Keeping the aspect ratio of a sub-classed QWidget during resize

Question

I\'m creating a new widget, by subclassing the QWidget class. I\'d like to be able to set a ratio (for its height and its width) for this widget, which will always be mainta

Answer 1

Create a parent widget (e.g., AspectRatioWidget) in which to place your widget. For the parent widget, subclass QWidget and give it a QBoxLayout. Put your widget into the center, and QSpacerItems on either side. Then in the parent widget's QWidget::resizeEvent adjust the direction and stretches as needed. I've provided an example below. To use, just create an instance of AspectRatioWidget and pass the constructor a pointer to your widget and the desired aspect ratio.

// header
class AspectRatioWidget : public QWidget
{
public:
    AspectRatioWidget(QWidget *widget, float width, float height, QWidget *parent = 0);
    void resizeEvent(QResizeEvent *event);

private:
    QBoxLayout *layout;
    float arWidth; // aspect ratio width
    float arHeight; // aspect ratio height
};

// cpp
AspectRatioWidget::AspectRatioWidget(QWidget *widget, float width, float height, QWidget *parent) :
    QWidget(parent), arWidth(width), arHeight(height)
{
    layout = new QBoxLayout(QBoxLayout::LeftToRight, this);

    // add spacer, then your widget, then spacer
    layout->addItem(new QSpacerItem(0, 0));
    layout->addWidget(widget);
    layout->addItem(new QSpacerItem(0, 0));
}

void AspectRatioWidget::resizeEvent(QResizeEvent *event)
{
    float thisAspectRatio = (float)event->size().width() / event->size().height();
    int widgetStretch, outerStretch;

    if (thisAspectRatio > (arWidth/arHeight)) // too wide
    {
        layout->setDirection(QBoxLayout::LeftToRight);
        widgetStretch = height() * (arWidth/arHeight); // i.e., my width
        outerStretch = (width() - widgetStretch) / 2 + 0.5;
    }
    else // too tall
    {
        layout->setDirection(QBoxLayout::TopToBottom);
        widgetStretch = width() * (arHeight/arWidth); // i.e., my height
        outerStretch = (height() - widgetStretch) / 2 + 0.5;
    }

    layout->setStretch(0, outerStretch);
    layout->setStretch(1, widgetStretch);
    layout->setStretch(2, outerStretch);
}

Answer 2

I have rewritten Anthony's code in Python/PySide2:

from PySide2.QtWidgets import QBoxLayout, QSpacerItem, QWidget


class AspectRatioWidget(QWidget):
    def __init__(self, widget, parent):
        super().__init__(parent)
        self.aspect_ratio = widget.size().width() / widget.size().height()
        self.setLayout(QBoxLayout(QBoxLayout.LeftToRight, self))
        #  add spacer, then widget, then spacer
        self.layout().addItem(QSpacerItem(0, 0))
        self.layout().addWidget(widget)
        self.layout().addItem(QSpacerItem(0, 0))

    def resizeEvent(self, e):
        w = e.size().width()
        h = e.size().height()

        if w / h > self.aspect_ratio:  # too wide
            self.layout().setDirection(QBoxLayout.LeftToRight)
            widget_stretch = h * self.aspect_ratio
            outer_stretch = (w - widget_stretch) / 2 + 0.5
        else:  # too tall
            self.layout().setDirection(QBoxLayout.TopToBottom)
            widget_stretch = w / self.aspect_ratio
            outer_stretch = (h - widget_stretch) / 2 + 0.5

        self.layout().setStretch(0, outer_stretch)
        self.layout().setStretch(1, widget_stretch)
        self.layout().setStretch(2, outer_stretch)