How to determine total size needed by QML scene?

[Asperamanca]

I display the following QML file inside a QQuickView:

import QtQuick 2.0
import QtQuick.Layouts 1.12
import QtQml 2.12

Item 
{
    ColumnLayout
    {
        anchors.fill: parent
        Text
        {
            text: "First row"
        }
        Text
        {
            text: "Second row"
        }
        Text
        {
            text: "Third row"
        }
    }
}

How do I determine on the C++ side how much size the QML scene needs in order to display fully (without scaling)?

In the QQuickView::statusChanged signal with status "Ready", I get

• Null size for QQuickView::initialSize()
• the current size of the view for QQuickView::size() , not the size needed to display
• Invalid size for renderTargetSize()

EDIT: Also contentItem().implicitWidth / Height or .size() are null sizes

[KroMignon]

@Asperamanca You have to use implicitHeight and implicitWidth from the ColumnLayout component, like this:

import QtQuick 2.0
import QtQuick.Layouts 1.12
import QtQml 2.12

Item 
{
    implicitWidth: _layout.implicitWidth
    implicitHeight: _layout.implicitHeight
       
    ColumnLayout
    {
        id: _layout
        ...
    }
    Component.onCompleted: {
        console.log("Preferred size: " + _layout.implicitWidth + "x" + _layout.implicitHeight);
    }
}

[GrecKo]

Why do you use Item as a root with a single child?

[Asperamanca]

@KroMignon said in How to determine total size needed by QML scene?:

@Asperamanca You have to use implicitHeight and implicitWidth from the ColumnLayout component, like this:

import QtQuick 2.0
import QtQuick.Layouts 1.12
import QtQml 2.12

Item 
{
    implicitWidth: _layout.implicitWidth
    implicitHeight: _layout.implicitHeight
       
    ColumnLayout
    {
        id: _layout
        ...
    }
    Component.onCompleted: {
        console.log("Preferred size: " + _layout.implicitWidth + "x" + _layout.implicitHeight);
    }
}

EDIT: If that's what I need to do (and I don't have reason to doubt you): Why does it display correctly and doesn't show any warnings at design time?

[Asperamanca]

@GrecKo said in How to determine total size needed by QML scene?:

Why do you use Item as a root with a single child?

It used to hold more than that. But my C++ code should still be able to figure out how big the scene is, ideally...

[KroMignon]

@Asperamanca said in How to determine total size needed by QML scene?:

If that's what I need to do (and I don't have reason to doubt you): Why does it display correctly and doesn't show any warnings at design time?

I am not sure to really understand your questionings, I hope my reply is not out of scope!

I set up implicitWidth and implicitHeight of the top component, so if width/height are not defined, those values will be use are size for the item.
ColumnLayout (and RowLayout or GridLayout) is updating his implicitWidth/implicitHeight according to all items which it is holding.

This is how it works, of course if you are using anchors.

I don't understand your question about warnings? Do you have warnings or do you expect warnings?

[Asperamanca]

@KroMignon
Ah, I think I understand (but please correct me if I'm wrong): There is no rule saying that the root component has to be big enough to contain all it's children. It just would look bad e.g. if the root component was a rectangle, and you would try to fill the background...

[KroMignon]

@Asperamanca said in How to determine total size needed by QML scene?:

There is no rule saying that the root component has to be big enough to contain all it's children

There are some aspect to know about QML:

• root component size has not link with the elements in side it.
• an Item can contains elements which are outside from visible space (eg. x > root.width or y >root.height)
• all component for which visible = true are rendered, even if they are not visible because out of visible space or opacity = 0

==> To speed up drawing, always set visible=false for component which you want to hide.

[Asperamanca]

Thanks. Works much better now following the simple rules:

1. Children provide implizitWidth/implicitHeight to be used by parent
2. Parent sets sizes using width/height, taking the children's wishes into account