I have model
class Model {
int counter;
}
now I get a Model object at runtime, so I mark it as dynamic.
dynamic model = new Model();
int counter = 3;
What would be possible way to do equivalent of
model['counter'] = counter;
one thing to mention is that counter from Model is used in the app somewhere, so tree shaking wont remove it.
I am looking for any solution.
class Model {
int counter;
var counterPath = "any way to reference counter field, so I can give the value to it";
}
open to any suggestion or hack :)
One way is to implement a mapping between string name and actual field like
class Model {
int counter;
// reading: print(model['counter']);
operator [](String name) {
switch(name) {
case 'counter':
return counter;
default: throw 'Field $name doesn't exist' in $runtimeType';
}
// writing: model['counter'] = counter;
operator []=(String name, dynamic value) {
switch(name) {
case 'counter':
counter = value;
break;
default: throw 'Field $name doesn't exist' in $runtimeType';
}
}
I'd not recommend this, as using model['string'] makes you loose a lot in maintainability.
But if you truly need it, the best way to achieve this is by using code generation.
In your case, json_serializable may be a good idea. As it will generate a toJson method and a MyClass.fromJson constructor ; without having to change your model in any way.
In the end you'd have
#JsonSerializable()
class Model extends Object with _$ModelSerializerMixin {
int couter;
Model();
factory Model.fromJson(Map<String, dynamic> json) => _$ModelFromJson(json);
}
Which allows you to run the following :
final model = new Model()
..counter = 42;
final modelJson = model.toJson();
modelJson['counter'] = 43;
final result = new Model.fromJson(modelJson); // counter = 43
This may not be the most optimal in term of performances. But it's far less likely to have a bug or require constant edit.
That was for the easy solution.
But that's ugly ; please don't do that.
If you have a real use case where you need reflection then instead of loosing all type check ; create a custom code generator.
There are a few awesome toolings for code generators in dart.
Starting with the lower layer source_gen followed by build to name a few.
Dart team created a lot of tools for code generation ; go take a look !
Related
Is there a way to return several values in a function return statement (other than returning an object) like we can do in Go (or some other languages)?
For example, in Go we can do:
func vals() (int, int) {
return 3, 7
}
Can this be done in Dart? Something like this:
int, String foo() {
return 42, "foobar";
}
Dart doesn't support multiple return values.
You can return an array,
List foo() {
return [42, "foobar"];
}
or if you want the values be typed use a Tuple class like the package https://pub.dartlang.org/packages/tuple provides.
See also either for a way to return a value or an error.
I'd like to add that one of the main use-cases for multiple return values in Go is error handling which Dart handle's in its own way with Exceptions and failed promises.
Of course this leaves a few other use-cases, so let's see how code looks when using explicit tuples:
import 'package:tuple/tuple.dart';
Tuple2<int, String> demo() {
return new Tuple2(42, "life is good");
}
void main() {
final result = demo();
if (result.item1 > 20) {
print(result.item2);
}
}
Not quite as concise, but it's clean and expressive code. What I like most about it is that it doesn't need to change much once your quick experimental project really takes off and you start adding features and need to add more structure to keep on top of things.
class FormatResult {
bool changed;
String result;
FormatResult(this.changed, this.result);
}
FormatResult powerFormatter(String text) {
bool changed = false;
String result = text;
// secret implementation magic
// ...
return new FormatResult(changed, result);
}
void main() {
String draftCode = "print('Hello World.');";
final reformatted = powerFormatter(draftCode);
if (reformatted.changed) {
// some expensive operation involving servers in the cloud.
}
}
So, yes, it's not much of an improvement over Java, but it works, it is clear, and reasonably efficient for building UIs. And I really like how I can quickly hack things together (sometimes starting on DartPad in a break at work) and then add structure later when I know that the project will live on and grow.
Create a class:
import 'dart:core';
class Tuple<T1, T2> {
final T1 item1;
final T2 item2;
Tuple({
this.item1,
this.item2,
});
factory Tuple.fromJson(Map<String, dynamic> json) {
return Tuple(
item1: json['item1'],
item2: json['item2'],
);
}
}
Call it however you want!
Tuple<double, double>(i1, i2);
or
Tuple<double, double>.fromJson(jsonData);
You can create a class to return multiple values
Ej:
class NewClass {
final int number;
final String text;
NewClass(this.number, this.text);
}
Function that generates the values:
NewClass buildValues() {
return NewClass(42, 'foobar');
}
Print:
void printValues() {
print('${this.buildValues().number} ${this.buildValues().text}');
// 42 foobar
}
The proper way to return multiple values would be to store those values in a class, whether your own custom class or a Tuple. However, defining a separate class for every function is very inconvenient, and using Tuples can be error-prone since the members won't have meaningful names.
Another (admittedly gross and not very Dart-istic) approach is try to mimic the output-parameter approach typically used by C and C++. For example:
class OutputParameter<T> {
T value;
OutputParameter(this.value);
}
void foo(
OutputParameter<int> intOut,
OutputParameter<String>? optionalStringOut,
) {
intOut.value = 42;
optionalStringOut?.value = 'foobar';
}
void main() {
var theInt = OutputParameter(0);
var theString = OutputParameter('');
foo(theInt, theString);
print(theInt.value); // Prints: 42
print(theString.value); // Prints: foobar
}
It certainly can be a bit inconvenient for callers to have to use variable.value everywhere, but in some cases it might be worth the trade-off.
you can use dartz package for Returning multiple data types
https://www.youtube.com/watch?v=8yMXUC4W1cc&t=110s
Dart is finalizing records, a fancier tuple essentially.
Should be in a stable release a month from the time of writing.
I'll try to update, it's already available with experiments flags.
you can use Set<Object> for returning multiple values,
Set<object> foo() {
return {'my string',0}
}
print(foo().first) //prints 'my string'
print(foo().last) //prints 0
In this type of situation in Dart, an easy solution could return a list then accessing the returned list as per your requirement. You can access the specific value by the index or the whole list by a simple for loop.
List func() {
return [false, 30, "Ashraful"];
}
void main() {
final list = func();
// to access specific list item
var item = list[2];
// to check runtime type
print(item.runtimeType);
// to access the whole list
for(int i=0; i<list.length; i++) {
print(list[i]);
}
}
I am trying to understand the rational behind using writeByte(3) in the write method in Hive TypeAdapter.
Please consider the class:
#HiveType()
class Person{
#HiveField(0)
String name;
#HiveField(1)
int age;
}
In the TypeAdapter below It is easy to understand the read method, since it is just reads sequentially each field.
However, I'm trying to figure out why the same mechanism does not apply to the write, instead of using ..writeByte(...) just before each field. And, what is the meaning of the first ..writeByte(2)?
class PersonAdapter extends TypeAdapter<Person> {
#override
Person read(BinaryReader reader) {
var numOfFields = reader.readByte();
var fields = <int, dynamic>{
for (var i = 0; i < numOfFields; i++) reader.readByte(): reader.read(),
};
return Trips()
..name = fields[0] as String
..age = fields[1] as int;
}
#override
void write(BinaryWriter writer, Person obj) {
writer
..writeByte(2) // Why this here? (sometimes I see writeByte(3) !! )
..writeByte(0)
..write(obj.name)
..writeByte(1)
..write(obj.age);
}
}
Thanks for any clarification.
I know nothing about Hive but if you take a look at the builder which create this write method you can see the following:
String buildWrite() {
var code = StringBuffer();
code.writeln('writer');
code.writeln('..writeByte(${getters.length})');
for (var field in getters) {
var value = _convertIterable(field.type, 'obj.${field.name}');
code.writeln('''
..writeByte(${field.index})
..write($value)''');
}
code.writeln(';');
return code.toString();
}
https://github.com/hivedb/hive/blob/59ad5403593283233d922f62f76832c64fa33a3b/hive_generator/lib/src/class_builder.dart#L122
So based on this we can conclude the first writeByte is the length of getters. The next one is the index of the first getter (0) following by the value and next getter (1) with value and so on.
This makes sense since the protocol properly needs to know how many fields it can expect to get.
I would like to clone a complex object (copy values), not referencing, using Dart 2.
Example:
class Person {
String name;
String surname;
City city;
}
class City {
String name;
String state;
}
main List<String> args {
City c1 = new City()..name = 'Blum'..state = 'SC';
Person p1 = new Person()..name = 'John'..surname = 'Xuebl'..city = c1;
Person p2 = // HERE, to clone/copy values... Something similar to p1.clone();
}
What would be the way (best practice) to do this?
Update note: This How can I clone an Object (deep copy) in Dart? was posted some time ago. The focus here is to understand if Dart 2 that is bringing many improvements, has a facility for copying complex objects.
With the classes you have shown us here, there is nothing shorter than
Person p2 = Person()
..name = p1.name
..surname = p1.surname
..city = (City()..name = p1.city.name..state = p1.city.state);
If you add a clone method to Person and City, then you can obviously use that.
There is nothing built in to the language to allow you to copy the state of an object.
I would recommend changing the classes, at least by adding a constructor:
class Person {
String name;
String surname;
City city;
Person(this.name, this.surname, this.city);
}
class City {
String name;
String state;
City(this.name, this.state);
}
Then you can clone by just writing:
Person P2 = Person(p1.name, p1.surname, City(p1.city.name, p1.city.state));
(And ob-link about names)
I say that there is no language feature to copy objects, but there actually is, if you have access to the dart:isolate library: Sending the object over a isolate communication port. I cannot recommend using that feature, but it's here for completeness:
import "dart:isolate";
Future<T> clone<T>(T object) {
var c = Completer<T>();
var port = RawReceivePort();
port.handler = (Object o) {
port.close();
c.complete(o);
}
return c.future;
}
Again, I cannot recommend using this approach.
It would work for simple objects like this, but it doesn't work for all objects (not all objects can be sent over a communication port, e.g., first-class functions or any object containing a first class function).
Write your classes to support the operations you need on them, that includes copying.
My simpler solution just let clone() return a new Person with the current values:
class Person {
String name;
String surname;
City city;
Person(this.name, this.surname, this.city);
clone() => Person(name, surname, city);
}
You might further need to recursively clone the objects in your Person. as an example by creating a similar clone() function in the City and using it here as city.clone().
For the strings you will need to check their behavior or also create / add a way for cleaning them.
As said, there is no built in solution for that, but if the ideia is to accomplish immutable value types you can check built_value.
https://medium.com/dartlang/darts-built-value-for-immutable-object-models-83e2497922d4
I noted that using Map.from() do a shallow copy and not a deep copy.
To do a deep copy of a class containing a Map of anoter Class, one solution can be to use a nammed constructor
class MyClassB {
int myVar;
// Constructor
MyClassB(this.id);
// Named Constructor to do a deep clone
MyClassB.clone(MyClassB b){
id = b.id;
}
}
class MyClassA {
Map<int,MyClassB> mapOfClassB;
// Constructor
MyClassA(this.myClassB)
// Named constructor to do a deep clone
MyClassA.clone(MyClassA a){
Map<int,myClassB> m = {};
myClassB = a.mapOfClassB.forEach((k,v)=> m[k] = MyClassB.clone(v)); // Use the clone constructor here, if not the maps in MyClassA and MyClassB will be linked
}
}
main() {
var b1 = MyClassB(20);
var a1 = MyClassA({0:b1});
var a2 = MyClass1A.clone(a1);
a2.mapOfClassB[0].id = 50;
print(a1.mapOfClassB[0].id); // Should display 20
print(a2.(a1.mapOfClassB[0].id) // Should display 50
}
Using a package like freezed, you could make deep copies of the complex objects.
Although one downside is that the objects are immutable and you cannot make shallow copies of it. But again, it depends on your use case and how you want your objects to be.
I'm wondering if it's possible to filter logs by properties of passed arguments to a specific function. To be more specific, here's where I'm starting:
_dispatcher.getLogs(callsTo("dispatchEvent", new isInstanceOf<PinEvent>()));
I'd like to further filter this by PinEvent.property = "something"
In pseudo-code, I guess it'd look like this:
_dispatcher.getLogs(callsTo("dispatchEvent", new isInstanceOf<PinEvent>("property":"something")));
Any ideas? I know I can loop through the entire log list, but that seems dirty, and I would think there'd be a better way.
Thanks :-)
You can write your own matcher and use it instead of isInstanceOf
or derive from isInstanceOf and extend this matcher with the missing functionality.
I don't know of a matcher that does this out of the box (but I must admit that I didn't yet work very much with them).
While waiting for responses I whipped this up real quick. It's probably not perfect, but it does allow you to combine an isInstanceOf with this matcher using allOf. ex:
allOf(new isInstanceOf<MyThing>(), new ContainsProperty("property", "propertyValue"));
And here's the Matcher:
class ContainsProperty extends Matcher {
final String _name;
final dynamic _value;
const ContainsProperty(String name, dynamic value):this._name = name, this._value = value;
bool matches(obj, Map matchState) {
try {
InstanceMirror objMirror = reflect(obj);
InstanceMirror propMirror = objMirror.getField(new Symbol(_name));
return propMirror.reflectee == _value;
} catch (e) {
return false;
}
}
Description describe(Description description) {
description.add("contains property \"${_name}\" with value \"${_value}\"");
}
}
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.