Why can I not assign this Qt property?

问题

Qt throws the following error at runtime.

Unable to assign LIMITS_T to LIMITS_T

I assume that Qt needs more meta data information, but I don't know what I missing. I have done everything to declare the metatype:

limits.h

class LIMITS_T : public QObject{
    Q_OBJECT
    Q_PROPERTY(float min READ readMin WRITE writeMin NOTIFY minChanged)

public:
    LIMITS_T() : QObject() {}
    LIMITS_T(const LIMITS_T& limit) : QObject()
    {
        this->min = limit.min;
    }

    float min = 0;

    float readMin() { return min; }
    void writeMin(float min) { this->min = min; }

    bool operator = (const LIMITS_T &limit)
    {
        this->min = limit.min;
    }

signals:
    void minChanged();
};

Q_DECLARE_METATYPE(LIMITS_T)

This is a simplified version of the splitBarGauge class

splitDialGauge.h

class SplitDialGauge : public QQuickPaintedItem
{
    Q_OBJECT
    Q_PROPERTY(LIMITS_T limits READ getLimits WRITE setLimits NOTIFY limitsChanged)

public:
    SplitDialGauge(QQuickItem *parent = 0);

protected:
    LIMITS_T limits;
    virtual LIMITS_T getLimits();
    virtual void setLimits(LIMITS_T value);
}

splitDialGauge.cpp

#include "splitBarGauge.h"

SplitDialGauge::SplitDialGauge(QQuickItem *parent = 0);
    : QQuickPaintedItem(parent)
{
}

LIMITS_T SplitDialGauge::getLimits()
{
    return this->limits;
}

void SplitDialGauge::setLimits(LIMITS_T limits)
{
    this->limits = limits;
    update();
}

And I register the class with the Qt Metadata system

#include "limits.h"
#include "splitDialGauge.h"
int main(int argc, char *argv[])
{
    QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
    QGuiApplication app(argc, argv);

    qmlRegisterType<SplitDialGauge>("ExampleModule", 1, 0, "SplitDialGauge");
    qmlRegisterType<LIMITS_T>("ExampleModule", 1, 0, "Limits");
    qRegisterMetaType<LIMITS_T>();

    QQmlApplicationEngine engine;
    engine.load(QUrl(QLatin1String("qrc:/main.qml")));
    if (engine.rootObjects().isEmpty())
        return -1;

    return app.exec();
}

Here is a snipet from the QML file

import ExampleModule 1.0

ApplicationWindow {
    visible: true
    width: 800
    height: 480

    SplitDialGauge {
        Limits {
            id: qtyLimits
            min: 4
        }
        height: 200
        width: 50
        limits: qtyLimits
    }
}

回答1:

You must declare the property as a pointer. All QObjects are supposed to be manipulated as pointers, according to the docs:

No Copy Constructor or Assignment Operator QObject

has neither a copy constructor nor an assignment operator. This is by design. Actually, they are declared, but in a private section with the macro Q_DISABLE_COPY(). In fact, all Qt classes derived from QObject (direct or indirect) use this macro to declare their copy constructor and assignment operator to be private. The reasoning is found in the discussion on Identity vs Value on the Qt Object Model page.

The main consequence is that you should use pointers to QObject (or to your QObject subclass) where you might otherwise be tempted to use your QObject subclass as a value. For example, without a copy constructor, you can't use a subclass of QObject as the value to be stored in one of the container classes. You must store pointers.

In your case:

#ifndef SPLITDIALGAUGE_H
#define SPLITDIALGAUGE_H

#include "limits_t.h"

#include <QPainter>
#include <QQuickPaintedItem>    

class SplitDialGauge : public QQuickPaintedItem {
    Q_OBJECT
    Q_PROPERTY(LIMITS_T *limits READ getLimits WRITE setLimits NOTIFY limitsChanged)
    LIMITS_T *limits;    
public:
    SplitDialGauge(QQuickItem *parent = 0) : QQuickPaintedItem(parent), limits(nullptr) { }
    void paint(QPainter *painter) {
        [...]
    }    
    LIMITS_T *getLimits() const { return limits; }
    void setLimits(LIMITS_T *value) {
        if (limits == value) return;
        limits = value;
        [...]
        emit limitsChanged();
    }     
signals:
    void limitsChanged();
};
#endif // SPLITDIALGAUGE_H

A functional example can be found in the following link.

来源：https://stackoverflow.com/questions/47803531/why-can-i-not-assign-this-qt-property