How to maintain widgets aspect ratio in Qt?

Calling resize() from within resizeEvent() has never worked well for me -- at best it will cause flickering as the window is resized twice (as you have), at worst an infinite loop.

I think the "correct" way to maintain a fixed aspect ratio is to create a custom layout. You'll have to override just two methods, QLayoutItem::hasHeightForWidth() and QLayoutItem::heightForWidth().

The right answer is to create your custom layout manager. That is possible by subclassing QLayout.

Methods to implement when subclassing QLayout

void addItem(QLayoutItem* item);

Adds item to layout.

int count() const;

Returns the item count.

QLayoutItem* itemAt(int index) const;

Returns item reference at index or 0 if there's none.

QLayoutItem* takeAt(int index);

Takes and returns item from the layout from index or returns 0 if there is none.

Qt::Orientations expandingDirections() const;

Returns the layouts expanding directions.

bool hasHeightForWidth() const;

Tells if the layout handles height for width calculation.

QSize minimumSize() const;

Returns the layouts minimum size.

void setGeometry(const QRect& rect);

Sets the geometry of the layout and the items inside it. Here you have to maintain the aspect ratio and do the centering.

QSize sizeHint() const;

Returns the preferred size for the layout.

Further reading

• Maintaining square form for a widget in Qt @ Forum Nokia
• Implementing a layout manager in Qt @ Forum Nokia
• Writing custom layout managers @ Qt documentation

I too was trying to achieve the requested effect: a widget that keeps a fixed aspect ratio while staying centred in its allocated space. At first I tried other answers from this question:

• implementing heightForWidth and hasHeightForWidth as suggested by marc-mutz-mmutz simply didn't work for me.
• I briefly looked at implementing a custom layout manager, but all Bleadof's links were dead, and when I found the documentation and read through it, it looked way too complicated for what I was trying to achieve.

I ended up creating a custom widget that responds to resizeEvent and uses setContentsMargin to set margins such that the remaining content area keeps the desired ratio.

I found I also had to set the widget's size policy to QSizePolicy::Ignored in both directions to avoid odd resizing issues resulting from the size requests of child widgets—the end result is that my widget accepts whatever size its parent allocates to it (and then sets its margins as described above to keep the desired aspect ratio in its content area).

My code looks like this:

from PySide2.QtWidgets import QWidget, QSizePolicy


class AspectWidget(QWidget):
    '''
    A widget that maintains its aspect ratio.
    '''
    def __init__(self, *args, ratio=4/3, **kwargs):
        super().__init__(*args, **kwargs)
        self.ratio = ratio
        self.adjusted_to_size = (-1, -1)
        self.setSizePolicy(QSizePolicy(QSizePolicy.Ignored, QSizePolicy.Ignored))

    def resizeEvent(self, event):
        size = event.size()
        if size == self.adjusted_to_size:
            # Avoid infinite recursion. I suspect Qt does this for you,
            # but it's best to be safe.
            return
        self.adjusted_to_size = size

        full_width = size.width()
        full_height = size.height()
        width = min(full_width, full_height * self.ratio)
        height = min(full_height, full_width / self.ratio)

        h_margin = round((full_width - width) / 2)
        v_margin = round((full_height - height) / 2)

        self.setContentsMargins(h_margin, v_margin, h_margin, v_margin)

(Obviously, this code is in Python, but it should be straightforward to express in C++ or your language of choice.)

You don't have to implement your own layout manager. You can do with inheriting QWidget and reimplementing

int QWidget::heightForWidth( int w ) { return w; }

to stay square. However, heightForWidth() doesn't work on toplevel windows on X11, since apparently the X11 protocol doesn't support that. As for centering, you can pass Qt::AlignCenter as the third parameter of QBoxLayout::addWidget() or the fifth parameter of QGridLayout::addWidget().

Note: In newer versions of Qt at least, QWidget does not have the heightForWidth or widthForHeight anymore (so they cannot be overriden), and therefore setWidthForHeight(true) or setHeightForWidth(true) only have an effect for descendants of QGraphicsLayout.