Random "Invalid grouped property access" error

[GPBeta]

Hello everyone,
I got a problem confusing me these days when I try to create my own Button which directly inherits QQuickAbstractButton via C++ .

First, I create a new C++ class:

class MyButtonTemplate : public QQuickAbstractButton {
    // class definitions
};

Register it:

const char uri[] = "MyModule.Templates";

qmlRegisterModule(uri, 1, 0);
qmlRegisterType<MyButtonTemplate>(uri, 1, 0, "Button");

Refer it in MyModule:

import QtQuick 2.11
import MyModule.Templates 1.0 as T

T.Button {
    // invalid grouped property (icon) access
    icon.width: 18
    icon.height: 18
}

Then I got a run-time error:

• QQmlApplicationEngine failed to load component
• qrc:/main.qml:17 Type My.Button unavailable
• qrc:///imports/MyModule/Button.qml:10 "T.Button.icon" is not available in MyModule.Templates 1.0.
• InvalidAccess exited with code -1

So I register the corresponding revision of QQuickAbstractButton for my module:

qmlRegisterRevision<QQuickAbstractButton, 4>(uri, 1, 0);

Now, my app runs as expected......sometimes :( The program will have around 50% chances to raise an error:

• QQmlApplicationEngine failed to load component
• qrc:/main.qml:15 Type Button unavailable
• file:///F:/DEV/Library/QT/5.11.0/msvc2015_64/qml/QtQuick/Controls.2/Button.qml:56 Invalid grouped property access
• InvalidAccess exited with code -1

Here's my main.qml:

import QtQuick 2.11
import QtQuick.Controls 2.4
import QtQuick.Window 2.11

import MyModule 1.0 as My

Window {
    visible: true

    title: "Invalid Access"

    Column {
        anchors.centerIn: parent

        Button { text: "Qt Button" }

        My.Button { text: "My Button" }
    }
}

If I remove the QtQuick.Controls's Button, program will always raise that error.

Environment: Windows 10, Qt 5.11 x64
Here's a minimum demo project:
https://drive.google.com/file/d/1XCjSXlXu4SfulP_hwRVwkoDjejdComHV/view?usp=sharing

Is this a bug or something I missed?
Thank you for any ideas :)

[devDawg]

How are you assigning icon? Perhaps you have an undefined property somewhere. Post that code if you have it please.

But in the mean time, use something like this:

T.Button {
    Component.onCompleted: {
       
        // check properties here. Nothing should be undefined, unless it's supposed to be.
        //for each property, do something like : 
        
if (!this_property.isUndefined()) { assign value }    //elegant pseudocode                                  

else { handle undefined value (avoid crash) } //more elegant pseudocode

}
} 

Let me know how this goes.

Cheers

[GPBeta]

@devDawg Thank you for your reply :)
Please checkout the completed minimal project to reproduce this issue:

https://drive.google.com/file/d/1XCjSXlXu4SfulP_hwRVwkoDjejdComHV/view?usp=sharing

There's no other property access will raise this error, except for this revision tagged and grouped icon property which is defined in QQuickAbstractButton.
The Component.onCompleted() solution is not very suitable for my requirement because it will override bindings created before component completed.

[Diracsbracket]

Hi @GPBeta
I tried your example, it works without problems on my system (Win10, VS2017, Qt 5.11.0).
The "randomness" of the problem seems not reproducible here?

        My.Button {
            text: "My Button"
            onClicked: {
                console.debug("T.Button: " + icon.height + ", " + icon.width)
            }
        }

gives:

qml: T.Button: 18, 18
qml: T.Button: 18, 18
qml: T.Button: 18, 18
...
qml: T.Button: 18, 18
qml: T.Button: 18, 18
qml: T.Button: 18, 18

No warnings nor errors are produced.

[GPBeta]

Hi @Diracsbracket
The error occurs randomly only on startup, so please try to start it as much as possible, or just remove

Button { text: "Qt Button" }

and the program will never start here.

[Diracsbracket]

@GPBeta
What is MyModule.Templates? You import it in imports\MyModule\Button.qml but it's defined nowhere.

[GPBeta]

@Diracsbracket
MyModule.Templates is registered via qmlRegisterModule("MyModule.Templates", 1, 0);
T.Button is registered via qmlRegisterType<MyButtonTemplate>("MyModule.Templates", 1, 0, "Button");
They're both C++ codes in main.cpp

[Diracsbracket]

@GPBeta
My bad... should have read your post more carefully...
I have never defined or even used templates before, so I will just watch and learn from this thread...

[Diracsbracket]

@GPBeta
By adding:

import QtQuick.Templates 2.2

to your Button.qml template file, it seems to work?

import QtQuick 2.11
import QtQuick.Templates 2.2
import MyModule.Templates 1.0 as T

Apparently, without it, the required QQuickIcon type from

qtquickcontrols2/src/quicktemplates2/qquickicon_p.h

does not get registered, hence the errors you get?

[GPBeta]

@Diracsbracket Great! It works perfectly now!

The QQuickIcon is registered in the QtQuick.Templates qml plug-in(qtquicktemplates2plugin.dll), while my C++ module only linked against Qt5QuickTemplates2.dll.

So if we don't import that module explicitly, the icon property may be randomly registered later than the binding access.

I also tried to add depends QtQuick.Templates 2.4 in the qmldir but with no luck.

Whatever, I believe this is the best solution for me, thank you so much :)

[Diracsbracket]

@GPBeta
Thank you for the explanation. Now the randomness makes sense.
Btw. I just checked your webpage. It's quite awesome!

[GPBeta]

@Diracsbracket lol, thank you :P