modify a QML Text from C++

Hi,
I see data in application output of Qt Creator, but I can not bind this data to text control.
Please guide me.
Thanks

main.cpp:

Code:

---

int main(int argc, char *argv[])
{
    QApplication app(argc, argv);
 
    QQmlApplicationEngine engine;
    engine.load(QUrl(QStringLiteral("qrc:/main.qml")));
 
    MySerialPort iSerialPort;
 
    QThread * iTH = new QThread;
    iSerialPort.moveToThread(iTH);
    iSerialPort.openSerialPort();
    iTH->start();
 
    return app.exec();
}

---

myserialport.cpp:

Code:

---

void MySerialPort::readData()
{
    QByteArray data = serial->readAll();
 
    qDebug() << data;
 
    QQmlEngine engine;
    QQmlComponent component(&engine, QUrl("qrc:MyItem.qml"));
 
    QObject *object = component.create();
 
    object->setProperty("text1Text",data);
 
}

---

MyItem.qml:

Code:

---

import QtQuick 2.0
import QtQuick.Controls 1.2
 
Item {
    id: item1
    width: 400
    height: 400
    property alias text1Text: text1.text
 
    Text {
        id: text1
        width: 400
        height: 29
        color: "red"
        text: "This text should change..."
        font.pixelSize: 12
    }
 
}

---

main.qml:

Code:

---

import QtQuick 2.3
import QtQuick.Controls 1.2
import QtQuick 2.3
 
ApplicationWindow {
    visible: true
    width: 640
    height: 480
    title: qsTr("Hello World")
 
    MyItem{
 
    }
}

---

MySerialPort::readData() runs in a worker thread, it can't instantiate UI elements.

The cleanest way would be to have a model for the strings and use a Repeater in your main.qml to create MyItem instances for each model entry.

The model would need a slot that takes a string and appends it to its data.

You would then connect this slot to a signal in your serial port class which is emitted when it has received a new string.

Cheers,
_

Quote:

---

Originally Posted by anda_skoa

MySerialPort::readData() runs in a worker thread, it can't instantiate UI elements.

The cleanest way would be to have a model for the strings and use a Repeater in your main.qml to create MyItem instances for each model entry.

The model would need a slot that takes a string and appends it to its data.

You would then connect this slot to a signal in your serial port class which is emitted when it has received a new string.

Cheers,
_

---

Thanks for your reply.
Please guide me with simple code.

I change my code but I don't see any change to text control.
main.cpp:

Code:

---

int main(int argc, char *argv[])
{
    QApplication app(argc, argv);
 
    QQmlApplicationEngine engine;
    engine.load(QUrl(QStringLiteral("qrc:/main.qml")));
 
    MySerialPort iSerialPort;
QThread* thread = new QThread(app);
 iSerialPort.moveToThread(thread);
thread->start();
    iSerialPort.myText= engine.rootObjects().at(0)->findChild<QObject*>("text1Text");
    iSerialPort.openSerialPort();
 
    return app.exec();
}

---

serialport.cpp:

Code:

---

void MySerialPort::readData()
{
    QByteArray data = serial->readAll();
 
    qDebug() << data;
 
 
    myText->setProperty("text", data);
}

---

myserialport.h

Code:

---

#ifndef MYSERIALPORT_H
#define MYSERIALPORT_H
#include <QtSerialPort/QtSerialPort>
#include <QObject>
#include <QApplication>
#include <QQmlApplicationEngine>
 
class MySerialPort: public QSerialPort
{
     Q_OBJECT
public:
    MySerialPort();
    QObject *myText;
 
public slots:
    void openSerialPort();
    void closeSerialPort();
 
    void writeData(const QByteArray &data);
    void readData();
 
    void handleError(QSerialPort::SerialPortError error);
 
private:
    void showStatusMessage(const QString &message);
 
    QSerialPort *serial;
 
};
 
#endif // MYSERIALPORT_H

---

main.qml:

Code:

---

ApplicationWindow {
    visible: true
    width: 640
    height: 480
    title: qsTr("Hello World")
 
 
    Text {
        id: text1Text
        objectName: text1Text
        width: 400
        height: 29
        color: "red"
        text: "This text should change..."
        font.pixelSize: 12
    }
 
}

---

That still tries to have the thread interact with the GUI, i.e. you still need the mediator object.

But if you only want to show a single text that is updated with the newest data, then this can be done with a simple property.

So

1) Add a signal to MySerialPort that emits the data in readData()

2) Create a QObject derived class that has a matching slot which updates a QString property, something like this

Code:

---

class SerialData : public QObject
{
    Q_OBJECT
    Q_PROPERTY(QString text READ text NOTIFY textChanged)
 
public slots:
    void setText(const QString &text)
    {
        if (text == m_text) return;
        m_text = text;
        emit textChanged();
    }
 
signals:
    void textChanged();
 
private:
    QString m_text;
};

---

3) Set an instance of that as a context property on the QQmlEngine's rootContext

4) Connect the serial port signal to the slot

5) Use the "text" property in QML as the value of the Text element

Cheers,
_

My problem is solved.
Thanks

main.cpp:

Code:

---

#include <QApplication>
#include <QQmlApplicationEngine>
#include <myserialport.h>
#include <QQmlContext>
 
int main(int argc, char *argv[])
{
    QApplication app(argc, argv);
 
    QQmlApplicationEngine engine;
    engine.load(QUrl(QStringLiteral("qrc:/main.qml")));
 
    MyDisplay myDisplay;
	myDisplay.setText("neda");
    engine.rootContext()->setContextProperty("myDisplay", &myDisplay);
 
    MySerialPort iSerialPort;
    iSerialPort.setDisplay(&myDisplay);
    iSerialPort.openSerialPort();
 
    return app.exec();
}

---

mydisplay.h:

Code:

---

#include <QObject>
 
#ifndef MYDISPLAY_H
#define MYDISPLAY_H
 
class MyDisplay : public QObject
{
    Q_OBJECT
    Q_PROPERTY(QString newText READ getText WRITE setText NOTIFY textChanged)
 
public:
    MyDisplay();
 
    MyDisplay(QString);
 
    Q_INVOKABLE QString getText() const;
public slots:
    void setText(QString text);
 
signals:
    void textChanged(QString);
 
private:
    QString newText;
};
 
#endif // MYDISPLAY_H

---

mydisplay.cpp:

Code:

---

#include "mydisplay.h"
#include "qstring.h"
MyDisplay::MyDisplay()
{
    newText = "";
}
 
MyDisplay::MyDisplay(QString text)
{
    newText = text;
}
 
QString MyDisplay::getText() const
{
    return newText;
}
 
void MyDisplay::setText(QString text)
{
    if (text != newText)
    {
        newText = text;
        emit textChanged(text);
    }
}

---

serialport.cpp:

Code:

---

void MySerialPort::readData()
{
 
    QByteArray data = serial->readAll();
 
 
    qDebug() << data;
 
    QString dataString=data;
 
    myDisplay->setText(dataString);
}
 
void MySerialPort::setDisplay(MyDisplay * m_display)
{
myDisplay = m_display;
}

---

main.qml:

Code:

---

ApplicationWindow {
    visible: true
    width: 640
    height: 480
    title: qsTr("Hello World")
 
    Text {
            id: text1Text
            //objectName: text1Text
            width: 400
            height: 29
            color: "red"
            text: myDisplay.newText
            font.pixelSize: 12
        }
}

---

myserialport.h

Code:

---

class MySerialPort: public QSerialPort
{
     Q_OBJECT
public:
    MySerialPort();
 
public slots:
    void openSerialPort();
    void closeSerialPort();
void setDisplay(MyDisplay * m_display);
    void writeData(const QByteArray &data);
    void readData();
 
    void handleError(QSerialPort::SerialPortError error);
 
private:
    void showStatusMessage(const QString &message);
    MyDisplay * myDisplay;
    QSerialPort *serial;
 
};

---

Quote:

---

Originally Posted by neda

My problem is solved.
Thanks

---

I am surprised that this works.

In any case you should not call MyDisplay::setText() from the secondary thread, at least not without proper locking.
See my original suggestion of using a signal/slot connection.

Cheers,
_

> I am surprised that this works.

Why? Which part of the code is illogical?

> In any case you should not call MyDisplay::setText()
Why?

> from the secondary thread
secondary thread? I did not use thread.

> at least not without proper locking.

Please guide me, I do not know about proper locking.

> See my original suggestion of using a signal/slot connection.

> 1) Add a signal to MySerialPort that emits the data in readData()

Code:

---

connect(serial, SIGNAL(error(QSerialPort::SerialPortError)), this,
            SLOT(handleError(QSerialPort::SerialPortError)));

---

Code:

---

MySerialPort::MySerialPort()
{
    serial = new QSerialPort(this);
 
    connect(serial, SIGNAL(readyRead()), this, SLOT(readData()));
    connect(serial, SIGNAL(error(QSerialPort::SerialPortError)), this,
            SLOT(handleError(QSerialPort::SerialPortError)));
 
    //connect(socket, SIGNAL(setText(QString)), this, SLOT(readData()));
 
}

---

> 2) Create a QObject derived class that has a matching slot which updates a QString property

mydisplay.h:

Code:

---

#include <QObject>
 
#ifndef MYDISPLAY_H
#define MYDISPLAY_H
 
class MyDisplay : public QObject
{
    Q_OBJECT
    Q_PROPERTY(QString newText READ getText WRITE setText NOTIFY textChanged)
 
public:
    MyDisplay();
 
    MyDisplay(QString);
 
    Q_INVOKABLE QString getText() const;
 
public slots:
    void setText(QString text);
 
signals:
    void textChanged(QString);
 
private:
    QString newText;
};
 
#endif // MYDISPLAY_H

---

> Set an instance of that as a context property on the QQmlEngine's rootContext

Code:

---

engine.rootContext()->setContextProperty("myDisplay", &myDisplay);

---

> 5) Use the "text" property in QML as the value of the Text element

Code:

---

Text {
        id: text1Text
        width: 400
        height: 29
        color: "red"
        text: myDisplay.newText        
        font.pixelSize: 12
    }

---

I am new in Qt, please guide me to write better code.
Thank you for your kindness.

Quote:

---

Originally Posted by neda

> I am surprised that this works.
Why? Which part of the code is illogical?

---

You load the QML file before setting the context property but the QML code accesses the context property.
I would have expect that to cause the QML file to fail loading.

Usually one would set the context property before loading.

Quote:

---

Originally Posted by neda

> In any case you should not call MyDisplay::setText()
Why?

---

It is OK now, I didn't see you had removed the usage of QThread.

Quote:

---

Originally Posted by neda

> from the secondary thread
secondary thread? I did not use thread.

---

Right, you had thread usage earlier, didn't see that this changed.

That also makes the signal/slot approach unnecessary.

Since you are not using threading anymore you could even add the property to the MySerialPort class and set its instance as the context property.

Cheers,
_