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 have code that is something like this:
using variant_t = std::variant<MyObj1, MyObj2, MyObj3>;
auto foo(){
variant_t var;
if (condition1){
var = MyObj1{"A String"};
// Other stuff
} else if (condition2) {
var = MyObj2{123, 12345};
// Other stuff
} else if (condition3) {
var = MyObj3{SomeObject};
// Other stuff
} else {
throw std::runtime_error{};
}
return var;
}
int main(){
auto var = foo();
std::visit([&](auto& v){v.call_shared_function_name();}, var);
}
Assuming all of the MyObj's are non-trivial, is there a way to get this to work?
I know std::monostate will allow you to initialize the variant in this way and actually populate it later. But if I do so, I can't have the clean auto& lambda in the visitor and will have to create a visitor/lambda for each type.
I know this might looks silly, or not that clean, but you actually need to initialize with one of those objects:
variant_t var = MyObj1{"A String"};
And it compiles fine, if you can't, please let me know and i'll remove the answer
I've been looking through the code for a Flash game (link). However, I'm having trouble understanding how some of these functions work, especially because some of them have function definitions that I would think to fail to get past the compiler.
The following is some code from TodCommon.as that appears to conflict with itself (or at the very least uses bad naming conventions).
private static var gFlashingColor:Color = new Color();
final public static function ClampFloat(ClampInt:Number, ClampInt:Number, ClampInt:Number) : Number
{
if(ClampInt <= ClampInt)
{
return ClampInt;
}
if(ClampInt >= ClampInt)
{
return ClampInt;
}
return ClampInt;
}
final public static function ClampInt(gFlashingColor:int, gFlashingColor:int, gFlashingColor:int) : int
{
if(gFlashingColor <= gFlashingColor)
{
return gFlashingColor;
}
if(gFlashingColor >= gFlashingColor)
{
return gFlashingColor;
}
return gFlashingColor;
}
Also in the code is the weirdest syntax for a for-each loop that I've ever seen (this example also features a package name as a parameter name)
public function CountPlantByType(com.popcap.flash.framework.resources.fonts:int) : int
{
var _loc_3:CPlant = null;
var _loc_2:int = 0;
var _loc_4:int = 0;
var _loc_5:* = this.mPlants;
while(<to complete>)
{
_loc_3 = __nextvalue;
if(_loc_3.mSeedType != com.popcap.flash.framework.resources.fonts)
{
break;
}
_loc_2++;
}
return _loc_2;
}
Those are just a few examples of things that I think look super weird and am having trouble understanding. But these functions all work and are used extensively throughout the code. Can someone explain how the ClampFloat and ClampInt functions work, or why it's legal to use a package name as a parameter? Thanks
Resolved. Turns out the program I used to extract these files from the SWF also corrupted them in the process. Using JPEXS Free Flash Decompiler instead of ActionScriptExtractor fixed the code syntax.
I know basic Javascript, but am confronted with a problem in a Typescript file. I'm using Ionic framework to test a page where a user can theoretically "swipe" like they're on Tinder, just for fun.
I have all the JS written, because I'm moving this over from Codepen, but I can't seem to get past Typescript's syntax.
The Javascript:
var tinderContainer = document.querySelector('.tinder');
var allCards = document.querySelectorAll('.tinder--card');
var nope = document.getElementById('nope');
var love = document.getElementById('love');
function initCards(card, index) {
var newCards = document.querySelectorAll('.tinder--card:not(.removed)');
newCards.forEach(function (card, index) {
card.style.zIndex = allCards.length - index;
}
}
The Typescript (that I put together using Google and SOF answers):
export class TestPage {
constructor(public navCtrl: NavController) {
}
tinderContainer = document.querySelector('ion-content');
allCards = document.querySelector('.tinder--card');
nope = document.getElementById('nope');
love = document.getElementById('love');
declare initCards(card,index) {
newCards = document.querySelectorAll('.tinder--card:not(.removed)');
newCards.forEach((card,index)) {
card.style.zIndex = allCards.length - index;
}
}
}
some hints are:
Use let newCards in you function as you have to declare your variable.
Your forEach should be something like this.
newCards.forEach((card, index) => {
...
});
but in order to use syntax like card.style.zIndex and allCards.length you will have to declare variable types..
For unknown models you can use something like card['style']['zIndex']
Also you have to use this to access class properties, like this.allCards
I am new to Dart programming. I am trying to figure out what is the proper way (what everyone will do) to handle/guard those functions which are login required. The following is my first trial:
$ vim login_sample.dart:
var isLoggedIn;
class LoginRequiredException implements Exception {
String cause;
LoginRequiredException(this.cause);
}
Function loginRequired(Function f) {
if (!isLoggedIn) {
throw new LoginRequiredException("Login is reuiqred.");
}
return f;
}
void secretPrint() {
print("This is a secret");
}
void main(List<String> args) {
if (args.length != 1) return null;
isLoggedIn = (args[0] == '1') ? true : false;
try {
loginRequired(secretPrint)();
} on LoginRequiredException {
print("Login is required!");
}
}
then, run it with $ dart login_sample.dart 1 and $ dart login_sample.dart 2.
I am wondering if this is the recommended way to guard login required functions or not.
Thank you very much for your help.
Edited:
My question is more about general programming skills in Dart than how to use a plugin. In python, I just need to add #login_required decorator in the front of the function to protect it. I am wondering if this decorator function way is recommended in dart or not.
PS: All firebase/google/twitter/facebook etc... are blocked in my country.
I like the functional approach. I'd only avoid using globals, you can wrap it in a Context so you can mock then for tests and use Futures as Monads: https://dartpad.dartlang.org/ac24a5659b893e8614f3c29a8006a6cc
Passing the function is not buying much value. In a typical larger Dart project using a framework there will be some way to guard at a higher level than a function - such as an entire page or component/widget.
If you do want to guard at a per-function level you first need to decide with it should be the function or the call site that decides what needs to be guarded. In your example it is the call site making the decision. After that decision you can implement a throwIfNotAuthenticated and add a call at either the definition or call site.
void throwIfNotAuthenticated() {
if (!userIsAuthenticated) {
throw new LoginRequiredException();
}
}
// Function decides authentication is required:
void secretPrint() {
throwIfNotAuthenticated();
print('This is a secret');
}
// Call site decides authentication is required:
void main() {
// do stuff...
throwIfNotAuthenticated();
anotherSecreteMethod();
}