Binding C++ properties exposed to QML to dynamically created QML objects

Eeli K

@Obi-Wan Maybe it was me who was confused. You wrote "In C++ I create a QList<QVariant> that I expose to QML through the use of Q_PROPERTY, and this list contains information about the boxes received in the message." I interpreted it to mean that there's one list which has one element per one QML box object. Correct me if I was wrong. But in any case, if you have a binding

x: y

or the dynamic equivalent

x = Qt.binding(function() {return y})

y must be a property if you want it to evaluated automatically every time when y is changed. I think it this way: it's not just an ordinary function but the QML engine kind of keeps the whole function and every property which is inside it so that when one property inside that function changes the function can be evaluated anew. If y is not a property in C++ but just a normal C++ object or member, then the QML engine can't know when it's changed.

I'm not actually sure how deeply the binding can reach different things in QML, but when it comes to C++ objects it's quite clear when you think about it. In C++ you use the signal/slot for dynamic messages about changes, but you have always emit signals manually, or they have to be emitted by the library (written in the C++ code) for the connections to work. There's just no way anyone can know that something inside a QList is changed unless a signal is sent. There's no such signal in QList, and QVector2D doesn't have such signals. But when you create a Q_PROPERTY, for example:

Q_PROPERTY(QColor color MEMBER m_color NOTIFY colorChanged)

and then emit the signal when that member value is changed, all listeners who have been connected to the colorChanged signal are notified. The QML engine uses that possibility, using the meta object system, and can listen to property changes of C++ objects. When such a property is used inside a binding the whole function is evaluated when the C++ property is changed and the corresponding signal is emitted.

You wrote:

If i have a single QVector2D in C++ and use Q_PROPERTY to expose that to QML, I can indeed bind the x and y components of that vector to a property of any static QML object. I do not have to create a property of the x and y components in that case, why is that different now?

But you can't do that to achieve automatic binding evaluation. You can use x and y, yes, but the binding isn't evaluated automatically when the values are changed. The function is evaluated only once, with the values they have when the binding is made. If you actually have tested it and it works as you supposed, tell me I'm wrong.

Eeli K

@Obi-Wan I just noticed that there happens an implicit data type conversion between C++ and QML: QVector2D becomes vector2D. It doesn't have properties, it has members x and y. If x or y are used in bindings they don't trigger automatic evaluation. And when I try this (in main.cpp):

auto vec = QVector2D{3.4, 5.6};
QQmlApplicationEngine engine;
engine.rootContext()->setContextProperty("vector", vec);
vec.setX(6.7);

QML side gets only the first set x value 3.4, not 6.7. So it handles only a copy of the original. Your list probably becomes a javascript array which have vector2d elements and which is only a copy of the original C++ list.

So, in short, your problem isn't in dynamic creation of objects, it's in the C++ data. You just have to use your current method which worked for you or write much more complicated C++ code.

Obi-Wan

@Eeli-K said in Binding C++ properties exposed to QML to dynamically created QML objects:

Maybe it was me who was confused. You wrote "In C++ I create a QList<QVariant> that I expose to QML through the use of Q_PROPERTY, and this list contains information about the boxes received in the message." I interpreted it to mean that there's one list which has one element per one QML box object. Correct me if I was wrong.

No, this is correct.

@Eeli-K said in Binding C++ properties exposed to QML to dynamically created QML objects:

There's just no way anyone can know that something inside a QList is changed unless a signal is sent

Agreed. But there is a signal being sent whenever any part of the QList changes. In C++ I manually send a signal whenever the QList is updated with new data in order make NOTIFY work properly. And this does indeed work. Every time I change the QList in C++ a slot in QML is triggered, and I can do stuff with the updated data in the array.

@Eeli-K said in Binding C++ properties exposed to QML to dynamically created QML objects:

You can use x and y, yes, but the binding isn't evaluated automatically when the values are changed. The function is evaluated only once, with the values they have when the binding is made.

This is exactly what seems to be happening when I try to bind to dynamic objects.

@Eeli-K said in Binding C++ properties exposed to QML to dynamically created QML objects:

If you actually have tested it and it works as you supposed, tell me I'm wrong.

But yes, with a static object (not sure if static is the right word, but one that I declare in QML with an id, and that I know will be there from the start), I can simply bind any property to the QVector2D's x and y members.

Say I have a C++ class called "backend". It has a single QVector2D that I expose to QML through Q_PROPERTY and that has a manually defined signal that is emitted whenever the QVector2D changes. This QVector2D has the name "box". I am able to bind the text property of a static object in QML with the x member of the QVector2D as such:

Text 
{
      text: backend.box.x // This binding works
}

I don't have the code available for the weekend, but I will have to try some more come Monday. Thanks for all input in any case! :)

Eeli K

@Obi-Wan

text: backend.box.x // This binding works

OK. This probably works because backend.box.x is re-evaluated every time the boxChanged (or whatever the name is) signal is sent. Then behind the scenes backend.box() (or whatever the getter name is) function is called and you receive a new copy of the 2d vector. Therefore the x is correct. Maybe I now understand better: x doesn't need to be a property, it's enough that the expression is evaluated when x is changed. I have to test what it means in practice.

Obi-Wan

@Eeli-K It very well might be that you are right about the problem being something related to C++! I created a working example where I bind properties of dynamic objects using QML only, and it works! It does however use a method explicitly warned against in the documentation, so I think I should find an alternative. The documentation says about ListModel.get that():

Warning: The returned object is not guaranteed to remain valid. It should not be used in property bindings.

which is what I'm doing now.

I will have to check again on Monday why this doesn't work in the original code, and if the problem lies with the C++ property!

main.qml

import QtQuick 2.6
import QtQuick.Window 2.2
import QtQuick.Controls 2.1

Window {
    visible: true
    width: 640
    height: 480

    ListModel
    {
        id: listModel
    }

    Column
    {
        id: mainCol
        anchors.centerIn: parent
        spacing: 10

        Button
        {
            text: "Click to create object"

            onClicked:
            {
                var component = Qt.createComponent("Box.qml");
                var obj = component.createObject(mainCol);

                listModel.append({"obj": obj})
            }
        }

        TextField
        {
            id: inputTxt
        }

        Button
        {
            text: "Click to bind properties"

            onClicked:
            {
                for (var i = 0; i < listModel.count; ++i)
                {
                    var dynObj = listModel.get(i).obj;
                    dynObj.boxTxt = Qt.binding(function() {return inputTxt.text});
                }
            }
        }
    }
}

Box.qml

import QtQuick 2.0

Rectangle
{
    property alias boxTxt: txt.text

    width: 100
    height: 100
    color: "lightblue"

    Text
    {
        id: txt
        text: "Dynamic object"
    }
}

Eeli K

@Obi-Wan

It does however use a method explicitly warned against in the documentation, so I think I should find an alternative. The documentation says about ListModel.get that():
Warning: The returned object is not guaranteed to remain valid. It should not be used in property bindings.

I think it rather means using it in the right side of the binding. But I may be wrong. After all, I have demonstrably been wrong sometimes :)

But here is my code:
main.qml:

import QtQuick 2.7
import QtQuick.Controls 2.0
import QtQuick.Layouts 1.3

ApplicationWindow {
    visible: true
    width: 640
    height: 480
    title: qsTr("Hello World")

    ColumnLayout {
        id: layout
        anchors.fill:parent

        ListModel {
            id: boxListModel
        }

        Button {
            text: "create qml model"
            onClicked: {
                for (var i = 0; i < 1; ++i)
                {
                    var component = Qt.createComponent("Box.qml");
                    var obj = component.createObject(layout);
                    boxListModel.append({"obj": obj})
                    obj.boxTxt = Qt.binding(function() {return backend.proplist[i].x})
                }
            }
        }

        Button {
            id: button1
            text: "change backend"
            onClicked: {
                backend.changeList()
            }
        }
    }
}

backend.changeList() which you should be able to add to your backend class easily:

void MyClass::changeList() {
    float f = m_list.at(0).value<QVector2D>().x() + 1;
    m_list.clear();
    m_list.append(QVariant(QVector2D{f, f}));
    emit arrayChanged();
}

(The m_list is supposed to have at least one QVector2D in it.)

This doesn't work, but if you change

obj.boxTxt = Qt.binding(function() {return backend.proplist[i].x})

to

obj.boxTxt = Qt.binding(function() {return backend.proplist[0].x})

it works!

Eeli K

@Obi-Wan (Had to finish the previous post in a hurry, continuing...) So the problem is not in dynamic objects or C++ but that the property binding function can't handle variables taken from the immediate function context, in this case the loop counter. However, I got this working:

var f = function(indx) {return function(){return backend.array[indx].x}}
obj.boxTxt = Qt.binding(f(i))

Don't ask me why.

Obi-Wan

@Eeli-K Very interesting! I will give this a go tomorrow and report back! :)

Eeli K

@Eeli-K said in Binding C++ properties exposed to QML to dynamically created QML objects:

Don't ask me why.

I asked myself. This seems to be an example of a closure and functional programming in javascript. f() returns a function where indx is bound, it has a new value in each invocation of f(), and therefore the inner anonymous function works in the binding without the original scope. The backend object is in scope anyways (in QML) and the function is run every time when the array property is changed.

There's nothing miraculous in it, but the Qt documentation doesn't seem to give any advice about these kinds of situations where the binding function should handle variables which are in the outer function scope but not in the QML scope.

GrecKo

In the documentation it says:

The Repeater type creates all of its delegate items when the repeater is first created. This can be inefficient if there are a large number of delegate items and not all of the items are required to be visible at the same time. If this is the case, consider using other view types like ListView (which only creates delegate items when they are scrolled into view) or use the Dynamic Object Creation methods to create items as they are required.

The Dynamic Object Creation methods they refer to are the ones I am using :)

Why do you need that though ? A repeater can do what you are doing with less code.
Most of the time, handling dynamic object creation is counter-productive.

Eeli K

@GrecKo @Obi-Wan I agree with GrecKo. Although it would be nice to put my finding in real use, it's better to use standard means of creating several similar items, which is using views, models and delegates (in this case the Repeater may be the best).

Obi-Wan

@GrecKo @Eeli-K Thank you very much both of you!

I immediately got it working with your fix Eeli-K. I guess it goes to show that some knowledge of javascript is indeed useful! (I personaly have about zero, and just try to do what I would in C++ and adapt it from there ...)

I also got it working with the Repeater now, and it does indeed do what I want with a lot less code:

Repeater
{
model: backEnd.array.length // Adjusts number of boxes according to C++ list length
Box
{
// Binds x and y properties using Repeater's index in the array
pos_x: backEnd.array[index].x 
pos_y: backEnd.array[index].y
}
}

For some reason my mind was sort of set on the Repeater having to know the number of items on startup. It could just be that I'm not used to thinking QML'y and couldn't comprehend that binding the model property to the array length would create the number of objects as needed. (It isn't crucial, but I still don't understand the name of that property, what is the rationale behind "model" meaning roughly "number of items created by Repeater"?)

Googling Dynamic Object Creation QML leads you here which I guess led me to think this was the primary way of achieving dynamic object creation.

I guess I really should read up on models, views and delegates, and just the general QML "mindset"!

Again, thanks, I learned a lot from this little exercise! :)

GrecKo

@Obi-Wan a model can be a QAbstractItemModel, QList<QObject*>, QStringList, a js array, an integer, a single instance of an object, etc. You can read more about it here : http://doc.qt.io/qt-5/qtquick-modelviewsdata-modelview.html

Your example could be simplified by directly using the array, and not just its length

Repeater {
    model: backEnd.array
    Box {
        pos_x: modelData.x
        pos_y: modelData.y
    }
}

Obi-Wan

@GrecKo Ahh! Thanks! That explains the name! The code just keeps on getting shorter ...

And I guess this is what you meant in your original reply where you wrote:

Why not just use the QVariantList as a model in QML ?

?

GrecKo

Indeed :) , sorry for the brevity of my answer but I wasn't sure what you needed and was kinda lost by where the previous discussion was heading.
As you said, I feel that the Dynamic Object Creation in QML article is misguiding a lot of people (here and on StackOverflow) by not mentioning saner alternatives like model + repeater/view or even a declaratively created Component.

Ultimately I think that you should use imperative object creation only for temporary object needed by the UI layer, like showing a dialog for example. I have yet to see another legit usecase for it (or I don't remember it).

Obi-Wan

@GrecKo said in Binding C++ properties exposed to QML to dynamically created QML objects:

Ultimately I think that you should use imperative object creation only for temporary object needed by the UI layer, like showing a dialog for example. I have yet to see another legit usecase for it (or I don't remember it).

I will keep this in mind!

Hopefully this little discusion might help someone else struggling to understand the same concepts!