I am new to Dart and I wonder if I can, for example, extend the DivElement class to create custom elements in one line of code. I am looking for something like this;
class RedBox extends DivElement {
RedBox(text) {
this.text = text;
this.style.background = "red";
}
}
RedBox myBox = new RedBox("Hello World");
document.body.append(myBox);
Of course I will have much more complex elements with custom functions. But just in general, is this possible?
When I try to run this, i get:
implicit call to super constructor 'DivElement()'
You can extend HTML elements, but there are a few requirements. The one you're running into now is that you need a RedBox.created constructor, and it can only redirect to its super class. created must be the only generative constructor, though you can add factory constructors.
Another requirement is that the element is registered with document.registerElement.
Try adding this:
class RedBox extends HtmlElement {
RedBox.created() : super.created() {
style.background = "red";
}
factory RedBox(text) => new Element.tag('my-redbox')..text = text;
}
document.registerElement('my-redbox', RedBox);
Some note on extending HtmlElement.
Example:
https://api.dartlang.org/1.14.1/dart-html/HtmlDocument/registerElement.html
Pitfalls:
Extending non HtmlElement(e.g. PreElement) throws:
HierarchyRequestError: Custom element XXX is a native PreElement
should be derived from HtmlElement or SvgElement.
Using extendsTag option with registerElement suppresses the above error but causes "new Element.tag('xxx')" to return an instance of HtmElement.
document.registerElement('xxx', XXX, extendsTag:'pre');
Solution(Assuming extending PreElement):
Use 'document.registerElement('xxx', XXX, extendsTag:'pre');' and 'new Element.tag('pre','xxx');'
void main{
document.registerElement('xxx',
XXX,extendsTag: 'pre');
querySelectior('body').append(new XXX()..text = 'hello');
}
class XXX extends PreElement{
XXX.created():super.created(){}
factory XXX(){
return new Element.tag('pre','xxx');
}
}
Dart does not currently support library initialization. You must call document.registerElement in the main.
Tested with 1.14.0
Related
I'd like to use ES6 public class fields:
class Superclass {
constructor() {
// would like to write modular code that applies to all
// subclasses here, or similarly somewhere in Superclass
this.example++; // does NOT WORK (not intialized)
//e.g. doStuffWith(this.fieldTemplates)
}
}
class Subclass extends Superclass {
example = 0
static fieldTemplates = [
Foo,
function() {this.example++},
etc
]
}
Problem:
ES6 public fields are NOT initialized before the constructors, only before the current constructor. For example, when calling super(), any child field will not yet have been defined, like this.example will not yet exist. Static fields will have already been defined. So for example if one were to execute the code function(){this.example++} with .bind as appropriate, called from the superclass constructor, it would fail.
Workaround:
One workaround would be to put all initialization logic after all ES6 public classes have been properly initialized. For example:
class Subclass extends Superclass {
example = 0
lateConstructor = (function(){
this.example++; // works fine
}).bind(this)()
}
What's the solution?
However, this would involve rewriting every single class. I would like something like this by just defining it in the Superclass.constructor, something magic like Object.defineProperty(this, 'lateConstructor', {some magic}) (Object.defineProperty is allegedly internally how es6 static fields are defined, but I see no such explanation how to achieve this programatically in say the mozilla docs; after using Object.getOwnPropertyDescriptor to inspect my above immediately-.binded-and-evaluated cludge I'm inclined to believe there is no way to define a property descriptor as a thunk; the definition is probably executed after returning from super(), that is probably immediately evaluated and assigned to the class like let exampleValue = eval(...); Object.defineProperty(..{value:exampleValue})). Alternatively I could do something horrible like do setTimeout(this.lateConstructor,0) in the Superclass.constructor but that would break many things and not compose well.
I could perhaps try to just use a hierarchy of Objects everywhere instead, but is there some way to implement some global logic for all subclasses in the parent class? Besides making everything lazy with getters? Thanks for any insight.
References:
Run additional action after constructor -- (problems: this requires wrapping all subclasses)
Can I create a thunk to run after the constructor?
No, that is not possible.
How to run code after class fields are initialized, in a sane way?
Put the code in the constructor of the class that defines those fields.
Is there some way to implement some global logic for all subclasses in the parent class?
Yes: define a method. The subclass can call it from its constructor.
Just thought of a workaround (that is hierarchically composable). To answer my own question, in a somewhat unfulfilling way (people should feel free to post better solutions):
// The following illustrates a way to ensure all public class fields have been defined and initialized
// prior to running 'constructor' code. This is achieved by never calling new directly, but instead just
// running Someclass.make(...). All constructor code is instead written in an init(...) function.
class Superclass {
init(opts) { // 'constructor'
this.toRun(); // custom constructor logic example
}
static make() { // the magic that makes everything work
var R = new this();
R.init(...arguments);
return R;
}
}
class Subclass extends Superclass {
subclassValue = 0 // custom public class field example
init(toAdd, opts) { // 'constructor'
// custom constructor logic example
this.subclassValue += toAdd; // may use THIS before super.init
super.init(opts);
// may do stuff afterwards
}
toRun() { // custom public class method example
console.log('.subclassValue = ', this.subclassValue);
}
}
Demo:
> var obj = Subclass.make(1, {});
.subclassValue = 1
> console.log(obj);
Subclass {
subclassValue: 1
__proto__: Superclass
}
I am trying to create a class (working as factory class) in my Zend Expressive APP as follows:
declare(strict_types=1);
namespace App\Install\Factory;
use App\Install\Model as Models;
use App\Install\Abstracts\AttributeInterface;
class AttributeEntityFactory{
public static function create($type1 ='Attribute') : AttributeInterface
{
$resolvedClass = "Models\\$type1";
$resolvedClass1 = 'Models\\'.$type1;
//return new $resolvedClass();
//return new $resolvedClass1();
return new Models\Attribute();
}
}
The above code works perfectly for me. However, if try to use any of the other two return statements it shows
Class 'Models\Attribute' not found
How can I achieve dynamic instantiation?
The attribute class code is as follows:
namespace App\Install\Model;
use App\Install\Abstracts\AttributeInterface;
class Attribute implements AttributeInterface
{
protected $attribute;
public function setAttribute($attribute)
{
$this->attribute = $attribute;
}
public function getAttribute()
{
return $this->attribute;
}
}
My PHP version is:
PHP 7.2.13 (cli) (built: Dec 14 2018 04:20:16) ( NTS )
you may need to pass in the full namespace?
"App\Install\Model\" . $type1;
and more...
the model Attribute you have is in the namespace App\Install\Model, and the object you are trying to create is from Models\\ . $type1
maybe you need to change Models to Model
Personally, I would avoid such factory implementation because of several reasons:
It involves magic.
Less predictable code.
Harder to read for both humans and IDE's (E.g: PHPStorm would not find the usages of Attribute class in such code when you need to find it)
Harder to analyze using static analyzers
Instead, I would rewrite this to a more explicit factory, even if I had dozens of different classes in App\Install\Model namespace:
<?php declare(strict_types=1);
namespace App\Install\Factory;
use App\Install\Model as Models;
class AttributeEntityFactory
{
public static function create($type = 'Attribute') : AttributeInterface
{
switch ($type) {
case 'Attribute':
return new Models\Attribute();
case 'SomethingElse':
return new Models\SomethingElse();
default:
throw new \InvalidArgumentException(
sprintf('An unknown type %s requested from %s', $type, __METHOD__)
);
}
}
}
As a rule of thumb:
Never compose classnames / namespaces using strings concatenated with variables / parameters / constants whatever.
Never call methods in such way, too.
You'll thank me when your application/business/codebase grows enough.
I am designing a simple game in Flash and have come across this error. I have no idea how to go about this in actionscript and would appreciate any help.
Basically, I have a switch statement which creates an object of different type depending on each case (as I would prefer not to duplicate the same ten lines of code for each case) and I am getting a "conflict exists with definition in namespace internal" compiler error and I think I understand why.
switch(power){
case 1:
var Pow:objectOne = new objectOne();
break;
case 2:
var Pow:objectTwo = new objectTwo();
break;
}
My question however is this - what is the proper way of going about this?
I initially thought of declaring the variable before the switch statement which results in an "implicit coercion of a value of type object(One/Two) to an unrelated type Class" error. What am I missing here?
Aside from the compiler error you are experiencing, another problem here is that you are planning on using the pow variable later in your code, yet they are of different types. My suggestion is to use the benefits of Inheritance in OOP and create a base class that your two custom classes can inherit from. That way they are both technically of the same base type, while still giving you the freedom to customize each custom class, while keeping similar functionality in the base class.
Remember, OOP is here to always help you and is there to avoid issues like the one you have come across, but here is how I would do it, and I tested the following implementation in Flash CC 2014 and it compiled successfully:
Example .FLA:
var pow:BaseClass;
var power = 1;
switch(power){
case 1:
pow = new ObjectOne();
break;
case 2:
pow = new ObjectTwo();
break;
}
pow.whichObjectAmI(); // this will simply trace what object pow is
Base Class
package {
public class BaseClass {
public function BaseClass() {
// constructor code
}
public function whichObjectAmI() {
trace("I am the base class");
}
}
}
Object One
package {
public class ObjectOne extends BaseClass {
public function ObjectOne() {
// constructor code
}
override public function whichObjectAmI() {
trace("I am Object One!");
}
}
}
Object Two
package {
public class ObjectTwo extends BaseClass {
public function ObjectTwo() {
// constructor code
}
override public function whichObjectAmI() {
trace("I am Object Two!");
}
}
}
You can always inherit from any of ActionScript's classes as well like MovieClip, Button, etc. And by doing so, you're adding custom functionality on top of their functionality so 1) you don't have to rebuild a bunch of functionality, and 2) giving you the chance to reuse their functionality while adding your own custom code!
Disclaimer: My AS3 is a little rusty ;)
Of what type would the variable Pow be after the switch statement? objectOne or objectTwo? From the compiler's perspective objectOne and objectTwo could be totally different from each other (read: methods, fields,...)
So:
A) Keep variable name for both assignments but declare it before the switch-statement AND use a common base-type (object, MovieClip,...)
B) Have 2 different variables: var PowOne: objectOne and var PowTwo: objectTwo
I think option A would be preferable...
I've got a method that accepts a parameter of type Class, and I want to only accept classes that extend SuperClass. Right now, all I can figure out to do is this, which does a run-time check on an instance:
public function careless(SomeClass:Class):void {
var instance:SomeClass = new SomeClass();
if (instance as SuperClass) {
// great, i guess
} else {
// damn, wish i'd have known this at compile time
}
}
Is there any way to do something like this, so I can be assured that a Class instance extends some super class?
public function careful(SomeClass:[Class extends SuperClass]):void {
var instance:SuperClass = new SomeClass();
// all is good
}
If you are going to instantiate it anyway, why not accept an object instead which allows you to type it to :SuperClass?
careless(SomeClass);
//vs.
careless(new SomeClass);
Not too much of a problem there as far as your code goes.
There are a few differences though:
The object has to be created, because an object is required. If your function does not instantiate the class under some circumstances, this can be a problem. Additional logic to pass either an object or null can bloat the function call.
If you cannot call the constructor outside that function, it won't
work either.
All that is solved by the factory pattern. Pass a factory as the parameter that produces SuperClass objects.
function careful(factory:SuperClassFactory)
Your requirements:
I want to only accept classes that extend SuperClass
and
I need to pass in a Class so that it can be instantiated many times
by other objects later
Can be met by passing in an instance of the class you need, and using the Object.constructor() method.
public function careful(someInstance:SuperClass):void {
//you probably want to store classRef in a member variable
var classRef: Class = someInstance.constructor();
//the following is guaranteed to cast correctly,
//since someInstance will always be a descendant of SuperClass
var myInst:SuperClass = new classRef() as SuperClass;
}
More reading here.
You can't do that in ActionScript 3. In languages like C# you can do something like (forgive me if the syntax is off):
public void Careless<T>() where T : SuperClass
But AS3 does not have 'generics'. Unfortunately the only way I know how to do what you want is the way you have already done.
A pattern that might be more suitable for your use case might be something like:
class SuperClass
{
public static function careless():void
{
var instance:SuperClass = new SuperClass();
// ...
}
}
The only way to have static type checking in ActionScript 3 is to provide an instance of a class.
It is possible but it's expensive. You can use on a Class (not instance) the:
flash.utils.describeType
You then get an XML with a bunch of information including inheritance for that class. Like I said it's an expensive process and probably creating an instance and checking it will be in most cases faster.
Possibly bad practice but I'm not well versed in software design anyway (I'm sure this question would have been asked before but I can't seem to find the right terminology)...Anyhow, it's just another curiosity of mine I'd like to have answered.
So I have worked in a way where I type a base class variable to type Object or Sprite or something similar so that in my subclasses, I can instantiate my custom classes into them and store it. And when I access it, I just cast that variable to ensure I can access the methods.
Take this example, so that you know what I'm talking about:
public class BaseClass
{
protected var the_holder_var:Object;
public function BaseClass()
{
//Whatever abstract implementation here...
}
}
Now, my subclasses of that base class usually use an interface but for simplicity sake, I'll just write it without it.
public class AnExtendedClass extends BaseClass
{
public function AnExtendedClass()
{
//Instantiate my own class into the base class variable
this.the_holder_var = new ACustomClassOfMine();
//Then I can use the 'hackish' getter function below to
//access the var's functions.
this.holder_var.somefunction()
}
private function get holder_var():ACustomClassOfMine
{
return this.the_holder_var as ACustomClassOfMine;
}
}
This works and I'm sure it will make some ppl cringe (I sometimes cringe at it too).
So now, my question, is there a way to recast/retype that base var in my extended subclass?
kinda like this:
public class ExtendedClass extends BaseClass
{
//Not possible I know, but as a reference to see what I'm asking about
//Just want to change the type....
override protected var the_holder_var:ACustomClassOfMine;
public function ExtendedClass()
{
//Then I can forget about having that hackish getter method.
this.the_holder_var = new ACustomClassOfMine();
this.the_holder_var.somefunction();
}
}
I was thinking of typing most of my base class vars that I use as holders as type * and retyping them as I extend the class. (I could use it here too but yeah...)
Thoughts? Comments? Ideas?
I actually think your code (apart from the hypothetical addition at the end) is pretty alright. The practise of adding accessors to solve the type issue you're dealing with is a solid one. I would advise to rename the accessor to show it is a cast, maybe get holderVarAsCustom():ACustomClassOfMine (I'm also not a big fan of the underscores, that's another language's convention), but that's personal preference. What I'd do to solve your last problem is just create a matching setter function:
private function set holderVarAsCustom(value:ACustomClassOfMine):void {
this.the_holder_var = value;
}
This way you can access the correctly typed holder var for both read and write operations with complete type safety:
holderVarAsCustom = new ACustomClassOfMine();
holderVarAsCustom.someFunction();
I would definately advise against dropping the type safety by including arrays and what not, that just makes it unstable.
I must admit that i'm a little confused as to why you want to do this, but here goes. Could you not utilise the fact that Array's can hold different data types. So something like this:
public class BaseClass
{
protected var customStorage:Array;
public function BaseClass()
{
//Whatever abstract implementation here...
}
}
You could then access it with an associative method and a property:
public class AnExtendedClass extends BaseClass
{
private static const myName:String = "myName";
public function AnExtendedClass()
{
//Instantiate my own class into the base class variable
customStorage[myName] = new ACustomClassOfMine();
objectIWant.somefunction()
}
private function get objectIWant():ACustomClassOfMine
{
return ACustomClassOfMine(customStorage[myName]);
}
}
Is that any better?
I would not try to tinker this behaviour, since you can't change the declared type of a variable once declared, no matter how hard you try.
What I do in such cases, I either cast the variable if I use it sparingly or the object it references may change, or I add another variable with the type I want and let the other variable point to the new one. Like this:
public class A {
protected var object:Object;
public function A() {
//Whatever abstract implementation here...
}
}
and
public class B extends A {
protected var other:MyClass;
public function B() {
super();
this.other = new MyClass();
this.object = this.other;
}
}
Having it this way, class A uses the object via the this.object reference, and class B can use the this.other or both. But both references point to the same object. The only issues with this are:
having two references for in the same class to the same object is ugly (so are untyped variables and casts)
if the object one of them may point can change during runtime, you must be really carefull to synchronize these changes