[Development] Making QObject::dumpObjectTree() and QObject::dumpObjectInfo() invokable

[Mitch Curtis]

From: Development [mailto:development-bounces+mitch.curtis=qt.io at qt-project.org] On Behalf Of Pierre-Yves Siret
Sent: Tuesday, 6 March 2018 11:29 AM
To: André Somers <andre at familiesomers.nl>
Cc: Qt development mailing list <development at qt-project.org>
Subject: Re: [Development] Making QObject::dumpObjectTree() and QObject::dumpObjectInfo() invokable



2018-03-06 11:12 GMT+01:00 André Somers <andre at familiesomers.nl<mailto:andre at familiesomers.nl>>:


On 06/03/2018 11:04, Mitch Curtis wrote:
https://codereview.qt-project.org/#/c/221758/ makes QObject::dumpObjectTree() and QObject::dumpObjectInfo() invokable so that they can be used from QML. I think that this could be useful to debug issues, but being such a widely used and important class, I'm a bit unsure about whether it's worth the extra overhead. Here's what Olivier has to say about the overhead (taken from the review comments):

"The overhead here is that QObject, which is the base class of all objects, gets two more methods. (out of the 4 it has currently.) This means that QMetaObject::invoke might be slightly slower if it does not find the method. (But since it is currently not really optimized right now, i don't think we should care about this.) I don't know what that means for QML lookups, but probably does not matter."

So, I'm wondering what others think.

Would you use these from QML?

Would these be better off as a helper function in the Qt singleton? E.g. Qt.dumpObjectTree(object) and Qt.dumpObjectInfo(object).
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To be honest: no, I would probably never use them from QML. Nor do I use often from C++ either. I usually resort to external tooling such as GammaRay that give me all these methods can give me and much, much more.

André


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I won't either, I just tried it and it gave me too low-level information that I can't really exploit afterwards. I don't see a usecase where that'll help me, maybe you do?

I prefer my low-tech solution of recursively iterating the children or resources lists from QML or like André said, external tooling like GammaRay.

A helper function seems like a good compromise if it's needed.

Thanks for the feedback guys.

With regards to it giving too low-level information, I’m not sure what you mean. The idea is to be able to see the children of an object/item, so output matching that description would be on exactly the right level. :)

It seems to give more information than recursively iterating the children from QML:

import QtQuick 2.7
import QtQuick.Controls 2.3

ApplicationWindow {
    visible: true
    width: 640
    height: 480

    function printTree(item, indent) {
        if (indent === undefined)
            indent = 0
        else
            ++indent

        if (indent === 0)
            print(item)

        for (var childIndex = 0; childIndex < item.children.length; ++childIndex) {
            var pad = ""
            for (var i = 0; i < indent; ++i)
                pad += " "
            print(pad, item.children[childIndex])
            printTree(item.children[childIndex], indent)
        }
    }

    StackView {
        id: stackView
        initialItem: Button {
            Image {}
        }
        Component.onCompleted: {
            stackView.dumpObjectTree()
            printTree(stackView)
        }
    }
}

Output from dumpObjectTree():

QQuickStackView::
    QQuickTransition::
    QQuickTransition::
    QQuickTransition::
    QQuickTransition::
    QQuickTransition::
    QQuickTransition::
    QQuickButton::
        QQuickImage::
        QQuickRectangle::
    QQmlComponentAttached::

Output from printTree():

qml: QQuickStackView(0x25e95f08020)
qml:  QQuickButton(0x25e95f09100)
qml:   QQuickImage(0x25e95f08890)
qml:   QQuickRectangle(0x25e95f091f0)

The idea is not necessarily to print the QObjects, but since there was already a function that printed everything (dumpObjectInfo()), I figure it would be simple enough to expose it to QML.

Shawn has a similar patch that is aimed at items:

https://codereview.qt-project.org/#/c/193285/

The use case that led me to push the patch was a focus issue, if I remember correctly. An unnamed (no objectName) item had focus, and I couldn’t see where it came from.

I’m starting to lean towards a singleton helper for this, if we went ahead with it at all.
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