Avoid unqualified access with required property implementation trouble

Aymeric_Qt · 24 Jul 2023, 13:16

Hello,

I got a question about QML best practices implementation. My case is simple, opening a dialog when ckicking on a toolbar button.
In the first place I wrote this:

main.qml:

ApplicationWindow {
    readonly property ApplicationWindow appWindow: this

    width: 640
    height: 480
    visible: true
    // ...
}

MenuAbout.qml:

Menu {
    title: qsTr("A propos")
    MenuItem {
        text: qsTr("&A Propos")
        display: AbstractButton.TextOnly
        onTriggered: aboutDialog.open()
    }
    DialogAbout {
        id: aboutDialog
    }
}

DialogAbout.qml:

Dialog {
    id: aboutDialog

    anchors.centerIn: parent

    parent: appWindow.contentItem
    modal: false
    title: "About this"
    // ...
}

This was based on what I've found (on stackoverflow) at the time to solve the "problem" about the need of the Dialog component to have a parent to open in.
So this is what I've done to get a reference to a parent component even if I had concerns about the concept of a global property "floating" arround, accessible to any children.

Then I found this video of a talk made during the QtWS 2021. And there is a section named 'Avoid unaqualifed access - Outside property' where I think I've found a better solution.

So as it is done in the video I've modify main.qml:

ApplicationWindow {
    id: appWindow
    
    width: 640
    height: 480
    visible: true
    // ...
}

and AboutDialog.qml:

Dialog {
    required property ApplicationWindow appWindow

    id: aboutDialog

    anchors.centerIn: parent

    parent: appWindow.contentItem
    modal: false
    title: "About this"
    // ...
}

But in this case the application crash when lauching it with the following error:
"Required property appWindow was not initialized".

And that's where I get confused. For me, as 'appWindow' is the id I gave to the application window in the main.qml file it should be initialized when the application start, no?
So I don't know if I got in the wrong way (required property should not be use in this case) or if I just missing a piece.

Thanks for reading.

Aymeric_Qt · replied to JKSH on 25 Jul 2023, 08:17

Hello @JKSH
Thank you for your reply therefore there still something that I don't understand.
In my main.qml I added

AboutDialog {
    appWindowRef: appWindow //I've changed the required property's name in AboutDialog to avoid confusion
}

But I still get the same error message "Required property appWindowRef was not initialized". I feel that I'm missing something obvious, sorry about that.

And I got an other question in terms of best practices is it ok to "intialize" all the component which will need a reference to the Application Window in the main.qml file ? Or is there a cleaner way ?

Edit: There is something that I've forgot to mention.
I've also tried to initialoze a required property in an other component (just to test) which is also declared in the main.qml and in this case it is working perfectly.

main.qml

ApplicationWindow {

id: appWindow

//...
    MainStackLayout {
        appWindowRef: appWindow
        anchors.top: mainToolBar.bottom
        anchors.bottom: mainTabBar.top
        currentIndex: mainTabBar.currentIndex
    }

MainStackLayout.qml

StackLayout {
    id: layout

    required property ApplicationWindow appWindowRef

   //...
}

So is it supposed to be different with Dialog ?

And concerning my question about good pratices, I've look to some projects examples (those available in Qt Creator Examples tab) and it seems to be done that way. So I guess it is the right way at least for someone whith my level of knowledge which is not very high.

JKSH · 25 Jul 2023, 08:17

@Aymeric_Qt said in Avoid unqualified access with required property implementation trouble:

For me, as 'appWindow' is the id I gave to the application window in the main.qml file it should be initialized when the application start, no?
So I don't know if I got in the wrong way (required property should not be use in this case) or if I just missing a piece.

If your AboutDialog has a required property called appWindow, that means your main.qml needs to assign something to the dialog's appWindow property:

AboutDialog {
    appWindow: <something> // This line is required. Removing this line will give you an error.
}

The name of the dialog's property is completely unrelated to the ID that you assign to your root window.

Aymeric_Qt · replied to JKSH on 25 Jul 2023, 08:17