Based on the "How data binding tracks template instances" section in the Polymer Data Binding docs, it sounds like JavaScript equality is used to determine if the data is the "same" between template invocations. So value types have value equality, but objects and arrays use reference equality. If I make two calls to a service which returns an array of objects, and some of those objects have the same values between calls, the template instances will be recreated as both the array and objects are deserialized into different instances (I have verified this behavior in version 0.3.3).
Is it possible to specify an identity function similar to how D3 employs a key function?
I don't know if there is such a thing but I would come up with a different solution. Let's assume your code looks something like this (pseudocode)
<ajax onResponse="{{onResponse}}" response="{{ajaxResponse}}" url="/users"></ajax>
<template repeat="{{user in ajaxResponse}}">
<span>Hello from {{user.name}}</span>
</template>
If I got you right, you said, that when the request is made the template get's reevaluated no matter if there were changes in the data or not.
To fix this, we can represent our data in a different way which evaluates not by reference but by value.
class UserListElement extends PolymerElement {
#observable List userIds = toObservable([]);
Map<int, Object> users = {};
onResponse(var response) {
users = {};
for(var user in response) {
users[user.id] = user;
if (!users.contains(user.id)) userIds.add(user.id);
}
var toRemove = userIds.where((user) => !_userData.containsKey(user.id));
for(var userId in toRemove) userIds.remove(userId);
}
}
-
<ajax onResponse="{{onResponse}}" url="/users"></ajax>
<template repeat="{{userId in users}}">
<span>Hello from {{users[userId].name}}</span>
</template>
As I said non-tested pseudocode. But this is the only idea I can come up with.
Related
Given that you can't use arrow functions when you need to yield in its body, is it possible to set the this value for use in side the body.
I have made myself a database library which extends the "tedious" library that allows me to do something like the following
const self = this;
db.exec(function*(connection) {
let sql = 'SELECT * FROM myTable WHERE id = #id';
let request = connection.request(sql);
request.addParameter('id',db.TYPE.Int, myIdValue);
let count = yield connection.execSql(function*() {
let row = yield;
while(row) {
//process row with somthing like self.processRow(row);
row=yield;
}
});
if (count > 0) {
request = connection.request('some more sql');
//etc
}
return something;
}).then(something => {
//do some more things if the database access was a success
}).catch(error => {
// deal with any errors.
}) ;
I find I am increasingly needing to access the this value from the outside and am constantly doing the trick of assigning it to self at the head of the surrounding function.
Is it possible to set the this value with something like bind? inside the function* (at multiple levels down!)
Since I have full access to the iterators that I use to implement db.exec and connection.execSql I can change them if it's possible. to support it.
Generator use this as normal functions would.
You have few solutions:
use .bind on generator expression
pass this as first/second argument to generator named self
make db.exec take second argument thisArg, similar to array methods
If a thisArg parameter is provided to forEach(), it will be passed to callback when invoked, for use as its this value. Otherwise, the value undefined will be passed for use as its this value. The this value ultimately observable by callback is determined according to the usual rules for determining the this seen by a function.
I would suggest going with the last solution.
I have a class
class Person{
String _fn, _ln;
Person(this._fn, this._ln);
}
Is there a way to get a list of variables and then serialize it? Essentially i want to make a toJson, but i wanted to have it generic enough such that key is the variable name, and the value is the value of the variable name.
In javascript it would be something like:
var myObject = {}; //.... whatever you want to define it as..
var toJson = function(){
var list = Object.keys(myObject);
var json = {};
for ( var key in list ){
json[list[key]] = myObject[list[key]] ;
}
return JSON.stringify(json);
}
Dart doesn't have a built in functionality for serialization. There are several packages with different strategies available at pub.dartlang.org. Some use mirrors, which is harmful for client applications because it results in big or huge JS output size. The new reflectable packages replaces mirrors without the disadvantage but I don't know if serialization packages are already ported to use it instead. There are also packages that use code generation.
There is a question with an answer that lists available solutions. I'll look it up when I'm back.
I have a main view using several partial Views.
Each of these partials use a different model and have post action.
My problem is I need one property from my main view's model to be used in one of my partials.
The partial view which I need to pass this property view is the last stage in the process.
The application reaches a partial view that contains a switch statement , based on the status on the item being queried, decides which partial will be rendered.
I have the property passing that far and even have it included in the Renderaction for the partial but I don't know how to retrieve it in the controller, PartialViewResult.
In the main view:
#{Html.RenderPartial("StatusForm", Model.HeadingDataModel.Status, new ViewDataDictionary { { "PurchaseOrderNumber", Model.AccordionModel.LtsSpecific.PurchaseOrderNumber } });}
PurchaseOrderNumber is what I'm after. The value gets passed to the next stage:
#{
var obj = ViewData["PurchaseOrderNumber"];
}
And within the same view:
Html.RenderAction("FinishedCalibrationForm", obj);
How can I retreive this in my controller ?? The following is not correct I know, but you get the idea.
public PartialViewResult FinishedCalibrationForm( string obj)
All help is appreciated.
Calling Html.RenderAction or Html.Action is largely the same as Url.Action. There's many different overloads, but essentially, the first parameter is the action name, the second parameter is going to be either the controller name or an anonymous object of route values, and the third parameter will be an anonymous object of route values if the second parameter was used for the controller name.
Anyways, whatever you pass in the route values will be used to find and call the associated action, which includes parameters for the action. So, for your example:
Html.RenderAction("FinishedCalibrationForm", new { obj = obj })
Would properly pass obj into your action method. As you have it now, it's going to interpret the value of obj as the controller name the action is within, which is obviously not correct.
I am trying grab all the member variables in AS3, and then foreach one i would like to process it in various ways. I would need the name and then if it is a collection of some type I would like to loop through that collection as well. I am attempting to essentially serialize in a somewhat custom fashion.
Thanks!
If you're looking to serialize an object, you will definitely want to use JSON.
JSON basically converts objects into strings and also the other way round using an encode()/serialize() and decode()/deserialize() function.
There is a built-in JSON class in AS3, and it's really easy to use.
Once you do something like:
var myObject:Object = {};
var myObjectString:String = JSON.serialize(myObject);
After getting the string, you can do all your switch logic to manipulate each of your different variables and convert it back into an object via the deserialize() function.
You could use describeType. That returns information about the object as XML. By default, you can iterate over public properties in objects. You could try something like...
// the object to iterate over
var someObj:Object = {};
for(var prop:String in someObj) {
// check to see if its something you want to iterate over
if (someObj[prop] is Array) {
// iterator over the property here
}
}
I hope this answers your question.
I've been spending some of my spare time working a set of collections for ActionScript 3 but I've hit a pretty serious roadblock thanks for the way ActionScript 3 handles equality checks inside Dictionary Objects.
When you compare a key in a dictionary, ActionScript uses the === operator to perform the comparison, this has a bit of a nasty side effect whereby only references to the same instance will resolve true and not objects of equality. Here's what I mean:
const jonny1 : Person = new Person("jonny", 26);
const jonny2 : Person = new Person("jonny", 26);
const table : Dictionary = new Dictionary();
table[jonny1] = "That's me";
trace(table[jonny1]) // traces: "That's me"
trace(table[jonny2]) // traces: undefined.
The way I am attempting to combat this is to provide an Equalizer interface which looks like this:
public interface Equalizer
{
function equals(object : Object) : Boolean;
}
This allows to to perform an instanceOf-esq. check whenever I need to perform an equality operation inside my collections (falling back on the === operator when the object doesn't implement Equalizer); however, this doesn't get around the fact that my underlying datastructure (the Dictionary Object) has no knowledge of this.
The way I am currently working around the issue is by iterating through all the keys in the dictionary and performing the equality check whenever I perform a containsKey() or get() operation - however, this pretty much defeats the entire point of a hashmap (cheap lookup operations).
If I am unable to continue using a Dictionary instance as the backing for map, how would I go about creating the hashes for unique object instances passed in as keys so I can still maintain equality?
How about you compute a hash code for your objects when you insert them, and then look them up by the hash code in your backing dictionary? The hashcode should compare === just fine. Of course, that would require you to have a Hashable interface for your object types instead of your Equalizer interface, so it isn't much less work than you are already doing, but you do get the cheap lookups.
How about rather doing this:
public interface Hashable {
function hash():String;
}
personally, I ask myself, why you want to do this ... hashing objects to obtain keys makes little sense if they are mutable ...
also, you might consider using a different approach, as for example this factory:
package {
public class Person {
/**
* don't use this!
* #private
*/
public function Person(name:String, age:int) {
if (!instantiationAllowed)
throw new Error("use Person.getPerson instead of constructor");
//...
}
private static var instantiationAllowed:Boolean = false;
private static var map:Object = {};
private static function create(name:String, age:int):Person {
instantiationAllowed = true;
var ret:Person = new Person(name, age);
instantiationAllowed = false;
}
public static function getPerson(name:String, age:int):Person {
var ageMap:Array = map[name];
if (ageMap == null) {
map[name] = ageMap = [];
return ageMap[age] = Person.create(name, age);
}
if (ageMap.hasOwnProperty(age))
return ageMap[age];
return ageMap[age] = Person.create(name, age);
}
}
}
it ensures, there's only one person with a given name and age (if that makes any sense) ...
Old thread I know, but still worth posting.
const jonny1 : Person = new Person("jonny", 26); const jonny2 : Person = new Person("jonny", 26);
is creating two completely different objects that will not compare using ==, guess I don't see why it's any more of a road block because of as3
The problem with AS3/JavaScript/EcmaScript is not that they create two different, equivalent objects.
The problem is that they cannot equate those two equivalent objects--only identity works, since there is no equals or hashCode methods that can be overriden with class-specific comparison logic.
For Map implementations such as dynamic Object or Dictionary, this means that you have to either use Strings or references as keys: you cannot recover objects from a map using different but equivalent objects.
To work around that problem, people either resort to strict toString implementations (for Object maps) which is undesirable, or to instance control for Dictionaries, as in #back2dos example, which introduces different problems (Also, note that #back2dos solution does not really guarantee unique Person instances since there is a time window during which asynchronous threads will be allowed to instantiate new Persons).
#A.Levy's solution is good except that in general, hashCodes are not strictly required to issue unique values (they are meant to map entries to buckets allowing for fast lookups, wherein fine-grained differentiation is done through equals method).
You need both a hashCode and an equals method, e.g.
public interface IEquable
{
function equals(object : Object) : Boolean;
function hash():String;
}
In any programming language,
const jonny1 : Person = new Person("jonny", 26);
const jonny2 : Person = new Person("jonny", 26);
is creating two completely different objects that will not compare using ==, guess I don't see why it's any more of a road block because of as3