QML and JSON



  • I want to create a QML model from (downloaded) JSON data. I notice there's a XMLListModel but no similar mechanism for JSON data. How can I do this?

    Thanks

    Simon


  • Moderators

    There are several 3rd party JSON libraries that play nicely with Qt. Could using one of those be an option?



  • I assume you mean Javascript libraries. Is there one you know works ok with Qt?

    Thanks

    Simon


  • Moderators

    Actually I was referring to C++ libraries:-)



  • I assume there is (or will be) a better way to do all with javascript but if not you can't find out or wait you can always use the script module on c++ to parse the json output and expose the result to qml as a model.


  • Moderators

    I am not convinced "all Javascript" is a better way! Javascript is much slower than C++ and with a animation running at 60 frames per second you do not have all that much time to update your state...



  • I agree that is will not be the most efficient way but one of the goals for QML must be to make easier for the web developers to start writing qml code.

    In theory all that we need is to make a http request that will give us a string and an "evaluateJavascript" function :)

    Kidding apart, I would just try to create a qml extension in c++ that would return a "generic" model for any URI. I can only imagine that soon or later "qt" will have to provide a standard implementation for this.



  • I'm agreed with Tobias. I think that I will prefer QAbstractItemModel that will parse JSON (with one of libraries Tobias told about) and use it as model for QML ListView instead of doing all work in JS.



  • Parsing JSON is easy and no additional libraries are required, you can just use eval(), JSON structure is a valid javascript code, right?



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



  • 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



  • 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.



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



  • [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.



  • 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});

    }
    @



  • 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...



  • 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 ! :)



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



  • 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.



  • 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



  • 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



  • 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.



  • 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


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