在 QML 可访问的 C++ QObject 子类中创建成员变量？

Question

Say I have a QObject subclass Foo defined in C++:

class Foo : public QObject {
    Q_OBJECT
public:
    /* ... */
};

and say I attach it to the root context of a QQmlApplicationEngine as like:

Foo foo;
QQmlApplicationEngine engine;
engine.rootContext()->setContextProperty("foo", (QObject*)&foo);

so now I can call a Q_INVOKABLE member function (or slot it seems) of Foo , say called doStuff() , from QML with the call foo.doStuff() :

Item {
   onWhatever: { foo.doStuff(); }
}

Is it possible to define as well a member variable on Foo in C++ in such a fashion that it is visible in QML and when that member variable changes any usages of it in QML are also updated?

As in:

class Foo : public QObject {
    Q_OBJECT
public:

    Q_??? int bar = 40;
};

In QML:

Item {
    width: foo.bar + 100
}

So initially the width of this item is 140 , and then later in C++:

foo.bar = 80

And now the width of the item automatically changes to 180

Is there something that does what I'm trying to do above? ( Q_??? )

Answer 1

You have to create a Q_PROPERTY:

#ifndef FOO_H
#define FOO_H

#include <QObject>

class Foo : public QObject
{
    Q_OBJECT
    Q_PROPERTY(int bar READ bar NOTIFY barChanged)
public:
    explicit Foo(QObject *parent = nullptr);
    int bar() const;
    void setBar(int bar);
    Q_INVOKABLE void doStuff();
Q_SIGNALS:
    void barChanged(int bar);
private:
    int m_bar;
};

#endif // FOO_H

#include "foo.h"

Foo::Foo(QObject *parent) : QObject(parent), m_bar(40){}

int Foo::bar() const{
    return m_bar;
}

void Foo::setBar(int bar){
    if (m_bar == bar)
        return;
    m_bar = bar;
    emit barChanged(m_bar);
}

void Foo::doStuff()
{
    setBar(80);
}

With the above, the "bar" property is readOnly from QML, if you want it to be writable then you must also expose the setter:

Q_PROPERTY(int bar READ bar WRITE setBar NOTIFY barChanged)

Answer 2

I wrote a macro so this can be done with one line:

PROPERTY(int, bar)

The definition is:

#define PROPERTY(type, name)                                            \
  Q_PROPERTY(type name READ name WRITE set_##name NOTIFY name##Changed) \
 public:                                                                \
  type name() { return _property_member_##name; }                       \
  void set_##name(type _in) {                                           \
    if (_property_member_##name == _in) return;                         \
    _property_member_##name = _in;                                      \
    name##Changed(_property_member_##name);                             \
  }                                                                     \
  Q_SIGNAL void name##Changed(type name);                               \
                                                                        \
 private:                                                               \
  type _property_member_##name;