way it's the consumer's responsibility to ensuring the uniqueness of the type name), that might be the only way.