Layout classes


A tour of the standard layout managers and an introduction to custom layouts.

The Qt layout system provides a simple and powerful way of specifying the layout of child widgets.

By specifying the logical layout once, you get the following benefits:

  • Positioning of child widgets.
  • Sensible default sizes for windows.
  • Sensible minimum sizes for windows.
  • Resize handling.
  • Automatic update when contents change:
    • Font size, text or other contents of child widgets.
    • Hiding or showing a child widget.
    • Removal of child widgets.

The disadvantage of hand-written layout code is that it isn't very convenient when you're experimenting with the design of a form and you have to go through the compile, link and run cycle for each change. Our solution is \l{Qt Designer}, a GUI visual design tool which makes it fast and easy to experiment with layouts and which generates the Java layout code for you.

Qt Jambi's layout classes were designed for hand-written Java code, so they're easy to understand and use. The code generated for forms created using \l{Qt Designer} also uses the layout classes.

Topics

Horizontal, Vertical, and Grid Layouts

The easiest way to give your widgets a good layout is to use the built-in layout managers: QHBoxLayout, QVBoxLayout, and QGridLayout. These classes inherit from QLayout, which in turn derives from QObject (not QWidget). They take care of geometry management for a set of widgets. To create more complex layouts, you can nest layout managers inside each other.

  • A QHBoxLayout lays out widgets in a horizontal row, from left to right (or right to left for right-to-left languages).

    A QHBoxLayout with five child widgets

  • A QVBoxLayout lays out widgets in a vertical column, from top to bottom.

    A QVBoxLayout with five child widgets

  • A QGridLayout lays out widgets in a two-dimensional grid. Widgets can occupy multiple cells.

    A QGridLayout with five child widgets

The following code creates a QHBoxLayout that manages the geometry of five \l{QPushButton}s, as shown on the first screenshot above:

        QWidget window = new QWidget();
        QPushButton button1 = new QPushButton("One");
        QPushButton button2 = new QPushButton("Two");
        QPushButton button3 = new QPushButton("Three");
        QPushButton button4 = new QPushButton("Four");
        QPushButton button5 = new QPushButton("Five");
        QHBoxLayout layout = new QHBoxLayout();
        layout.addWidget(button1);
        layout.addWidget(button2);
        layout.addWidget(button3);
        layout.addWidget(button4);
        layout.addWidget(button5);

        window.setLayout(layout);
        QWidget window = new QWidget();

The code for QVBoxLayout is identical, except the line where the layout is created. The code for QGridLayout is a bit different, because we need to specify the row and column position of the child widget:

        QWidget window = new QWidget();
        QPushButton button1 = new QPushButton("One");
        QPushButton button2 = new QPushButton("Two");
        QPushButton button3 = new QPushButton("Three");
        QPushButton button4 = new QPushButton("Four");
        QPushButton button5 = new QPushButton("Five");
        QGridLayout layout = new QGridLayout();
        layout.addWidget(button1, 0, 0);
        layout.addWidget(button2, 0, 1);
        layout.addWidget(button3, 1, 0, 1, 2);
        layout.addWidget(button4, 2, 0);
        layout.addWidget(button5, 2, 1);

        window.setLayout(layout);
        window.show();

The third QPushButton spans 2 columns. This is possible by specifying 2 as the fifth argument to QGridLayout::addWidget().

Important: Widgets in a layout are children of the widget on which the layout is installed, \e not of the layout itself. Widgets can only have other widgets as parent, not layouts.

You can nest layouts using \c addLayout() on a layout; the inner layout then becomes a child of the layout it is inserted into. The \l{Basic Layouts} example uses this feature to create a complex dialog.

Adding Widgets to a Layout

When you add widgets to a layout, the layout process works as follows:

  1. All the widgets will initially be allocated an amount of space in accordance with their QWidget::sizePolicy().
  2. If any of the widgets have stretch factors set, with a value greater than zero, then they are allocated space in proportion to their stretch factor (explained below).
  3. If any of the widgets have stretch factors set to zero they will only get more space if no other widgets want the space. Of these, space is allocated to widgets with an \l{QSizePolicy::Expanding}{Expanding} size policy first.
  4. Any widgets that are allocated less space than their minimum size (or minimum size hint if no minimum size is specified) are allocated this minimum size they require. (Widgets don't have to have a minimum size or minimum size hint in which case the strech factor is their determining factor.)
  5. Any widgets that are allocated more space than their maximum size are allocated the maximum size space they require. (Widgets don't have to have a maximum size in which case the strech factor is their determining factor.)

Stretch Factors

Widgets are normally created without any stretch factor set. When they are laid out in a layout the widgets are given a share of space in accordance with their QWidget::sizePolicy() or their minimum size hint whichever is the greater. Stretch factors are used to change how much space widgets are given in proportion to one another.

If we have three widgets laid out using a QHBoxLayout with no stretch factors set we will get a layout like this:

Three widgets in a row

If we apply stretch factors to each widget, they will be laid out in proportion (but never less than their minimum size hint), e.g.

Three widgets with different stretch factors in a row

Custom Widgets in Layouts

When you make your own widget class, you should also communicate its layout properties. If the widget has a QLayout, this is already taken care of. If the widget does not have any child widgets, or uses manual layout, you can change the behavior of the widget using any or all of the following mechanisms:

  • Reimplement QWidget::sizeHint() to return the preferred size of the widget.
  • Reimplement QWidget::minimumSizeHint() to return the smallest size the widget can have.
  • Call QWidget::setSizePolicy() to specify the space requirements of the widget.

Call QWidget::updateGeometry() whenever the size hint, minimum size hint or size policy changes. This will cause a layout recalculation. Multiple consecutive calls to QWidget::updateGeometry() will only cause one recalculation.

If the preferred height of your widget depends on its actual width (e.g., a label with automatic word-breaking), set the \l{QSizePolicy::hasHeightForWidth()}{height-for-width} flag in the widget's \l{QWidget::sizePolicy}{size policy} and reimplement QWidget::heightForWidth().

Even if you implement QWidget::heightForWidth(), it is still a good idea to provide a reasonable sizeHint().

For further guidance when implementing these functions, see the \l{http://doc.trolltech.com/qq/qq04-height-for-width.html}{Trading Height for Width} article in \e{Qt Quarterly}.

Layout Issues

The use of rich text in a label widget can introduce some problems to the layout of its parent widget. Problems occur due to the way rich text is handled by Qt's layout managers when the label is word wrapped.

In certain cases the parent layout is put into QLayout::FreeResize mode, meaning that it will not adapt the layout of its contents to fit inside small sized windows, or even prevent the user from making the window too small to be usable. This can be overcome by subclassing the problematic widgets, and implementing suitable sizeHint() and minimumSizeHint() functions.

In some cases, it is relevant when a layout is added to a widget. When you set the widget of a QDockWidget or a QScrollArea (with QDockWidget::setWidget() and QScrollArea::setWidget()), the layout must already have been set on the widget. If not, the widget will not be visible.

Manual Layout

If you are making a one-of-a-kind special layout, you can also make a custom widget as described above. Reimplement QWidget::resizeEvent() to calculate the required distribution of sizes and call \l{QWidget::setGeometry()}{setGeometry()} on each child.

The widget will get an event of type QEvent::LayoutRequest when the layout needs to be recalculated. Reimplement QWidget::event() to handle QEvent::LayoutRequest events.

Writing Custom Layout Managers

An alternative to manual layout is to write your own layout manager by subclassing QLayout. The \l{layouts/borderlayout}{Border Layout} and \l{layouts/flowlayout}{Flow Layout} examples show how to do this.

Here we present an example in detail. The class CardLayout is inspired by the Java layout manager of the same name. It lays out the items (widgets or nested layouts) on top of each other, each item offset by QLayout::spacing().

To write your own layout class, you must define the following:

  • A data structure to store the items handled by the layout. Each item is a \link QLayoutItem QLayoutItem\endlink. We will use a QList in this example.
  • \link QLayout::addItem() addItem() \endlink, how to add items to the layout.
  • \link QLayout::setGeometry() setGeometry() \endlink, how to perform the layout.
  • \link QLayout::sizeHint() sizeHint() \endlink, the preferred size of the layout.
  • \link QLayout::itemAt() itemAt() \endlink, how to iterate over the layout.
  • \link QLayout::takeAt() takeAt() \endlink, how to remove items from the layout.

In most cases, you will also implement \link QLayout::minimumSize() minimumSize\endlink().

The Implementation of CardLayout

We take a short look at the memebers of the \c CardLayout class; we will examine their implementation shortly.

First we have two functions that iterate over the layout: itemAt() and takeAt(). These functions are used internally by the layout system to handle deletion of widgets. They are also available for application programmers.

itemAt() returns the item at the given index. takeAt() removes the item at the given index, and returns it. In this case we use the list index as the layout index. In other cases where we have a more complex data structure, we may have to spend more effort defining a linear order for the items.

            public QLayoutItem itemAt(int idx)
            {
                return list.get(idx);
            }

            public QLayoutItem takeAt(int idx)
            {
                return idx >= 0 && idx < list.size() ? list.remove(idx) : null;
            }

addItem() implements the default placement strategy for layout items. It must be implemented. It is used by QLayout::add(), by the QLayout constructor that takes a layout as parent. If your layout has advanced placement options that require parameters, you must provide extra access functions such as the row and column spanning overloads of \l QGridLayout::addItem(), addWidget(), and addLayout().

            public void addItem(QLayoutItem item)
            {
                list.add(item);
            }

The layout takes over responsibility of the items added. Since QLayoutItem does not inherit QObject, we must delete the items manually. The function QLayout::deleteAllItems() uses takeAt() defined above to delete all the items in the layout.

        /* Destructor for c++ (Not relevant for Jambi)
        CardLayout::~CardLayout()
        {
            deleteAllItems();
        }*/

The setGeometry() function actually performs the layout. The rectangle supplied as an argument does not include margin(). If relevant, use spacing() as the distance between items.

            public void setGeometry(QRect r)
            {
                super.setGeometry(r);

                if (list.size() == 0)
                    return;

                int w = r.width() - (list.size() - 1) * widgetSpacing();
                int h = r.height() - (list.size() - 1) * widgetSpacing();
                int i = 0;
                while (i < list.size()) {
                    QLayoutItem o = list.get(i);
                    QRect geom = new QRect(r.x() + i * widgetSpacing(),
                                           r.y() + i * widgetSpacing(), w, h);
                    o.setGeometry(geom);
                    ++i;
                }
            }

sizeHint() and minimumSize() are normally very similar in implementation. The sizes returned by both functions should include spacing(), but not margin().

            public QSize sizeHint()
            {
                QSize s  = new QSize(0,0);
                int n = list.size();
                if (n > 0)
                    s = new QSize(100, 70); //start with a nice default size
                
                int i = 0;
                while (i < n) {
                    QLayoutItem o = list.get(i);
                    s = s.expandedTo(o.sizeHint());
                    ++i;
                }
                return s.add(new QSize(widgetSpacing() * n, widgetSpacing() * n));
            }

            public QSize minimumSize()
            {
                QSize s = new QSize(0, 0);
                int n = list.size();
                int i = 0;
                while (i < n) {
                    QLayoutItem o = list.get(i);
                    s = s.expandedTo(o.minimumSize());
                    ++i;
                }
                return s.add(new QSize(widgetSpacing() * n, widgetSpacing() * n));
            }

Further Notes

This layout does not handle height for width.

We ignore QLayoutItem::isEmpty(), this means that the layout will treat hidden widgets as visible.

For complex layouts, speed can be greatly increased by caching calculated values. In that case, implement QLayoutItem::invalidate() to mark the cached data as dirty.

Calling QLayoutItem::sizeHint(), etc. may be expensive, so you should store the value in a local variable if you need it again later in the same function.

You should not call QLayoutItem::setGeometry() twice on the same item in the same function. That can be very expensive if the item has several child widgets, because it must do a complete layout every time. Instead, calculate the geometry and then set it. (This doesn't only apply to layouts, you should do the same if you implement your own resizeEvent().)

This page was updated at 17.05.2016