QML and JSON

[DenisKormalev]

Cezary Tomczak, yes, but you need to build model from object you will receive after eval, it will take processor time, not JSON parsing.

[SimonJudge]

Having done some more research, Cezary is correct. While it would be nice to have a JSONListModel, something similar can be achieved using the sample shown at http://bugreports.qt.nokia.com/browse/QTBUG-12117

After other comments it would be useful to know the performance implications of doing this vs doing it in Qt c++

Simon Judge

[DenisKormalev]

SimonJudge, all depends on your JSON size. If it is not big, than JS way is ok for you. But if it contains a lot of elements, then using c++ will be better.

[luizpaulo]

Hey a good Qt/C++ JSON parser is avaliable with a LGPL licence: http://qjson.sourceforge.net/
Might be what you need.

[fcrochik]

[quote author="luizpaulo" date="1291470389"]Hey a good Qt/C++ JSON parser is avaliable with a LGPL licence: http://qjson.sourceforge.net/
Might be what you need.[/quote]

I saw it when I was working on a simple json parser but decided against it. Depending on the task you can get the same result w/o any additional dependencies - just using the script module.

[dridk]

Hello here is my point of view! I tested everything! XMLModel, javascript XML parser, C++ side parser etc...

Do Not use C++ Json parser : Because exchange data between c++ and QML is slow

Make a Javascript parser ! It's more easy with eval ! Here is an example :

Json data
@
{"index":["all"],"flux":{"all":[{"data":{"title":"boris","icon":"icon.png"}]}}
@

QML side : main.qml
@
import QtQuick 1.0
import "parser.js" as JS

Item {
id:root
width: 360
height: 640

Component.onCompleted: JS.load()


ListModel {  id:listModel }

ListView {
    id:view
    anchors.fill:parent
    model : listModel
    delegate: Rectangle {
         width:parent.width
         height:80
         Text {
         anchors.center:parent
         text: title
         }
       
    }
}

}
@

javascript side : parser.js

@
function load() {

listModel.clear();
var xhr = new XMLHttpRequest();

xhr.open("GET","http://data.json",true);
xhr.onreadystatechange = function()
{
if ( xhr.readyState == xhr.DONE)
{
if ( xhr.status == 200)
{
var jsonObject = eval('(' + xhr.responseText + ')');
loaded(jsonObject)
}
}
}
xhr.send();
}

function loaded(jsonObject)
{
for ( var index in jsonObject.flux.all )
{
listModel.append({
"title" : jsonObject.flux.all[index].data["title"],
"icon" : jsonObject.flux.all[index].data["icon"]});
}

// get directly the json object. Should work but not tested
//listModel.append({jsonObject.flux.all});

}
@

[fcrochik]

It is a little surprising to me that you found that exchanging data between C++ and QML to be slow.

I can imagine that maybe the C++ xml parser you tried is slow but I can't imagine creating the model in c++ can be any slower than creating it using Javascript.

Another scenario I can imagine it would slow things is if you are adding elements to a model that is already in use.

I am just curious of the why...

[dridk]

That what I m thinking before! But a a guy said me that, and I trust him ... He said me C++ parsing is faster, but exchanging between binary and script is slower.

So, no matter, the javascript method is a little bit more easy than creating a c++ model ! :)

[remy_david]

I am also not so happy with the performances of my C++ model but I can't precisely say what's slowing down.

[Alicemirror]

Hi to all,

I see that the interest in json parsing is always present. I tried to manage qjson but it seems too complex for what can be done and I have developed a method that I am experiencing very efficient and reliable. It concerns of a C++ class and a very short function in js to be used by QML. If someone is interested I can explain it in details in a wiki page. For now the method is integrated in a project but it will be one of the new features of Qt-Complex 2.0 that I am preparing.

Take a look to http://projects.developer.nokia.com/pricemyhouse.

[peppelorum]

Please don't use eval when parsing json-data, you are actually running the code, so if something nasty is in the reply you get, well...

I've made a small library that plays nice with QML, for making the whole ajax-thingie a bit easier, you can check it out at https://github.com/peppelorum/ajaxmee

[Chuck Gao]

I think the simplest way to parse and turn json data to C++ model is using QScriptEngine. Please see:
@
QScriptValue json = m_scriptEngine->evaluate("JSON.parse").call(QScriptValue(), QScriptValueList() << jsonString);
QVariantList xxxList = json.toVariant().toList();
or
QVariantMap xxxMap = json.toVariant().toMap();
.....
Then, do parse
.....
@

And, in the code, m_scriptEngine is your QScriptEngine, and jsonString is your json data

[fcrochik]

QML "includes" a JSON parser. You can just use, without having to include anything:

@var obj = JSON.parse( myJSONString );@

"See json.org for more details":http://www.json.org/js.html

Of course eval can do the trick as well but it is risky and the JSON parser maybe optimized.

[kromain]

For anybody still interested, I've just published a JSONListModel component that works just like XMLListModel, but for JSON data instead. It's all pure QML/Javascript, and it even supports queries via JSONPath (XPath for JSON).

You can read more about it there: "Devnet article":http://qt-project.org/wiki/JSONListModel