Attached properties in QML - c++

Question

Can I create my own attached for everything property like a Component?
Item{
Component.onCompleted : {} // Component is attached to everyone Items
}

Answer 1

Yes, for example:
#include <QQmlEngine>
#include <QTimer>
class TestAttached : public QObject
{
Q_OBJECT
// Declare property with setter and getter
Q_PROPERTY(int val READ getVal WRITE setVal)
public:
TestAttached() Q_DECL_EQ_DEFAULT;
explicit TestAttached(QObject *parent):
QObject{parent},
m_val{100500}
{
Q_ASSERT(parent);
qDebug() << "* We just have created the object of attached properties for" << parent->metaObject()->className();
}
~TestAttached() Q_DECL_EQ_DEFAULT;
// define static function qmlAttachedProperties(QObject *object)
// which returns pointer to instance of TestAttached class
static TestAttached *qmlAttachedProperties(QObject *object)
{
TestAttached* testAttached { new TestAttached{object} };
QTimer* timer { new QTimer{testAttached} };
connect(timer, &QTimer::timeout,
[testAttached] {
testAttached->setVal(testAttached->getVal()+1);
emit testAttached->testDone(testAttached->getVal());
});
timer->start(3000);
return testAttached;
}
inline int getVal() const
{
return m_val;
}
inline void setVal(int val)
{
m_val = val;
}
signals:
void testDone(int val);
private:
int m_val;
};
// Makes the type TestAttached known to QMetaType (for using with QVariant)
QML_DECLARE_TYPE(TestAttached)
// declares that the TestAttached supports attached properties.
QML_DECLARE_TYPEINFO(TestAttached, QML_HAS_ATTACHED_PROPERTIES)
Register attached type in QML system (e.g. at main.cpp):
qmlRegisterUncreatableType<TestAttached>("my.test", 1, 0, "Test", QObject::tr("Test is an abstract type that is only available as an attached property."));
And try it in main.qml:
import QtQuick 2.15
import QtQuick.Window 2.15
//import our module
import my.test 1.0
Window {
width: 640
height: 480
visible: true
title: qsTr("Hello World of Attached properties")
Rectangle {
color: focus ? Qt.lighter("red") : "red"
width: parent.width/2; height: parent.height
anchors.left: parent.left
Component.onCompleted: {
console.log("Rectangle is completed", Test.val)
}
Keys.onReturnPressed: {
console.log("Rect 1: Attachet value is", Test.val)
}
Test.val: 20000
Test.onTestDone: function(num) {
console.log("We received", num, "from attached object")
}
MouseArea {
anchors.fill: parent
onClicked: parent.focus = true
}
}
Rectangle {
color: focus ? Qt.lighter("blue") : "blue"
width: parent.width/2; height: parent.height
anchors.right: parent.right
focus: true
Keys.onReturnPressed: {
// The attached object will created after return pressed
console.log("Rect 2: Attachet value is", Test.val)
}
MouseArea {
anchors.fill: parent
onClicked: parent.focus = true
}
}
}

Answer 2

You may not be able to attach properties to Items or Components you did not create. But why would you want to do that anyway?
Instead you could consider using signals and global properties.
For global properties that you can access from anywhere you can set the context property of the root context of the declarative view.
i.e,
QmlApplicationViewer viewer;
MyPropertyClass myProp;
viewer->rootContext()->setContextProperty("MyPropClass", &myProp);
Now, in your QML file you can access the properties of this class as
Rectangle {
Text {
text: MyPropClass.getMyPropText()
}
MouseArea {
onClicked: { MyPropClass.text = "Clicked" }
}
}
This will invoke the Q_INVOKABLE method getMyPropText() from the MyPropertyClass. and Q_PROPERTY 'text' can be set when some signal is emitted.
Would this suit you needs?