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

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().

Call QQmlComponent::beginCreate()
Set the parent
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
}