Integrating QML and C++ problem

[Alexey Moiseev]

I do as described in the tutorial: https://doc.qt.io/qt-5/qtqml-cppintegration-topic.html

I am writing a simple logger transmitting messages from within the program to the text area of the application.:
.h

#ifndef LOGGER_H
#define LOGGER_H

#include <QObject>

class Logger : public QObject
{
    Q_OBJECT
    Q_PROPERTY(QString message READ message WRITE setMessage NOTIFY messageChanged)

public:
    explicit Logger(QObject *parent = nullptr);

signals:
    void messageChanged();

public:
    QString message() const;
    Logger& operator << (QString value);
    void setMessage(const QString& value);

private:
    QString log_message;
};

#endif // LOGGER_H

.cpp

#include "include/logger.h"

Logger::Logger(QObject *parent) : QObject(parent)
{
}

QString Logger::message() const
{
    return log_message;
}

Logger& Logger::operator << (QString value)
{
    setMessage(value);

    return *this;
}

void Logger::setMessage(const QString& value)
{
    if (log_message != value) {
        log_message = value;

        emit messageChanged();
    }
}

main.cpp

qmlRegisterType<Logger>("edu.logger", 0, 1, "Logger");

And trying to get the value in the form.

Logger {
     id: log;
     onMessageChanged: logWidget.append(log.message)  <------- This does not work
}

TextArea {
     id: logWidget
     placeholderText: qsTr("Clear log")
     text: log.message        <------- It works
}             

For some reason, onMessageChanged event does not occur. There are no error messages, variables are changing.

[sierdzio]

@Alexey-Moiseev said in Integrating QML and C++ problem:

text: log.getLastMessage <------- It works

How? There is no getLastMessage property in your Logger class!

How do you call your setMessage() method or modify message property?

[Alexey Moiseev]

@sierdzio Sorry, too many versions of the same I wrote too. Mixed when inserted. Fixed the code in the initial post. Existing functions are called.
I call setMessage from an overloaded operator "<<", where setMessage changes the variable. If you display the values through qDebug, then you can see that the values come and change. But the signal does not call. As I pointed out in the code, when log.message is called separately, it works. But the onMessageChanged event does not occur.

[sierdzio]

That's weird. I can think of only 3 explanations:

• you're trying to log the same message, so if (log_message != value) { condition is not met, and signal is not emitted (BTW. it's quite bizarre to have a logger which prevents the same message from being logged twice!)
• TextArea.append() somehow does not work... but that's unlikely. Or maybe text is added below your viewport? append() adds a newline so you might miss it if your UI is small
• log.message does not return correct value when invoked in a slot... but that's unlikely

One possible solution: pass message as argument in messageChanged(const QString &msg) signal, then use it in slot onMessageChanged: logWidget.append(msg).

[fcarney]

Why did the question get downvoted? This isn't SO.

[Alexey Moiseev]

@sierdzio

1. That was the first thing I checked.
2. The text is inserted where necessary, it works:

TextArea {
     text: log.message
}

3. log.message returns the correct value, because № 2.
4. This solution was the second that I tried.

[J.Hilk]

@Alexey-Moiseev

can you check the following? Maybe append is the issue here.

Logger {
id: log;
onMessageChanged: logWidget.append(log.message) <------- This does not work
onMessageChanged: logWidget.text = log.message <------- Does this work ?
}

[Alexey Moiseev]

@J-Hilk No. Like any other action written in onMessageChanged. For some reason, onMessageChanged is not called. I also can’t check the emit function call, because it's void.

[J.Hilk]

@Alexey-Moiseev and when you simply print something to the console ?

onMessageChanged:console.log("onMessageChanged", log.message)