How to maintain widgets aspect ratio in Qt?

Question

How is it possible to maintain widgets aspect ratio in Qt and what about centering the widget?

Answer 1

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.

Answer 2

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