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How do you access properties of a QML component in C++?

  • Okay so I understand you can connect signals and slots between QML and C++ objects and how to pass stuff generated in C++ to QML but how would I go about calling property of QML object for use in C++. Basically what I am trying to do is take text input from the TextInput component in QML and use it to set the currentPath() of an object which in the QDir class. I am sure I messed up a lot syntax and stuff there but I am pretty new to this.

    I was thinking of doing something like
    QObject *object = view.rootObject();
    QObject textIn = object->findChild<QObject>("textIn");
    and then calling text property like so and using QString to make a string.
    QString textholdervariable(textIn.text());
    and then doing the following to set my QDir path
    QDir dir;
    But this doesn't work as I get following error

    request for member ‘text’ in ‘textIn’, which is of non-class type ‘QObject*’

    Anyone know the proper way to go about retrieving properties of QML objects for use in C++?
    I would also be okay with being able to set the a variable in QML equal to some variable and then being able to call that in C++.
    I would also be okay with being told I am asking a stupid question and what I am talking about is impossible or there is a better way to do such a thing any help would be great.

    Edit: use @ tags to markup code sections please; Andre

  • I have seen that but isn't that for changing the properties in QML? I am trying to take properties from the QML object and use them as inputs for C++ functions.

  • The last post helped quite a bit I am having trouble using it though can't seem to find any examples of people actually using it so not sure how I am supposed to implement that piece of code. In the form it is presented it isn't working for me. Do I need to use Q_PROPERTY macros or anything to make it work? Not really sure if what I need to make it work.

  • Yes you need Q_PROPERTY in order for the moc (Meta Object Compiler) to add it to the generated MetaObject, so you can access it via these methods. If you have any further problems, just ask.

  • @QObject *object = view.rootObject();
    QObject textIn = object->findChild<QObject>(“textIn”);
    textIn->setProperty("text","sampleText"); @

    See if this works.

  • i have a similar problem. i managed to set a value from c++ in my qml file. the value is the x position of a qml rectangle, but when i change the value in c++ the rectangle isnt moved. What am i doing wrong?

  • [quote author="Nazgul" date="1308039647"]i have a similar problem. i managed to set a value from c++ in my qml file. the value is the x position of a qml rectangle, but when i change the value in c++ the rectangle isnt moved. What am i doing wrong?[/quote]

    How should we know, if you are not showing us what you are actually doing?

  • I'll check to see if that works but here is something that does work really well

    QDeclarativeProperty property([name of element], "[name of property]");

    and then to call the property just do
    and you can even do stuff like etc. this is what I was really looking for I think but thanks for all the help wouldn't have been able to find it without the leads.

    You still need stuff like
    QObject object = object->findChild<QObject>("rect");
    to use this though

    Edit: use @ tags to markup code sections please.

  • c++ code

    @QDeclarativeEngine engine;
    QDeclarativeComponent component(&engine, url);
    QObject *object = component.create();
    if (object) {
    object->setProperty("currentValue", newVal);
    delete object;
    object = NULL;

    and my qml file:
    @Item {
    id: line
    objectName: "line"
    property real currentValue: 100
    property real minimum: 0
    property real maximum: 200
    property int xMax: line.width - rect.width - 4
    onCurrentValueChanged: { rect.x = 2 + (currentValue - minimum) * line.xMax / (maximum - minimum) }
    signal valueChanged(real val)
    width: 400
    height: 20
    Rectangle {
    id: rect
    x: 0; y: 0
    width: 100; height: 20
    color: "red"
    MouseArea {
    anchors.fill: parent
    drag.axis: Drag.XAxis rect
    drag.minimumX: 0
    drag.maximumX: line.width - rect.width
    onPositionChanged: {
    line.currentValue = (maximum - minimum) * (rect.x-2) / line.xMax + minimum

    thanks in advance

  • What is the delete statement doing on line 6 of your code? What do you expect to happen if you first set a property, and then immediately delete the object?

    What you are doing here, is creating an object, setting a property on it, and then immediately disposing of it again. That seems like a rather useless thing to do. Instead, you should try to find the object you wish to manipulate, and then modify its properties. Or, perhaps better from a QML/declarative way of thinking kind-of-perspective, you should create a QObject derived object, give it properties for the position, insert it into your QML environment, and bind to these properties.

  • @QObject* obj = mDeclarativeView->rootObject();
    if (obj) {
    obj->setProperty("currentValue", newVal);

    ok, i wasnt sure about a memory leak because the method was create.

    i changed my code to the above and made sure obj != null. obj is a qml object with name = "line"
    thats fine so far but still the setProperty has no effect.

  • no ideas about my problem?

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