Understanding QAbstractListModel in python/QML program and C++ equivalent

[Hodenbrecher]

Hi everyone !

I have to change a QML human machine interface from python to C++. But I'm new to python AND qml so I'm struggling as the things are getting tough.

I'll ask my questions before showing the code :

there's a python class derived from QQAbstractListModel that looks like containing some QtQuick Items. (code below) It's PresetModel. First, I kind of understand the setPresetBase method but... that way to put everything in a table like it's nothing... how can I convert it into C++ ? I guess parameters is a QList ? So I could store my items as QVariant in a QList ? If so, then what ?

How can I share such a thing with QML ? I see that they just put it in setContextProperty. I tried to do a simple one and do the same, but I couldn't get the data(index) from my QAbstractListModel, or call a method called apply...

i see that they defined apply as a pyqt slot. What is the equivalent in C++ ? I tried to define it as a public slot : but it changed nothing...
I'm lost.
Could you explain me how to create and use a QAbstractListModel such as this on but in C++ ?

Thanks in advance. Here is the code (not everything, just what I think being useful)

class PresetModel( QtCore.QAbstractListModel ):
    NameRole = QtCore.Qt.UserRole + 1

    _roles = {
        NameRole: b"name",
        }

    def __init__( self, view, parent=None ):
        super().__init__( parent )
        self._preset_list = []
        self._preset_base = None
        self.view = view
        self._name_id = 0

    def rowCount( self, parent=QtCore.QModelIndex() ):
        return len( self._preset_list )

    def data( self, index, role=QtCore.Qt.DisplayRole ):
        try:
            preset_item = self._preset_list[index.row()]
        except IndexError:
            return QVariant()
        if role == self.NameRole:
            return preset_item.name
        return QVariant()

    def roleNames( self ):
        return self._roles

    def setPresetBase( self ):
        object = self.view.rootObject()
        items = object.childItems()
        parameters = []
        for i in items:
            if type(i) == QtQuick.QQuickItem and i.objectName() != "" and i.objectName() != "EditPanel" and i.objectName() != "EditGrid":
                parameters.append( [ i.objectName(), i.x(), i.y(), i.z(), i.width(), i.height(), i.isVisible() ] )
        self._preset_base = Preset( "base", parameters )

    @QtCore.pyqtSlot( int )
    def removePreset( self, index ):
        if len( self._preset_list ) > 0:
            preset = self._preset_list[ index ]
            self.beginRemoveRows( QtCore.QModelIndex(), index, index )
            self._preset_list.remove( preset )
            self.endRemoveRows()
            self.savePresetsToFile()

    @QtCore.pyqtSlot( int )
    def applyPreset( self, index ):
        if index != -1 and len( self._preset_list ) > index:
            preset = self._preset_list[index]
        else:
            preset = self._preset_base
        for i in preset.config:
            item = self.view.rootObject().findChild( QtQuick.QQuickItem, i[0] )
            if item != None:
                item.setProperty( "x", i[1] )
                item.setProperty( "y", i[2] )
                item.setProperty( "z", i[3] )
                item.setProperty( "width", i[4] )
                item.setProperty( "height", i[5] )
                item.setProperty( "visible", i[6] )

The class is instanciated like this

self.preset_model = preset_model.PresetModel( view )
        self.view.rootContext().setContextProperty( 'presetModel', self.preset_model )

    @QtCore.pyqtSlot( QtCore.QVariant )
    def onStatusViewChanged( self, status ):
        if status == QtQuick.QQuickView.Ready:
            self.preset_model.setPresetBase()
            self.preset_model.loadPresetsFromFile()

[Pablo J. Rogina]

@Hodenbrecher said in Understanding QAbstractListModel in python/QML program and C++ equivalent:

they defined apply as a pyqt slot. What is the equivalent in C++ ? I tried to define it as a public slot : but it changed nothing...

Not that I'm proficient with Python (yet...) but please remember that a slot (in C++, QML, Python, whatever) is meant to be called/activated by a signal being emitted...
That said, you may need to check the whole Python code for "connect" entries like these:

buttonX.clicked.connect(buttonX_clicked())

or

QObject.connect(buttonY, SIGNAL("clicked()"), buttonY_clicked)

[KillerSmath]

@Hodenbrecher

. that way to put everything in a table like it's nothing... how can I convert it into C++ ? I guess parameters is a QList ? So I could store my items as QVariant in a QList ? If so, then what ?

You could create a custom data type to store all variables of QQuickItem, then define a list of this custom data type

// definition
typedef struct ObjectDataType{
    QString objectName;
    qreal x, y, z, width, height;
    bool visible;
} ObjectDataType;

// usage
QList<ObjectDataType> parameters;
parameters.append({objName, xValue, yValue, zValue, wValue, hValue, vBoolean});

---

How can I share such a thing with QML ? I see that they just put it in setContextProperty. I tried to do a simple one and do the same, but I couldn't get the data(index) from my QAbstractListModel, or call a method called apply...

Are you passing the correct parameters ? Take a look at data function signature.

QVariant data(const QModelIndex &index, int role = Qt::DisplayRole) const

Can you share a snippet of your code to understand how you are trying to access the data from QML ?

---

i see that they defined apply as a pyqt slot. What is the equivalent in C++ ? I tried to define it as a public slot : but it changed nothing...

Have you defined Q_OBJECT macro on your class definition ? It is necessary to allow the Signal and Slot feature on c++

https://doc.qt.io/qt-5/qobject.html#Q_OBJECT

[Hodenbrecher]

@KillerSmath Thanks for your answer

The struct is a good idea. What I started to do is a QVector<QVariant>. So I could use it like this:

//CTOR of the dataModel I will store in the QList
Preset(QString& air_name, QVector<QVariant>& air_config);

//definition of the QList in PresetModel
QList<Preset> m_preset_list

what do you think about ? Is the struct more efficient or convenient ?

---

i'm using the right definitio, copied it fro the doc.
Here the use of the class. It's used to personalize the interface by moving the items into the main window.

GridView {
    id: container
    width: 300; height: 200
    clip: true
//    [...]
    onCurrentIndexChanged: presetModel.applyPreset( currentIndex )
//    [...]
    model: presetModel
    delegate: delegatePreset
    highlight: highlight
    remove: Transition {
        ParallelAnimation {
            NumberAnimation { property: "scale"; to: 0; duration: 500 }
            NumberAnimation { properties: "x,y"; to: 100; duration: 500 }
        }
    }
    currentIndex: -1
}

I have to say that I don't understand it yet... I just know that I have to create PresetModel. Else I can't launch the QML application.
What is the model, delegate... I don't know if I need to understand it to write the C++

So I think that I have to use

PresetModel* johnnyBoy = new PresetModel(myView);
QQmlContext::setContextProperty("presetModel", johnnyBoy);

but heard about passing it as a QVariant containing the QList... Maybe I missunderstood

---

Yes, everything that needs to be defined as a Q_OBJECT is.
What I am not sure of is if I have to define apply as a

public slot: 

or as a

Q_INVOKABLE

?

---

Subsidiary question
what is this doing and how to write it in C++ ? I kind of understand the role of the role but this is too much for me and the doc is not very helpful

class PresetModel( QtCore.QAbstractListModel ):
    NameRole = QtCore.Qt.UserRole + 1      #+1 ??? The docs says it's value is 0x011

    _roles = {                             #so I guess its an int...
        NameRole: b"name",                 #b"name" ????
        }
#[...]
    def data( self, index, role=QtCore.Qt.DisplayRole ):
          try:
              preset_item = self._preset_list[index.row()]#ok
          except IndexError:
              return QVariant()
          if role == self.NameRole:#not ok
              return preset_item.name     #Probably used to find the QQuickItem linked later
          return QVariant()

Maybe I should create a new topic specific to this ?

Thanks a lot !

---

---

[edit] here is my code for data

QVariant PresetModel::data(const QModelIndex & air_index, int ai_role) const
{
	if (air_index.row() < 0 || air_index.row() >= m_preset_list.size())
		return QVariant();
	const Preset& wr_preset_item = m_preset_list[air_index.row()];//definition : QList<Preset> m_preset_list;
	if (ai_role == m_nameRole)
		return wr_preset_item.getName();//in CTOR : m_nameRole = Qt::UserRole + 1;
	return QVariant();
}

[JonB]

@Hodenbrecher
Your Python to C++ translation looks broadly correct. However, a couple of comments, if you want to understand/improve:

• NameRole = QtCore.Qt.UserRole + 1: the + 1 just ensures it's unique. I can't recall, but somewhere in the docs something talks about starting your own from QtCore.Qt.UserRole + 1 instead of from QtCore.Qt.UserRole. The value will actually be 0x0101 rather than what you wrote. An int should be fine (Qt::ItemDataRole is actually an enum, but I don't think you can extend its values in C++.)

• b"name": Python "b string". Byte literal, producing bytes type rather than str type. Whatever that NameRole is used for is it perhaps supposed to be a perhaps a ByteArray rather than a QString? Alternatively, if your original is Python 2 not 3 it might have something to do with Python char type vs Qt QString/C++ char [], I don't know, but I imagine you'll soon know if it's wrong.

• //in CTOR : m_nameRole = Qt::UserRole + 1;: Note that in Python, anything inside a class but not inside a def --- your NameRole & _roles --- are not instance variables. They are class variables. Your choice of name m_nameRole and setting it in constructor are therefore not great. Your NameRole should therefore be something like a const int defined in the class declaration, and similarly with _roles = ... (can't do that in my C# but I think you can in C++), rather than anything in constructor.

[Hodenbrecher]

@JonB Thank you very much, that was helpful.

Note that in Python, anything inside a class but not inside a def --- your NameRole & _roles --- are not instance variables. They are class variables.

I'm not sure to understand. Do you mean that NameRole and _roles should be static variables in my C++ ?

---

Whatever that NameRole is used for is it perhaps supposed to be a perhaps a ByteArray

That's logical. We set the name as a byte array and use the numerical values of the ASCII char to use it's value.
But I still don't understand this :

_roles = {
        NameRole: b"name",
        }

if _roles is a byte array maybe the synthax to set it's value is using the { } but then... nameRole : b"name"... So all the PresetModel instances (probably only one thought) will have their _role set to the value corresponding to "name" ? That's weird, i think I don't get it

Thank you very much for your help

---

another subsidiary question

@QtCore.pyqtSlot()
    def onPresetChanged( self ):
        row = 0
        for r in self._preset_list:
            if r == self.sender:
                row = self._preset_list.index( r )#index is part of a QAbstractListItem. But i think _preset_list is a QList or a QVector. and r a Preset object
                break
        self.dataChanged.emit( self.index(row, 0), self.index(row, 0), [self.NameRole] )

in the for loop the code assigns to row ( an int probably) a _preset_list.index(r) which means that _preset_list is derived from QAbstractItemModel ? and then there's r in the parameters. r is an element from _preset_list, right ? A Preset object.

[JonB]

@Hodenbrecher said in Understanding QAbstractListModel in python/QML program and C++ equivalent:

I'm not sure to understand. Do you mean that NameRole and _roles should be static variables in my C++ ?

Yes. static, or better still const since you don't want to alter them after they are set. My C++ is a bit ropey, but you can have something like:

class PresetModel
{
    static const int s_nameRole = Qt::UserRole + 1;
}

The important thing is that this value is available outside/independent of any instance (via PresetModel::s_nameRole), and other code might require that.

if _roles is a byte array

It isn't. roles is (like) a C++ struct, hence the { ... }. It is the NameRole member which is the byte array. You can do something more nowadays in C++ with these, but in C this would be something like:

struct {
    byteArray NameRole;
} _roles = {
    "name"
};

Again because it's at the class level you'll want something like static struct ...: it's to be shared by all instances. This is adding some kind of "name role" to this class, in addition to the in-built Qt::ItemDataRole ones. I don't think all on its own the _roles is used/does anything: I am expecting you to find it referenced somewhere in code (via PresetModel._roles) to "declare" a new role. Doubtless it will have something to do with https://doc.qt.io/qt-5/qabstractitemmodel-obsolete.html#setRoleNames (now defunct) & https://doc.qt.io/qt-5/qabstractitemmodel.html#roleNames, q.v..

[KillerSmath]

@Hodenbrecher

What I am not sure of is if I have to define apply as a
public slot:
or as a
Q_INVOKABLE

Slot and Invokable macros are used to different situation.

• Slot is used to allow a function to receive signals.
• Q_INVOKABLE allow the qml to directly call a function

---

Have you seen the QAbstractItemModel Qt Example ? It will improve your understanding of how to forward a custom c++ model to QML.

https://doc.qt.io/qt-5/qtquick-models-abstractitemmodel-example.html