Can't get geometry() in form constructor

[kartun]

I've QGridLayout on my widget and I need to add several elements to it during runtime.
Everything is fine, but I need to calculate and store proper height for added elements. I've tried to get it through

ui->setupUi(this);
    ...
    createRuler(minValue, maxValue, steps);

ui->setupUi(this);
    ...
    createRuler(minValue, maxValue, steps);

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in createRuler I've tried this :

int frame_height = ui->rulerLayout->geometry().height() / steps;

int frame_height = ui->rulerLayout->geometry().height() / steps;

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But it allways return 0; However inside setupUi there is a code :

gridLayoutWidget = new QWidget(rulerFrame);
    gridLayoutWidget->setObjectName(QString::fromUtf8("gridLayoutWidget"));
    gridLayoutWidget->setGeometry(QRect(0, 0, 61, 361));
    rulerLayout = new QGridLayout(gridLayoutWidget);
    rulerLayout->setSpacing(6);
    rulerLayout->setMargin(11);
    rulerLayout->setObjectName(QString::fromUtf8("rulerLayout"));
    rulerLayout->setSizeConstraint(QLayout::SetMinAndMaxSize);
    rulerLayout->setContentsMargins(0, 0, 0, 0);
 
    horizontalLayout->addWidget(rulerFrame);

gridLayoutWidget = new QWidget(rulerFrame);
    gridLayoutWidget->setObjectName(QString::fromUtf8("gridLayoutWidget"));
    gridLayoutWidget->setGeometry(QRect(0, 0, 61, 361));
    rulerLayout = new QGridLayout(gridLayoutWidget);
    rulerLayout->setSpacing(6);
    rulerLayout->setMargin(11);
    rulerLayout->setObjectName(QString::fromUtf8("rulerLayout"));
    rulerLayout->setSizeConstraint(QLayout::SetMinAndMaxSize);
    rulerLayout->setContentsMargins(0, 0, 0, 0);

    horizontalLayout->addWidget(rulerFrame);

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