Resizing child widget with keeping aspect ratio when parent widget resized

Question

I have four widgets in a QWidgets named FourWindowWidget I am trying to put these four widgets in FourWindowWidget as one at top and remaining under that while the top one should always have 70% of the size of FourWindowWidget. I tried following :

QSize size = ui->FourWindowWidget->size();

ui->View1->setFixedHeight(size.height()*0.70);

QHBoxLayout * lay = new QHBoxLayout();
lay->addWidget(ui->View2);
lay->addWidget(ui->View3);
lay->addWidget(ui->View4);
lay->setMargin(0);
lay->setContentsMargins(0,0,0,0);

QGridLayout * mainlay =  new QGridLayout;
mainlay->addWidget(ui->View1,0,0);
mainlay->setContentsMargins(0,0,0,0);
mainlay->setHorizontalSpacing(0);
mainlay->setSpacing(0);
mainlay->setMargin(0);
mainlay->addLayout(lay,1,0);

delete ui->FourWindowWidget->layout();
ui->FourWindowWidget->setLayout(mainlay);
this->update();

Now the problem is now View1 have fixed size but size of FourWindowWidget is not fixed. It changes when I remove any other widget around it like closing dock widget. This resize is expected, or say I don't want to make it fixed. So when ever FourWindowWidget get resized its child widgets should be as per ratio set. But I am unable to do this cause 1. There is no any signal that inform resize so the height of internal widget will be calculated. 2. No other way I found to set 70% height of top widget rather than setFixedHeight in grid layout.

I have tried with setting QSizePolicy to setHieghtForWidth but iit is not a working solution. Please suggest me to do this by any way.

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The Question is kind of duplicat of Resizing Qt Widgets based on Window size but there nobody answered.

Answer 1

Setting the stretch factor for your layout should work. I'm not sure why you are using a grid layout for this, when you are adding everything in the same column, but here is an example that works:

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    gridLayout = new QGridLayout;
    this->centralWidget()->setLayout(gridLayout);
    label1 = new QLabel("l1");
    label2 = new QLabel("l2");
    label3 = new QLabel("l3");
    label4 = new QLabel("l4");
    gridLayout->addWidget(label1, 0, 0);
    gridLayout->addWidget(label2, 1, 0);
    gridLayout->addWidget(label3, 2, 0);
    gridLayout->addWidget(label4, 3, 0);
    gridLayout->setRowStretch(0,7);
    gridLayout->setRowStretch(1,1);
    gridLayout->setRowStretch(2,1);
    gridLayout->setRowStretch(3,1);
}

In this example label1 will take 70% of the available vertical space, while the other labels will take combined 30% of the available vertical space.