How to get a ListModel to update from a signal

[flowolf]

So I have a ListView with delegates of Rectangles that contain Text. It uses a ListModel that which when completed, appends more ListElements that is read from a C++ class method which iterates a QStringList. Essentially it is a Logger that functions well at the start but only at the start of the application. I'm not sure if I want to use Component.onCompleted and need to use a signal from the C++ class to continue to update the ListModel. I've tried using onLogLinesChanged() as a signal but keep getting an error "Non-existent attached object". Here is a portion of my code so far.

ListView {
id: logList_Id
boundsBehavior: Flickable.StopAtBounds
model: listModel
clip: true
anchors.fill: parent
delegate: Rectangle {

                id: rectangle
                width: logList_Id.width
                height: textLogs_Id.height
                color: index % 2 == 0 ? "#bdbdbd" : "#ffffff"

                Text {
                    
                    id: textLogs_Id
                    text: logString
                    wrapMode: Text.WordWrap
                    width: parent.width
                }
            }

}

ListModel {

//    Connections {
//                    target: LibXpcWorkerApi
//                    onlogLinesChanged: {
//                        listModel.append()
//                    }
//                }

      id: listModel
      Component.onCompleted: {
                LibXpcWorkerApi.setLogLines();
                for(var i=0; i<LibXpcWorkerApi.logLinesSize(); i++)
                    append(createListElement(i));
            }

            function createListElement(i) {
                return { logString: LibXpcWorkerApi.logLinesAt(i)};
            }

//          LibXpcWorkerApi.onLogLinesChanged: listModel.append(LibXpcWorkerApi.logLinesAt(0));

}

[dheerendra]

You can expose the model from c++ itself. Please look at examples directory of qt installations. It is easy.

[flowolf]

That is an incredibly vague answer and does not help me whatsoever. I already have the signals connected and can call class members from qml. I do not know nor can find any information on what is the correct qml implementation for updating a ListModel once it receives a signal.

[RajeeshRaveendran]

Hi,

I have created a sample application. Hope this will help you to get what u want.

/// main.cpp

#include <QApplication>
#include <QQmlApplicationEngine>
#include <QQmlContext>
#include "myviewmodel.h"

int main(int argc, char *argv[])
{
QApplication app(argc, argv);

QQmlApplicationEngine engine;

QQmlContext* context = engine.rootContext();
MyViewModel myModel;
context->setContextProperty("myViewModelInstance", &myModel);

engine.load(QUrl(QStringLiteral("qrc:/main.qml")));

return app.exec();

}

/// myviewmodel.h
#ifndef MYVIEWMODEL_H
#define MYVIEWMODEL_H

#include <QObject>

class MyViewModel : public QObject
{
Q_OBJECT
public:
Q_PROPERTY(QStringList dataList READ dataList NOTIFY dataListChanged)
explicit MyViewModel(QObject *parent = 0);
Q_INVOKABLE void setLogLines();
Q_INVOKABLE void updateModel(const QString& value);
QStringList dataList();

signals:
void getLatest();
void dataListChanged();

public slots:
void onGetLatest();

private:
QStringList m_myData;
};

#endif // MYVIEWMODEL_H

/// myviewmodel.cpp

#include <QVariant>
#include "myviewmodel.h"

MyViewModel::MyViewModel(QObject *parent) : QObject(parent)
{
connect( this, SIGNAL( getLatest()),
this, SLOT( onGetLatest()),
Qt::DirectConnection );
}

void MyViewModel::setLogLines()
{
QString item("Name: %1");
for(int i = 0; i < 5; ++i)
{
m_myData.append(item.arg(i+1));
}
emit dataListChanged();
}

void MyViewModel::updateModel(const QString& value)
{
m_myData.append(value);
emit dataListChanged();
}

void MyViewModel::onGetLatest()
{
/// Update the model and emit the signal. So that QML will update.
updateModel("from C++");
}

QStringList MyViewModel::dataList()
{
return m_myData;
}

/// main.qml
import QtQuick 2.5
import QtQuick.Controls 1.4
import QtQuick.Dialogs 1.2

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

ListView {
id: logList_Id
boundsBehavior: Flickable.StopAtBounds
model: myViewModelInstance.dataList /// Propery exposed from C++
clip: true
anchors.fill: parent
delegate: Rectangle {

                id: rectangle
                width: logList_Id.width
                height: textLogs_Id.height
                color: index % 2 == 0 ? "#bdbdbd" : "#ffffff"

                Text {

                    id: textLogs_Id
                    text: modelData
                    wrapMode: Text.WordWrap
                    width: parent.width
                }
            }
}

ListModel {
      id: listModel
      Component.onCompleted: {
                //LibXpcWorkerApi.setLogLines();
                myViewModelInstance.setLogLines();
                for(var i=0; i<10/*LibXpcWorkerApi.logLinesSize()*/; i++)
                    append(createListElement(i));
            }

            function createListElement(i) {
                //return { logString: LibXpcWorkerApi.logLinesAt(i)};
                return ({"action": "preset"});
            }

}
Button {
    id: button1
    width: 100
    height: 50
    text: "ADD"
    anchors.bottom: parent.bottom
    onClicked: {
        myViewModelInstance.updateModel("from QML"); // Add an item in the list.
    }
}
Button {
    id: button2
    width: 100
    height: 50
    text: "ADD"
    anchors.bottom: parent.bottom
    anchors.left: button1.right
    onClicked: {
        myViewModelInstance.getLatest(); /// Trigger model c++ class to update the list.
    }
}

}

[Eeli K]

@flowolf I think "Non-existent attached object" comes because you have
target: LibXpcWorkerApi
LibXpcWorkerApi.setLogLines();
LibXpcWorkerApi.logLinesAt(0)

which begin with a capital letter. Either your LibXpcWorkerApi should be a type which actually has attached properties or it should be an object called libXpcWorkerApi.

[Wolosocu]

@Eeli-K said in How to get a ListModel to update from a signal:

@flowolf I think "Non-existent attached object" comes because you have
target: LibXpcWorkerApi
LibXpcWorkerApi.setLogLines();
LibXpcWorkerApi.logLinesAt(0)

which begin with a capital letter. Either your LibXpcWorkerApi should be a type which actually has attached properties or it should be an object called libXpcWorkerApi.

This capital letter requirement has gotten me a few times. It's one of the things I wish would change about QML.

[flowolf]

I actually figured it out on my own but thanks for taking the time to help me out.

I did not use QQml Context, rather qmlRegisterSingletonType. Inside the LibXpcWorkerApi ctor I connect a signal to a slot which also passes a string. Inside slot, it takes that string and modifies a private member QString mLog and it ends with emitting a signal that the log has changed.
Inside qml, for whatever reason, Connections can't be too far embedded in a qml file rather needs to be further out of scope towards the top.
Eeli K, your right about the capital letter. So for anyone else attempting to append strings to a qml component based on signals, here's the short.

LibXpcWorkerApi ctor { connect(mXpcLib, SIGNAL(signalLogLineAdded(const QString), this, SLOT(slotLogAdded(const QString))); }

void LibXpcWorkerApi::slotLogAdded(const QString logline)
{
if(mLog != "") mLog= "";
mLog = logline;
emit logChanged();
}

qml file:
Connections
{
target: LibXpcWorkerApi
onLogChanged: listModel_Id.append(showLogsRect_Id.addLogElement());
}

function addLogElement()
{
return { logString: LibXpcWorkerApi.getLog()};
}

append will not take strings, only objects so you have to set text inside a delegate as logString or whatever matches in your return statement.
I stored logs one string at a time for better memory management, for what I'm doing isn't required to stay in memory in a c++ class.