Access dynamic created ListModel elements from C++!

[Roland_R]

Hi,

there is no public API to conveniently access QML elements from C++. Even when using private impl. classes (a no go) it looks quite dirty as you can see in "tst_qquicklistmodel.cpp":http://qt.gitorious.org/qt/qtdeclarative/blobs/stable/tests/auto/qml/qquicklistmodel/tst_qquicklistmodel.cpp

You could write simple QML functions that provide you access to the model elements and then call QMetaObject::invokeMethod from C++ as explained in "Interacting with QML Objects from C++":http://doc-snapshot.qt-project.org/qt5-stable/qtqml/qtqml-cppintegration-interactqmlfromcpp.html#invoking-qml-methods. This may be a practical way for smaller projects/tasks.

But it seems to be much cleaner, you manage your model data completely in C++ and implement a reasonable QML/JS callable interface. Details are provided in "Integrating QML and C++":http://doc-snapshot.qt-project.org/qt5-stable/qtqml/qtqml-cppintegration-topic.html.

Beware:
I am not a QML expert. I just had a closer look at C++ interfacing too.

[utcenter]

It is a counter-productive thing to access QML from C++. It defeats the whole purpose of UI and logic abstraction. The idea is to be able to access C++ functionality from C++ but not the other way around. Even though it is possible to like... get a pointer to a specific QML element you should not get in the practice of reaching from C++ into the QML world.

[jackmack]

[quote author="utcenter" date="1363417941"]It is a counter-productive thing to access QML from C++. It defeats the whole purpose of UI and logic abstraction. The idea is to be able to access C++ functionality from C++ but not the other way around. Even though it is possible to like... get a pointer to a specific QML element you should not get in the practice of reaching from C++ into the QML world.[/quote]

I know the purpose of UI and logic abstraction! And i try to do that!
Based on that pupose I need a code example to imagine it by myself how I can do that. At the moment I don't know how I can solve my situation described above.

Please give me an example how I can call a JS function of "Rect2.qml" from C++. That's typical the logic layer is changing somewhat and the UI has to show it.

[andre]

You don't call from C++ into QML or JS. What you do is that you expose an object with a signal on it, and you emit that signal. Whatever happens in response to that, is out of the concern of your C++ code.

[Roland_R]

Adding a getChild function to your model first

@
ListModel {
id: myModel
objectName: "myModel"

    ListElement {
        source: "Rect1.qml"
    }
    ListElement {
        source: "Rect2.qml"
    }
    function getChild(index)
    {
        return get(index)
    }

@

Then from C++ you can access a model item this way

@
// get number of items
QVariant count = QQmlProperty::read( model, "count" );
if (count.isValid())
qDebug() << "count: " << count;

// get element #0
QVariant index = 0;
bool succeeded = QMetaObject::invokeMethod(
    model, "getChild", Q_RETURN_ARG(QVariant, retValue), Q_ARG( QVariant, index ) );
if (succeeded)
{
    const QObject *child = qvariant_cast<QObject *>( retValue );

    QVariant name = QQmlProperty::read( child, "name" );
    if (name.isValid())
        qDebug() << "child.name: " << name;
}

@

Ugly as hell but it should work.

[jackmack]

->Roland_R
The "ugly as hell" works. But there is an other issue! Communicate from QML to C++ doesn't work. I used an C++ exposed class with signals/slots.

I think the used Loader component in the delegate creates the *.qml components/objects with their own context - not in the main.qml context. Therefore I don't have access to my exposed class. Strange: Nothing is written to the console like "unknown to find my exposed class". Only no calls of event handlers or calling methods works :-(

Using of example:
@(.h)
...
signals:
void enableComponent();
...

(.cpp)
...
emit enableComponent();
...

(.qml)
...
Connections {
target: MyBinding
onEnableComponent: {
console.log("rect1")
}
}
...@

To avoid this and have fully access uand functionality of exposes classes I have to use VisualItemModel instead of ListModel.

->Andre:
Yes, I think this is the correct way. Using exposed classes can be uses for vice-versa communication QML/C++.

But in my case there is an evil restriction of using VisualItemModel: No append(), remove() methods!

For reminder:
My application should be able to set the number of pages of a ListView depending of a xml config file which is loaded at runtime.

VisualItemModel works only with static items like

@main.qml

VisualItemModel {
    id: myModel
    objectName: "myModel"

    Rect1 {}
}

@

But know I found a hack/workaround:
"Changing the model of the VisualItemModel" ;-)

Calling a JS function at runtime from C++ like that:

@ function changeModel()
{
var str;
str = "import QtQuick 1.1\n
Item {\n
id: myComp\n
property alias count: myModel.count\n
property alias model: myModel\n
\n
VisualItemModel {\n
id: myModel\n
objectName: "myModel"\n
Rect2 {}\n
Rect3 {}\n
}\n
}"

    var o = Qt.createQmlObject(str, mainRect)

    view.model = o.model;
}

@

That works!

At runtime I can prepare a string containing the qml code and pass it to the changeModel() as parameter.

Strange, but works... (await I will find next restrictions).

[Torgeir]

This works, but seems unnatural (or hacky/ugly :-) ) for me that the C++ code "reaches" into the QML world and depends on particular functions or named objects to be present.

I don't know the full scope of requirements for your project, but would it be possible to implement the model in C++? Derive from QAbstractListModel:

@
class MyCppModel : public QAbstractListModel
{
Q_OBJECT
public:
explicit MyCppModel(QObject *parent = 0) : QAbstractListModel(parent) {}
int rowCount(const QModelIndex & parent = QModelIndex()) const {
return itemSources.count();
}

QVariant data(const QModelIndex &index, int role) const {
    if (role == ItemSourceRole)
       return itemSources.at(index.row());
   else
       return QVariant();
  }

public slots:
void append(QString itemSource) {
beginInsertRows(itemSources.count(), itemSources.count());
itemSources.append(itemSource);
endInsertRows();
}

protected:
enum Roles { ItemSourceRole : Qt::UserRoles + 1 }
QHash<int, QByteArray> roleNames() const {
QHash<int, QByteArray> roles;
roles[ItemSourceRole] = "item_source";
return roles;
}
private:
QList<QString> itemSources;
@

in main.cpp:
@qmlRegisterType<MyCppModel>("my.components", 1, 0, "MyCppModel");@

in QML:
@
MyCppModel {
id: myModel
onComponentCompleted: {
myModel.append("Rect1.qml")
myModel.append("Rect2.qml")
}
}

Column {
Repeater {
model: myModel
Loader { source: item_source }
}
}
@

(Code not tested, just written to show the main idea)

Since the model is in C++, any other C++ code could also call append() to add items to the model.
This way you don't tie the C++ backend to any particular QML frontend.

[jackmack]

Thanks. When I have more time I will check your suggestion to implement a C++ model.

[xiangzhai]

Hi Torgeir, you are so cool to qmlRegisterType a customized C++ Model for QML usage.

But it is much easier to @findChild@ by @objectName@ and @setProperty@ the @QQuickListView@ property model.

Here is my example,

in QML:
@
...
ListView {
objectName: "categoryListView"
model: categoryModel
...
@

in C++:
@
QQmlApplicationEngine engine(QUrl("qml/main.qml"));
QObject *topLevel = engine.rootObjects().value(0);

QObject *categoryListView = topLevel->findChild<QObject *>("categoryListView");
qDebug() << categoryListView;
QStringList dataList;
for (int i = 1; i < 19; i++)
dataList.append("Category " + QString::number(i));
categoryListView->setProperty("model", QVariant::fromValue(dataList));
@

snapshot shown as https://www.dropbox.com/s/t03szfgyidohsoy/抓图2.png

[andre]

[quote author="xiangzhai" date="1393923065"]Hi Torgeir, you are so cool to qmlRegisterType a customized C++ Model for QML usage.

But it is much easier to @findChild@ by @objectName@ and @setProperty@ the @QQuickListView@ property model.
[/quote]
It depends on the situation what is easier. I think Torgeir is right that it is a bit weird to have to "reach into" the QML tree from C++. It is actually discouraged by the Qt documentation to reach deeply into the tree if you can avoid it. I find it more elegant to not have to depend on manually setting an objectName in the QML: way less brittle.

I'd advise your approach for small toy projects and experiments, but Torgeirs approach for anything serious.

[xiangzhai]

Hi Andre,

Yes, it depends :)

And I find another way to play with ListView model

1. qmlRegisterType a customized C++ Model such as:
   @
   qmlRegisterType<MyModel>("cn.com.isoft.demo", 1, 0, "MyModel");
   @

2. QStringList property such as:
   @
   Q_PROPERTY(QStringList myLists READ myLists NOTIFY myListsChanged)
   @

3. put MyModel in QML such as:
   @
   import cn.com.i-soft.demo 1.0
   ...
   ListView {
   ...
   MyModel {
   id: myModel
   }

   model: myModel.myLists
   delegate: Rectangle {
   ...
   Text { text: modelData }
   }
   }
   @

Regards,
Leslie Zhai