Q_GADGET可以是其他Q_GADGET的属性吗？

Question

When I tried to pass such object in qml to some QML component property, it's just crashed in release or hung up in debug on:

Currency& operator=(const Currency& that) = default;

currency.h:

#ifndef CURRENCY_H
#define CURRENCY_H

#include <QObject>

class Currency
{
    Q_GADGET

    Q_PROPERTY(QString name READ getName CONSTANT)

public:
    Currency() : name("") {}

    Currency(const Currency& that) = default;
    Currency& operator=(const Currency& that) = default;

    static Currency getUSD() {
        return Currency("USD");
    }

    QString getName() {
        return this->name;
    }

private:
    Currency(const QString& name) : name(name) {}

    QString name;
};

#endif // CURRENCY_H

money.h:

#ifndef MONEY_H
#define MONEY_H

#include <QObject>

#include "currency.h"

class Money {
    Q_GADGET

    Q_PROPERTY(int amount READ getAmount CONSTANT)
    Q_PROPERTY(Currency currency READ getCurrency CONSTANT)

public:
    Money() :
        Money(0, Currency())
    {

    }

    Money(int amount, const Currency& currency) :
        amount(amount),
        currency(currency)
    {

    }


    int getAmount() {
        return this->amount;
    }

    Currency getCurrency() {
        return this->currency;
    }

private:
    int amount;
    Currency currency;
};

#endif // MONEY_H

factory.h:

#ifndef FACTORY_H
#define FACTORY_H

#include <QObject>
#include <QVariant>

#include "money.h"

class Factory : public QObject {
    Q_OBJECT
    Q_DISABLE_COPY(Factory)

public:
    static Factory* instance() {
        static Factory factory;
        return &factory;
    }

    Q_INVOKABLE QVariant getMoney() {
        return QVariant::fromValue(Money(12345, Currency::getUSD()));
    }

private:
    explicit Factory(QObject* parent = nullptr) :
        QObject(parent)
    {

    }
};

#endif // FACTORY_H

main.cpp:

#include <QGuiApplication>
#include <QQmlApplicationEngine>

#include "factory.h"
#include "money.h"


int main(int argc, char *argv[])
{
    QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);

    QGuiApplication app(argc, argv);

    qmlRegisterUncreatableType<Money>("test", 1, 0, "Money", "error");
    qmlRegisterUncreatableType<Currency>("test", 1, 0, "Currency", "error");
    qmlRegisterSingletonType<Factory>("test", 1, 0, "Factory",
                                      [](QQmlEngine* engine, QJSEngine* scriptEngine) -> QObject* {
        Q_UNUSED(scriptEngine)

        Factory* instance = Factory::instance();
        engine->setObjectOwnership(instance, QQmlEngine::ObjectOwnership::CppOwnership);

        return instance;
    });

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

    return app.exec();
}

main.qml:

import QtQuick 2.11
import QtQuick.Window 2.11

import test 1.0

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

    MyItem {
        value: {
            var money = Factory.getMoney()
            console.log(money)

            return money
        }

        anchors.centerIn: parent
    }
}

MyItem.qml:

import QtQuick 2.10
import QtQuick.Controls 2.3


Item {
    id: root
    property var value

    width: label.width
    height: label.height

    Component.onCompleted: {
        console.log(root.value)
    }

    Label {
        id: label

        text: root.value.currency.name
    }
}

Answer 1

The error I get when executing your code is:

QMetaProperty::read: Unable to handle unregistered datatype 'Currency' for property 'Money::currency'
qml: Money(12345, )

According to the docs :

...
The property type can be any type supported by QVariant, or it can be a user-defined type. In this example, class QDate is considered to be a user-defined type.
...

In your case Currency is not supported by QVariant , so the solution is to use Q_DECLARE_METATYPE to be supported by QVariant :

class Currency
{
    ...
};

Q_DECLARE_METATYPE(Currency)

The complete example can be found in the following link .