Error in creating QML Component from C++. Why?

[bogong]

Hello!
I am trying to create QML Component from C++. Everything works fine, but I have this errors in log on start application (When I am trying to create it in QML - no errors):

qrc:/ButtonExit.qml:30: TypeError: Cannot read property 'width' of null
qrc:/ButtonExit.qml:32: TypeError: Cannot read property 'horizontalCenter' of null

on kill application

qrc:/ButtonExit.qml:32: TypeError: Cannot read property 'horizontalCenter' of undefined
qrc:/ButtonExit.qml:38: TypeError: Cannot read property 'horizontalCenter' of undefined

Every property that is predefined in button.qml within "parent" rising error but everything works fine:

Rectangle {

	id: buttonExit;
	objectName: "buttonExit";
	color: COLOR.blueDark();
	width: parent.width * 0.8;
	height: width * 0.2;
	anchors.horizontalCenter: parent.horizontalCenter;

	Text {

		id: buttonExitLabel;
		text: qsTr("Exit");
		anchors.horizontalCenter: parent.horizontalCenter;
		anchors.verticalCenter: parent.verticalCenter;
		color: COLOR.whiteClean();
		font.pointSize: 18;
	}

	MouseArea {

		id: buttonExitArea;
		anchors.fill: parent;
		onClicked: {
			Qt.quit();
		}
	}
}

main.cpp:

int main(int Counter, char *Arguments[]) {

	QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
	QGuiApplication Application(Counter, Arguments);

	QQmlApplicationEngine Engine;
	Engine.load(QUrl(Main));
	if (Engine.rootObjects().isEmpty()) {
		return -1;
	}

	QObject *oRootObject = dynamic_cast<QObject*>(Engine.rootObjects()[0]);
	QObject *oBottomBlock = oRootObject->findChild<QObject*>("bottomBlock");

	if (oBottomBlock) {

		QQmlComponent oComponent(&Engine,QUrl(QString("qrc:/ButtonExit.qml")));
		QObject *oButtonExit = oComponent.create();
		oButtonExit->setParent(oBottomBlock);
		QQuickItem *oItemButtonExit = qobject_cast<QQuickItem*>(oButtonExit);
		oItemButtonExit->setParentItem(qobject_cast<QQuickItem*>(oBottomBlock));

		aLOG << oBottomBlock;
		aLOG << oBottomBlock->children();
	}

	return Application.exec();
}

How to avoid this errors?

[bogong]

I found this thread https://stackoverflow.com/questions/34721748/is-it-possible-to-access-qml-anchors-from-c related to issue.

[JKSH]

@bogong see https://forum.qt.io/topic/53666/adding-qml-element-from-c

[bogong]

@jksh said in Error in creating QML Component from C++. Why?:

@bogong see https://forum.qt.io/topic/53666/adding-qml-element-from-c

I've seen it and upvoted it too before :) This the first thread that been reading. The problem that I have slightly different - when I mean "parent", I mean "parent" defined inside of QML, not in C++. This parent inside of QML working (thx to you first thread) but I have error messages in log for every property in QML that is defined through calling parent.

The question is why I have error messages but it's working how it has to be working? Why there are error messages only when I am trying to define for example "width" not like number but like "parent.width" inside of QML.

The global idea is to create adoptive ui component from C++ that is using relative value (parented) and anchors instead of numbers and constants.

This what do I mean in QML created from C++:

Rectangle {
    ...
    width: parent.width;
    anchors.horizontalCenter: parent.horizontalCenter;
    ...
}

When I am using this relative definition of "width" for example - it's working perfectly but I have error message. When I am using the same directly in QML - no any error and the same result, it's happening only when I am creating QML component that contain relative values from C++ only.

Rectangle {
    ...
    width: 300;
    ...
}

When I am using constant value instead of relative - it's working perfectly too and no any error messages.
Just tested on Qt 5.12.4 and Qt 5.13

[bogong]

The same issue appeared on Qt 5.13.1. The source of example published here.

QML debugging is enabled. Only use this in a safe environment.
qrc:/ButtonExit.qml:30: TypeError: Cannot read property 'width' of null
qrc:/ButtonExit.qml:32: TypeError: Cannot read property 'horizontalCenter' of null
qrc:/PaddingItem.qml:27: TypeError: Cannot read property 'width' of null
1567772412676 QQuickColumn(0x7ffaf842e570, name = "bottomBlock")
1567772412676 (QQuickRectangle(0x7ffb185b6a70, name = "buttonExit"), QQuickItem(0x7ffaf8469080, name = "paddingButtom"))

Is it bug or not?

[JKSH]

@bogong said in Error in creating QML Component from C++. Why?:

The question is why I have error messages but it's working how it has to be working? Why there are error messages only when I am trying to define for example "width" not like number but like "parent.width" inside of QML.

When you call QQmlComponent oComponentButtonExit(&Engine,QUrl(QString("qrc:/ButtonExit.qml")));, Qt creates your ButtonExit item and binds all the properties. However, it does not have a parent at this point, so the error message is correct: parent is null.

After the ButtonExit is created, you set the parent so the property bindings start working.

To see this in action, add some debug messages:

QQmlComponent oComponentButtonExit(&Engine,QUrl(QString("qrc:/ButtonExit.qml")));
QObject *oButtonExit = oComponentButtonExit.create();
QQuickItem *oItemButtonExit = qobject_cast<QQuickItem*>(oButtonExit);

qDebug() << "Before: " << oItemButtonExit; // <-- Add this

oItemButtonExit->setParentItem(qobject_cast<QQuickItem*>(oBottomBlock));
oButtonExit->setParent(oBottomBlock);

qDebug() << "After: " << oItemButtonExit; // <-- Add this

Notes:

• Do replace dynamic_cast with qobject_cast which is much more robust.
• QQuickItem inherits QObject, so oButtonExit is redundant. Just do all your operations on oItemButtonExit:

QQuickItem *oItemButtonExit = qobject_cast<QQuickItem*>(oComponentButtonExit.create());
oItemButtonExit->setParentItem(...);
oItemButtonExit->setParent(...);

[bogong]

@jksh SUPER-MEGA-HUGE Thanks to you for this explanation. It's making clear for me now. Is there any way to create with parent for avoiding this messages?

[JKSH]

You're welcome!

@bogong said in Error in creating QML Component from C++. Why?:

Is there any way to create with parent for avoiding this messages?

I haven't tried it myself, but I think you must avoid QQmlComponent::create().

1. Call QQmlComponent::beginCreate()
2. Set the parent
3. Call QQmlComponent::completeCreate()

See https://doc.qt.io/qt-5/qqmlcomponent.html#beginCreate

[bogong]

@jksh This works fine on creating:

QQmlComponent oComponentButtonExit(&Engine,QUrl(QString("qrc:/ButtonExit.qml")));
QQuickItem *oItemButtonExit = dynamic_cast<QQuickItem*>(oComponentButtonExit.beginCreate(Engine.rootContext()));
oItemButtonExit->setParentItem(dynamic_cast<QQuickItem*>(oBottomBlock));
oItemButtonExit->setParent(oBottomBlock);
oComponentButtonExit.completeCreate();

But there are on killing application still troubles:

QML debugging is enabled. Only use this in a safe environment.
1567855709534 QQuickColumn(0x7f9c09406a40, name = "bottomBlock")
1567855709535 (QQuickRectangle(0x7f9c2b089530, name = "buttonExit"))
qrc:/ButtonExit.qml:32: TypeError: Cannot read property 'horizontalCenter' of undefined
qrc:/ButtonExit.qml:38: TypeError: Cannot read property 'horizontalCenter' of undefined
qrc:/ButtonExit.qml:39: TypeError: Cannot read property 'verticalCenter' of undefined

[bogong]

Solution found. It's a little tricky but do not rising any error message.
Briefly - you have to define object creating function in QML and invoke it from C++. Only then it's working correctly, without any error messages. The example published here

[JKSH]

@bogong said in Error in creating QML Component from C++. Why?:

Solution found. It's a little tricky but do not rising any error message.
Briefly - you have to define object creating function in QML and invoke it from C++. Only then it's working correctly, without any error messages. The example published here

I'm glad you found a solution. Happy coding!

May I ask why you're using dynamic_cast instead of qobject_cast?

[GrecKo]

For others reading this post :
This is not idiomatic QML and generally leads to messy and not really maintenable code

https://forum.qt.io/post/548974

Don't access QML objects from C++. Instead expose your data from C++ to QML and present or react on it accordingly in QML.

[Dylan Bowie]

@bogong said in Error in creating QML Component from C++. Why?:

nchors.horizontalCenter: parent.horizontalCenter;

I'm glad you gays found a solution. Happy coding!

But there are other way that work very well and without any error messages too.:

Rectangle {
       id: buttonExit;
       anchors.horizontalCenter: parent ? parent.horizontalCenter : undefined
}