I'm learning Dart and was reading the article Using Dart with JSON Web Services, which told me that I could get help with type checking when converting my objects to and from JSON. I used their code snippet but ended up with compiler warnings. I found another Stack Overflow question which discussed the same problem, and the answer was to use the #proxy annotation and implement noSuchMethod. Here's my attempt:
abstract class Language {
String language;
List targets;
Map website;
}
#proxy
class LanguageImpl extends JsonObject implements Language {
LanguageImpl();
factory LanguageImpl.fromJsonString(string) {
return new JsonObject.fromJsonString(string, new LanguageImpl());
}
noSuchMethod(i) => super.noSuchMethod(i);
}
I don't know if the noSuchMethod implementation is correct, and #proxy seems redundant now. Regardless, the code doesn't do what I want. If I run
var lang1 = new LanguageImpl.fromJsonString('{"language":"Dart"}');
print(JSON.encode(lang1));
print(lang1.language);
print(lang1.language + "!");
var lang2 = new LanguageImpl.fromJsonString('{"language":13.37000}');
print(JSON.encode(lang2));
print(lang2.language);
print(lang2.language + "!");
I get the output
{"language":"Dart"}
Dart
Dart!
{"language":13.37}
13.37
type 'String' is not a subtype of type 'num' of 'other'.
and then a stacktrace. Hence, although the readability is a little bit better (one of the goals of the article), the strong typing promised by the article doesn't work and the code might or might not crash, depending on the input.
What am I doing wrong?
The article mentions static types in one paragraph but JsonObject has nothing to do with static types.
What you get from JsonObject is that you don't need Map access syntax.
Instead of someMap['language'] = value; you can write someObj.language = value; and you get the fields in the autocomplete list, but Dart is not able to do any type checking neither when you assign a value to a field of the object (someObj.language = value;) nor when you use fromJsonString() (as mentioned because of noSuchMethod/#proxy).
I assume that you want an exception to be thrown on this line:
var lang2 = new LanguageImpl.fromJsonString('{"language":13.37000}');
because 13.37 is not a String. In order for JsonObject to do this it would need to use mirrors to determine the type of the field and manually do a type check. This is possible, but it would add to the dart2js output size.
So barring that, I think that throwing a type error when reading the field is reasonable, and you might have just found a bug-worthy issue here. Since noSuchMethod is being used to implement an abstract method, the runtime can actually do a type check on the arguments and return values. It appears from your example that it's not. Care to file a bug?
If this was addressed, then JsonObject could immediate read a field after setting it to cause a type check when decoding without mirrors, and it could do that check in an assert() so that it's only done in checked mode. I think that would be a nice solution.
Related
I need to set the Xaml property of a RichTextBox user control via a binding expression in Windows Phone 8, and I found that it is not a DP, so I have decided to inherit from a RichTextBox and add a DP that will change the Xaml property with PropertyChanged event, anyways the code looks like this, stripped out irrelevant parts.
public class RichTextBoxWithBindableXaml : RichTextBox
{
public string BindableXaml
{
get { return (string)GetValue(BindableXamlProperty); }
set { SetValue(BindableXamlProperty, value); }
}
public static readonly DependencyProperty BindableXamlProperty =
DependencyProperty.Register("BindableXaml",
typeof(string),
typeof(RichTextBoxWithBindableXaml),
new PropertyMetadata(0));
}
//xaml code
<local:RichTextBoxWithBindableXaml BindableXaml="{Binding PageContent , Mode=OneWay}"> </local:RichTextBoxWithBindableXaml>
And I get the following dreaded exception message:
Object of type 'System.Windows.Data.Binding' cannot be converted to type 'System.String'.
I have checked many solutions to these exceptions and similar problems with data binding, and still going through the suggested similar questions on the right, and still cannot see why a simple thing wont work for me. The code I listed above is just the simplest implementations of a DP with a binding expression. Btw, the source PageContent is from a INotifyPropertyChanged object, and it works, I know because, it can bind to TextBlock's Text property.
Am I missing out something so obvious? I wouldn't want to post question for such a straightforward thing, but I cant seem to solve in any way.
EDIT:
Following P.S note turned out to be completely irrelevant.
P.S. My final doubt was on the way xmlns namespace local is loaded. It is loaded as clr assembly, could xaml parser think my custom inherited class as clr-only and confuse since clr properties are not dependency properties. Hope it doesnt sound stupid, i'm desperate. It is as such :
xmlns:local="clr-namespace:RumCli"
I found out that I should either provide a null PropertyMetadata (new PropertyMetadata(null) instead of 0), or a metadata with a default value type if the DP is supposed to be used in Xaml. For my sceneario, since I will make use of the PropertyChangedCallback, the propertymetadata that will passed to the Register method looks like this.
new PropertyMetadata(default(string), new PropertyChangedCallback(OnBindableXamlChanged))
hope, it helps to others.
For each dependency property one must supply a non subscribed value (not a C# term here) which suites the type of object which the consumer will access.
To quote MSDN Dependency Property Metadata
Dependency property metadata exists as an object that can be queried to examine the characteristics of a dependency property.
So for the value type results, a default for the different value types, such as a double is to use double.NaN. A decimal use decimal.Zero. While a string, string.empty is good as a base.
That allows whatever operation which may blindly reflect off of the property, it can determine what its true property type is and access it accordingly.
So assigning 0 to a string makes no sense in identifying that the property is a string which 0 identifies it as an integer. So the int as string is setting up a future runtime failures when objects try to assign bindings, styles and other items to it.
Let's say I have some C# DTO's and I want to convert them to TypeScript interfaces using T4 templates and neat little library called TypeLite
On the client side, I have some concrete TypeScript classes (that inherit from Backbone.Model but that's not important) that represent the same DTO defined on the server side.
The intended goal of the interfaces is to act as data contracts and ensure client and server DTOs are kept in sync.
However, this is problematic since TypeScript supports no run-time type checking facilities other than instanceof. The problem with instance of is when I fetch my DTOs from the server they are plain JSON objects and not instances of my model. I need to perform run-time type checking on these DTOs that come in from the server as JSON objects.
I know I can do something like this:
collection.fetch({...}).done((baseModels) => {
baseModels.forEach((baseModel) => {
if(baseModel&& baseModel.SomeProperty && baseModel.SomeOtherProperty){
//JSON model has been "type-checked"
}
});
});
However, there is obvious problems to this because now I need to update in three places if I change or add a property.
Currently the only thing I found is this but it's undocumented, not maintained, and uses node which I have zero experience with so I'll save myself the frustration. Does anybody know of anything similar to perform run-time type checking in TypeScript or some other way to accomplish what I'm after?
It would be great if this was built into TypeLite to generate the interfaces as well as a JSON schema for type checking at run-time. Being that this project is open source somebody should really go ahead and extend it. I'd need some pointers at the least if I would do it myself (thus the question).
More details about my particular problem here (not necessary but if needed extra context)
At runtime you are using plain JavaScript, so you could use this answer as it relates to plain JavaScript:
How do I get the name of an object's type in JavaScript?
Here is a TypeScript get-class-name implementation that can supply the name of the enclosing TypeScript class (the link also has a static separate version of this example).
class ShoutMyName {
getName() {
var funcNameRegex = /function (.{1,})\(/;
var anyThis = <any> this;
var results = (funcNameRegex).exec(anyThis.constructor.toString());
return (results && results.length > 1) ? results[1] : "";
}
}
class Example extends ShoutMyName {
}
class AnotherClass extends ShoutMyName {
}
var x = new Example();
var y = new AnotherClass();
alert(x.getName());
alert(y.getName());
This doesn't give you data about the inheritance chain, just the class you are inspecting.
I'm an architect from a strong JavaScript background, but I did some .NET and Java in the past.
However, I wanted to put a hand on ActionScript3, which I was promised that is very related to JavaScript.
As a startup project I took on myself to try port to ActionScript3 one of my favorite assertion utils - should.js - that makes your test codes really pleasant to read.
Updated: 2013-02-19
I saw I confuse with my abstract speaking, so I replaced some of the post with the concrete question in mind.
Here's the full picture:
Consider the following JavaScript code:
Object.defineProperty(Object.prototype, 'should'
, { set: function(){}
, get:
function(){
return new Assertion(Object(this).valueOf());
}
, configurable: true
, enumerable : false
}
);
That is part of the implementation of the JavaScript module Should. The other part is a definition of a the class Assertion, that is constructed with a value, and implements a wide and nice set of assertion methods, against that value. Methods like like
var o = Assertion(actualValue)
o.equals(expectedValue1)
o.moreThan(expectedValue2)
o.contains(expectedValue3)
and aliases to keep english grammer
var o = Assertion(actualValue)
o.equal(expectedValue1)
o.contain(expectedValue3)
and aliases for the lazy sharpshooters, like
o.eql(expectedValue)
o.gt(expectedValue) //greater then
o.gte(...) //greater then or equal
//and so on...
and some connectors that just return this, (which is the instance of Assertion constructed with the test value) like
o.be
o.and
What does it give you?
A test code that looks like this:
var person = getPerson();
Should.exist(person); //that's a static call, and that's easy
//but these are a member calls:
person.should.have("name","age","address","friends");
person.name.should.equal("John");
person.age
.should
.be.number()
.and.be.between(20,30);
person.address
.should
.be.string().and
.startWith("\d").and
.endWith(" st.")
//or even
.and.match(/^[0-9]{1,9}\s+[A-Z][a-z0-9 ]* st\.$/);
person.friends
.should
.be.array().and
.be.between(3,5).and
.containOnlyType(String);
Isn't that wonderful? it's plain English!
You could argue about aesthetics of indentation, where to put the and, and if they are at all necessary, but besides that - anybody can read or write it:
Once you took the 'should' attribute that exists on every object but does not spoil map iterations - you can go on chaining whatever you have to claim regarding the value you started from.
It could have more nifty iteration tools, reflection utilities, be augmented with test functions relevant for your object model, and so on and so forth, but lets just get over the first step :)
But for that, you need every object in the system to feature a non-enumerable smart property called should that in it's getter function returns an Assertion object constructed with the this as the tested value.
(you ain't seen nothing yet - wait to see the beautiful rejection messages it gives! Yummie!!
So yea - I would happily sacrifice the option to call an attribute "should"... and will happily give up intelisense as well - at least as long as it's plain English)
So, in comments, bfavaretto gave us the first step - we know how to prevent enumeration of an attribute - great & thanks!!
Now, can we make it a getter-attribute who's function can access the this?
When I'm done I'm going to put it in some public repo licensed under MIT, for all of us to have fun with :)
Help anybody?
You example is actually 90% correct - but define it like actionscript, not like javascript!
You can still define prototypes in AS3 and they will still work just like prototypes in AS2. The only difference in AS3 is the compiler. AVM2 for some reason does not cast prototypes to native classes (although I didn't test custom classes).
The Prototype Trick: Cast the class as an object.
Eg: if you create:
Array.prototype.random = function():void{}
Then create the object:
var myProtoArray:Array = new Array;
2 things will happen:
myProtoArray.random() //ERROR - this will fail, AVM2 did not map the prototype to Array
but
Object(myProtoArray).random() //WORKS
random() was cast to the Object class, then mapped to Array - I have no idea why!
Hope this helps, cheers.
I confess I'm not keenly familiar with how Javascript works, but if I'm understanding defineProperties purpose correctly, it is a runtime dictation of not just what a property should be, but also the associated namespace to which it belongs (or at least what AS3 considers a namespace).
Class properties are either predefined & only modifiable via custom get() set() functions, or dynamic. Once compiled, their namespace cannot be changed (to my knowledge), so any non-private property is implicitly enumerable, and modifiable whether or not you've written getter/setters (ie: foo.a = value). According to Adobe...
Properties that you create are enumerable, but built-in properties are
generally not enumerable.
That said, you can get a complete list of properties from a class by using describeType. Quite an exhaustive amount of info can be gleaned this way, and I suspect should suit your needs if you wanted to port Mozilla's recreated defineProperties example. Below is an example printing out only property values.
function showProps(obj:*):void {
var desc:XML= describeType(obj);
// public vars
for each (var n:XML in desc.variable){
trace(n.#name + ": " + obj[n.#name]);
}
// getters
for each (n in desc.accessor){
try {
trace(n.#name + ": " + obj[n.#name]);
} catch (error:Error) {
trace("Unable to read write-only property.");
}
}
}
I hope this helps, but I'm certain I don't fully understand what you're trying to accomplish. If you could elaborate, that'd be appreciated.
Ok, guys, thanks for all the help, 22+
I'll give a summary for the people that are interested in the original question, and after that - I'll show you the outcome of my efforts.
The challange was made of two parts:
1 - prevent the augmented (=added on runtime) property from being enumerated
To the first part - thanks to #bfavaretto, commented on the question level - Object.setPropertyIsEnumerable - did the trick great.
2 - make the augmented property operate a getter function with access to the this so it can use it on the constructor of the returned value.
About this second part - Basically - I could not find a way to augment (=add) a property getter to a prototype, and have it operate on instances that enjoy it's API through the inheritance tree.
Anyway, within these limits - here's the outcome:
https://github.com/osher/should.as
Not exact porting because of the platform differences,
and I still have some methods to catch up with the original should.js (like the HTTP testing methods)
but close enough.
The main difference is that instead
var o:Object =
{ name : "Radagast"
, color: "Brown"
}
o.should.have.properties("name","color")
.and.have.property("name","Radagast");
o.name.should.not.equal("Palandoo");
o.color.should.equal("Brown");
you have to go
o.should().have.properties("name","color")
and.have.property("name","Radagast");
o.name.should().not.equal("Palandoo");
o.color.should().equal("Brown");
(the brackets - no getter possible - so the should attribute is a method, and you have to invoke it yourself)
Now if you get stuck and need help from the intellisense, you have to do this:
var should:tdd.Should = o.color.should();
should. <ctrl+space>
which kind'a takes the sting out, but for a peek in the intelisense - it helps
Important
One more thing - you have to force the static constructor of Should as soon in execution as you can,
for example, I do it here:
[Suite]
[RunWith("org.flexunit.runners.Suite")]
public class my_awsome_test_suite
{
//forces the static constructor of tdd.Should
import tdd.Should;
private static var s:Should = new Should();
public var c1:testCase1;
public var c2:testCase2;
public var c3:testCase3;
public var c4:testCase4;
}
I'll probably add some propper README.md later, and more awsome member functions to tdd.Should
Have fun
For a game I'm attempting to develop, I am writing a resource pool class in order to recycle objects without calling the "new" operator. I would like to be able to specify the size of the pool, and I would like it to be strongly typed.
Because of these considerations, I think that a Vector would be my best choice. However, as Vector is a final class, I can't extend it. So, I figured I'd use composition instead of inheritance, in this case.
The problem I'm seeing is this - I want to instantiate the class with two arguments: size and class type, and I'm not sure how to pass a type as an argument.
Here's what I tried:
public final class ObjPool
{
private var objects:Vector.<*>;
public function ObjPool(poolsize:uint, type:Class)
{
objects = new Vector.<type>(poolsize); // line 15
}
}
And here's the error I receive from FlashDevelop when I try to build:
\src\ObjPool.as(15): col: 18 Error: Access of undefined property type.
Does anybody know of a way to do this? It looks like the Flash compiler doesn't like to accept variable names within the Vector bracket notation. (I tried changing constructor parameter "type" to String as a test, with no results; I also tried putting a getQualifiedClassName in there, and that didn't work either. Untyping the objects var was fruitless as well.) Additionally, I'm not even sure if type "Class" is the right way to do this - does anybody know?
Thanks!
Edit: For clarification, I am calling my class like this:
var i:ObjPool = new ObjPool(5000, int);
The intention is to specify a size and a type.
Double Edit: For anyone who stumbles upon this question looking for an answer, please research Generics in the Java programming language. As of the time of this writing, they are not implemented in Actionscript 3. Good luck.
I have been trying to do this for a while now and Dominic Tancredi's post made me think that even if you can't go :
objects = new Vector.<classType>(poolsize);
You could go something like :
public final class ObjPool
{
private var objects:Vector.<*>;
public function ObjPool(poolsize:uint, type:Class)
{
var className : String = getQualifiedClassName(type);
var vectorClass : Class = Class(getDefinitionByName("Vector.<" + className + ">"));
objects = new vectorClass(poolsize);
}
}
I tried it with both int and a custom class and it seems to be working fine. Of course you would have to check if you actually gain any speed from this since objects is a Vector.<*> and flash might be making some implicit type checks that would negate the speed up you get from using a vector.
Hope this helps
This is an interesting question (+1!), mostly because I've never tried it before. It seems like from your example it is not possible, which I do find odd, probably something to do with how the compiler works. I question why you would want to do this though. The performance benefit of a Vector over an Array is mostly the result of it being typed, however you are explicitly declaring its type as undefined, which means you've lost the performance gain. So why not just use an array instead? Just food for though.
EDIT
I can confirm this is not possible, its an open bug. See here: http://bugs.adobe.com/jira/browse/ASC-3748 Sorry for the news!
Tyler.
It is good you trying to stay away from new but:
Everything I have ever read about Vector<> in actionscript says it must be strongly typed. So
this shouldn't work.
Edit: I am saying it can't be done.
Here see if this helps.
Is it possible to define a generic type Vector in Actionsctipt 3?
Shot in the dock, but try this:
var classType:Class = getDefinitionByName(type) as Class;
...
objects = new Vector.<classType>(poolsize); // line 15
drops the mic
I don't really see the point in using a Vector.<*>. Might as well go with Array.
Anyhow, I just came up with this way of dynamically create Vectors:
public function getCopy (ofVector:Object):Object
{
var copy:Object = new ofVector.constructor;
// Do whatever you like with the vector as long as you don't need to know the type
return copy;
}
I asked a related question about findbugs, but let's ask a more general question.
Suppose that I am working with an object-oriented language in which polymorphism is possible.
Suppose that the language supports static type checking (e.g., Java, C++)
Suppose that the language does not allow variance in parameters (e.g., Java, again...)
If I am overriding the equality operation, which takes Object as a parameter, what should I do in a situation where the parameter is not the same type or a subtype as the LHS that equals had been called upon?
Option 1 - Return false because the objects are clearly not equals
Option 2 - Throw a casting exception because if the language actually supported variance (which would have been preferable), this would have been caught at compile time as an error; thus, detecting this error at runtime makes sense since a situation where another type is sent should have been illegal.
I vote for option 1. It is possible for two objects of different types to be equal -- for example, int and double, if first class objects, can validily be cast as each other and are comparable mathematically. Also, you may want to consider differing subclasses equal in some respects, yet neither may be able to be cast to the other (though they may be derived from the same parent).
Return false, because the objects are not equal.
I don't see how throwing a ClassCastException would be any better here.
There are contracts in interfaces such as Collection or List that actually depend on any object being able to check for equality with any other object.
It depends.
SomeClass obj1 = new SomeClass();
object other = (object)obj1;
return obj1.Equals(other); // should return "true", since they are really the same reference.
SomeClass obj1 = new SomeClass();
object other = new object();
return obj1.Equals(other); // should return "false", because they're different reference objects.
class SomeClass { }
class OtherClass { }
SomeClass obj1 = new SomeClass();
OtherClass obj2 = new OtherClass();
return obj1.Equals(obj2); // should return "false", because they're different reference objects.
If the types are two completely different types that don't inherit from one another, then there is no possible way they can be the same reference.
You shouldn't be throwing any type of casting exception because you're accepting the base object class as a parameter.
Hmmm.. I like Option 1 as well, for the following 4 reasons:
1) The objects were apparently not equal by whatever condition you used to check
2) Throwing ClassCastException requires a check for that exception every time you do a comparison, and I think that contributes to the code being less understandable or at least longer...
3) The class cast exception is merely a symptom of the problem, which is that the two objects were not equal, even at the type level.
4) As user "cbo" mentioned above, double/int being equal despite their types being different (4.0 == 4) and this applies to other types as well.
Disclaimer: I may be letting my Python ways colour a Java debate :)
Dr. Dobb's Java Q&A says the best practice is that they both are the same type. So I vote option 1.