I'm trying to port some Dart code to another language but I'm a little confused by the .new constructor.
Given this class:
class DynamicTreeNode {
final int id;
DynamicTreeNode(this.id);
}
and this simple main function:
void main(List<String> arguments) {
List<DynamicTreeNode> _nodes = List<DynamicTreeNode>.generate(
16,
DynamicTreeNode.new,
);
}
I can see in the debugger that each instance of DynamicTreeNode is given an incrementing id value matching the index in the List:
Can someone explain what is happening here? I thought the call to new would fail as the unnamed (and only constructor) requires an int to be passed to it.
If we look at the List.generate constructor we can see that the signature is
List<E>.generate(
int length,
E generator(
int index
),
{bool growable = true}
)
The generator parameter — in your case DynamicTreeNode.new — is a function that takes the position in the list it is generated into as an argument and returns the type of element of the list. So the first DynamicTreeNode in the list will be generated by DynamicTreeNode.new(0), the second one as DynamicTreeNode.new(1), etc.
I'm not familiar with dart at all, but it looks like you're passing the .new function itself, and the .generate function on List<T> will construct an object under the hood by calling the .new function with an incrementing integer.
In JavaScript, that would look like this:
class DynamicTreeNode {
constructor(id) {
this.id = id;
}
}
const generate = (len, constructor) => new Array(len).fill(0).map((_, idx) => new constructor(idx));
function main() {
const _nodes = generate(16, DynamicTreeNode);
console.log(_nodes);
}
main();
I have a class in AngularDart as followings:
abstract class Validator {
Map validate(AbstractControl c);
}
Looking closely, this used to be (before we added strong-mode support):
abstract class Validator {
validate(AbstractControl c);
}
The issue that it technically supports returning a Future or Map.
I'd like to refactor this and properly type it using FutureOr:
abstract class Validator {
FutureOr<T> validate(AbstractControl c);
}
Will I be able to use an is check at runtime? (In DDC and dart2js)
void runValidator(Validator v) {
var result = v.validate(...);
if (result is Future) {
// async...
} else {
// sync...
}
}
Am I thinking about this correctly?
EDIT: As mentioned below, I mean
if (result is Future<T>) {
} else if (result is T) {
}
One more question, would validate match these two typedefs:
Future<Map> AsyncValidate(AbstractControl c);
Map SyncValidate(AbstractControl c);
Yes, you can do result is Future<Map>. The actual value returned by the validate method is either a Future or it's not. The static type of the function doesn't affect that, and since FutureOr<Map> isn't an actual class, you can't have an object that is "a FutureOr". It's either a real Future<Map> or it's a Map.
For the second question, that depends on what yo mean by "match".
You can override the method with a method that returns either Map or FutureMap:
abstract class Validator {
FutureOr<Map> validate(abstractControl c);
}
class AsyncValidator extends Validator {
Future<Map> validate(AbstractControl c) {...}
}
class SyncValidator extends Validator {
Map validate(AbstractControl c) {...}
}
That is, you can use one of the function types you mention as a Validator.validate, but not in the other direction.
typedef FutureOr<Map> Validate(AbstractControl c);
typedef Future<Map> AsyncValidate(AbstractControl c);
typedef Map SyncValidate(AbstractControl c);
Validator v = ...;
Validate f0 = v.validate; // Safe.
AsyncValidate f1 = v.validate; // BAD assignment.
SyncValidate f2 = v.validate; // BAD assignment.
Map syncValidate(AbstractControl c) { ... }
Future<Map> asyncValidate(AbstractControl c) { ... }
v = syncValidate; // Good assignment.
v = asyncValidate; // Good assignment.
In practice, the concrete validate method of the validator v will probably be assignable to one of f1 or f2, but its static type doesn't say which one, so both are considered bad assignments.
You should only very rarely have a non-abstract method that is declared as returning FutureOr. In most cases, it's better to just always return a Future or a non-Future, and declare the method as such. Then you can always use the function as returning FutureOr if you need to, but use the more precise type in cases where you need it.
I'm playing around with the mirrors stuff in Dart. I can't find any way reflect a class and figure out if it has a constructor and, if so, what the parameters to this constructor are.
With the ClassMirror, it looks like the "declarations" collection of DeclarationMirror objects will include an entry for the constructor, but there's no way with a DeclarationMirror to tell if whether or not it's a constructor, and no way to see information about the parameters.
With the "instanceMembers" collection of MethodMirror objects, it looks like constructors are not even included. I assume this is because a constructor isn't a normal kind of method that one could invoke, but still, it's odd since MethodMirror has an "isConstructor" attribute.
Is there any way to, given an object type, figure out if it has a constructor and, if so, get information on the parameters to that constructor?
The below code illustrates the problem:
import 'dart:mirrors';
class Person {
String name;
int age;
Person(this.name, this.age);
string getNameAndAge() {
return "${this.name} is ${this.age} years old";
}
}
void main() {
ClassMirror classMirror = reflectClass(Person);
// This will show me the constructor, but a DeclarationMirror doesn't tell me
// anything about the parameters.
print("+ Declarations");
classMirror.declarations.forEach((symbol, declarationMirror) {
print(MirrorSystem.getName(symbol));
});
// This doesn't show me the constructor
print("+ Members");
classMirror.instanceMembers.forEach((symbol, methodMirror) {
print(MirrorSystem.getName(symbol));
});
}
Firstly, you need to find contructors in declarations map.
ClassMirror mirror = reflectClass(Person);
List<DeclarationMirror> constructors = new List.from(
mirror.declarations.values.where((declare) {
return declare is MethodMirror && declare.isConstructor;
})
);
Then, you can cast DeclarationMirror into MethodMirror and use getter MethodMirror.parameters to get all parameters of the constructor. Something like:
constructors.forEach((construtor) {
if (constructor is MethodMirror) {
List<ParameterMirror> parameters = constructor.parameters;
}
});
I would like to pass a primitive (int, bool, ...) by reference. I found a discussion about it (paragraph "Passing value types by reference") here: value types in Dart, but I still wonder if there is a way to do it in Dart (except using an object wrapper) ? Any development ?
The Dart language does not support this and I doubt it ever will, but the future will tell.
Primitives will be passed by value, and as already mentioned here, the only way to 'pass primitives by reference' is by wrapping them like:
class PrimitiveWrapper {
var value;
PrimitiveWrapper(this.value);
}
void alter(PrimitiveWrapper data) {
data.value++;
}
main() {
var data = new PrimitiveWrapper(5);
print(data.value); // 5
alter(data);
print(data.value); // 6
}
If you don't want to do that, then you need to find another way around your problem.
One case where I see people needing to pass by reference is that they have some sort of value they want to pass to functions in a class:
class Foo {
void doFoo() {
var i = 0;
...
doBar(i); // We want to alter i in doBar().
...
i++;
}
void doBar(i) {
i++;
}
}
In this case you could just make i a class member instead.
No, wrappers are the only way.
They are passed by reference. It just doesn't matter because the "primitive" types don't have methods to change their internal value.
Correct me if I'm wrong, but maybe you are misunderstanding what "passing by reference" means? I'm assuming you want to do something like param1 = 10 and want this value to still be 10 when you return from your method. But references aren't pointers. When you assign the parameter a new value (with = operator), this change won't be reflected in the calling method. This is still true with non-primitive types (classes).
Example:
class Test {
int val;
Test(this.val);
}
void main() {
Test t = new Test(1);
fn1(t);
print(t.val); // 2
fn2(t);
print(t.val); // still 2, because "t" has been assigned a new instance in fn2()
}
void fn1(Test t) {
print(t.val); // 1
t.val = 2;
}
void fn2(Test t) {
t = new Test(10);
print(t.val); // 10
}
EDIT
I tried to make my answer more clear, based on the comments, but somehow I can't seem to phrase it right without causing more confusion. Basically, when someone coming from Java says "parameters are passed by reference", they mean what a C/C++ developer would mean by saying "parameters are passed as pointers".
As dart is compiled into JavaScript, I tried something that works for JS, and guess what!? It worked for dart!
Basically, what you can do is put your value inside an object, and then any changes made on that field value inside that function will change the value outside that function as well.
Code (You can run this on dartpad.dev)
main() {
var a = {"b": false};
print("Before passing: " + a["b"].toString());
trial(a);
print("After passing: " + a["b"].toString());
}
trial(param) {
param["b"] = true;
}
Output
Before passing: false
After passing: true
One of the way to pass the variables by reference by using the values in List. As arrays or lists are Pass by reference by default.
void main() {
List<String> name=['ali' ,'fana'];
updatename(name);
print(name);
}
updatename(List<String> name){
name[0]='gufran';
}
Try this one, This one of the simplest way to pass by reference.
You can use ValueNotifier
And, you can pass it as ValueListenable to classes or methods that needs to know up-to-date value, but should not edit it:
class Owner {
final theValue = ValueNotifier(true);
final user = User(theValue);
...
}
class User {
final ValueListeneble<bool> theValue;
User(this.theValue);
...
}
It provides more functionality than actually needed, but solves the problem.
If ValueNotifier + ValueListenable do not work for you (you want to make sure the client does not listen to every change of the value, or your package is pure Dart package and thus cannot reference Flutter libraries), use a function:
class Owner {
int _value = 0;
int getValue() => _value;
void increase() => _value++;
}
void main() {
final owner = Owner();
int Function() obtainer = owner.getValue;
print(obtainer());
owner.increase();
print(obtainer());
}
Output will be:
0
1
This approach has memory usage related downside: the obtainer will hold the reference to the owner, and this, even if owner is already not referenced, but obtainer is still reachable, owner will be also reachable
and thus will not be garbage collected.
If you do not want the downside, pass the smaller container than the entire owner:
import 'package:flutter/foundation.dart';
class ListenableAsObtainer<T> implements ValueObtainer<T> {
ListenableAsObtainer(this._listenable);
final ValueListenable<T> _listenable;
#override
T get value => _listenable.value;
}
class FunctionAsObtainer<T> implements ValueObtainer<T> {
FunctionAsObtainer(this._function);
final T Function() _function;
#override
T get value => _function();
}
class ValueAsObtainer<T> implements ValueObtainer<T> {
ValueAsObtainer(this.value);
#override
T value;
}
/// Use this interface when the client needs
/// access to the current value, but does not need the value to be listenable,
/// i.e. [ValueListenable] would be too strong requirement.
abstract class ValueObtainer<T> {
T get value;
}
The usage of FunctionAsObtainer will still result in holding the owner from garbage collection, but two other options will not.
Just to make it clear:
void main() {
var list1 = [0,1,2];
var modifiedList1 = addMutable(list1, 3);
var list2 = [0,1,2];
var modifiedList2 = addImmutable(list2, 3);
print(list1);
print(modifiedList1);
print(list2);
print(modifiedList2);
}
List<int> addMutable(List<int> list, int element){
return list..add(element);
}
List<int> addImmutable(List<int> list, int element){
return [...list, element];
}
Output:
[0, 1, 2, 3]
[0, 1, 2, 3]
[0, 1, 2]
[0, 1, 2, 3]
All variables are passed by value. If a variable contains a primitive (int, bool, etc.), that's it. You got its value. You can do with it whatever you want, it won't affect the source value. If a variable contains an object, what it really contains is a reference to that object.
The reference itself is also passed by value, but the object it references is not passed at all. It just stayed where it was. This means that you can actually make changes to this very object.
Therefore, if you pass a List and if you .add() something to it, you have internally changed it, like it is passed by reference. But if you use the spread operator [...list], you are creating a fresh new copy of it. In most cases that is what you really want to do.
Sounds complicated. Isn't really. Dart is cool.
In Dart, is it possible for a function to have a prototype associated with it?
Example Javascript code:
doStuff.prototype.isDefined = true; //is there anything like Javascript's function prototypes in Dart?
function doStuff(){
console.log("The function doStuff was called!");
}
Is it possible to do the equivalent of this in Dart (i.e., create a list of properties for each function?)
Two things to address here:
First, Dart doesn't have prototypes or prototypal inheritance, and instead uses classical inheritance. Rather than a prototype, objects have a class, and instead of a prototype chain, objects have superclasses.
Second, for your specific case, I think we'd have to see more of what you need to do to figure out the idiomatic way to do it in Dart. It should soon be possible to emulate functions with objects so that you can invoke an object and still have state and other methods associated with it.
See this article for more: http://www.dartlang.org/articles/emulating-functions/
When that capability lands you'll be able to do this:
class DoStuff {
bool isDefined = true;
call() => print("The function doStuff was called!");
}
var doStuff = new DoStuff();
main() => doStuff();
Which works if you have a fixed set of metadata about your function that you need to keep track of. It's slightly different from JavaScript because each instance of the function in Dart will have its own state for isDefined. I'm not sure if it's possible or easy to get multiple instances of the function in JavasScript, but you might need to make isDefined static so that the value is shared across all instances.
Dart does not allow you to add or remove member variables from an instance of a class at runtime. Rewriting your example in Dart it might look something like this:
class doStuff {
bool isDefined;
doStuff() {
isDefined = true;
}
void stuff() {
print('The function stuff was called!');
}
}
main() {
new doStuff().stuff();
}
If you wanted to add a property bag to a class in Dart you would write:
class PropertyObject {
Map<String, Dynamic> properties;
PropertyObject() {
properties = new Map<String, Dynamic>();
}
Dynamic operator[](String K) => properties[K];
void operator[]=(String K, Dynamic V) => properties[K] = V;
}
main() {
PropertyObject bag = new PropertyObject();
bag['foo'] = 'world';
print('Hello ${bag['foo']}');
}
Note that you can't access map properties using the '.' operator.