Layout Management¶
A tour of the standard layout managers and an introduction to custom layouts.
The Qt layout system provides a simple and powerful way of automatically arranging child widgets within a widget to ensure that they make good use of the available space.
Introduction¶
Qt includes a set of layout management classes that are used to describe how widgets are laid out in an application’s user interface. These layouts automatically position and resize widgets when the amount of space available for them changes, ensuring that they are consistently arranged and that the user interface as a whole remains usable.
All
QWidget
subclasses can use layouts to manage their children. ThesetLayout()
function applies a layout to a widget. When a layout is set on a widget in this way, it takes charge of the following tasks:
Qt’s Layout Classes¶
Qt’s layout classes were designed for hand-written C++ code, allowing measurements to be specified in pixels for simplicity, so they are easy to understand and use. The code generated for forms created using Qt Designer also uses the layout classes. Qt Designer is useful to use when experimenting with the design of a form since it avoids the compile, link and run cycle usually involved in user interface development.
PySide2.QtWidgets.QGraphicsAnchorLayout
The QGraphicsAnchorLayout class provides a layout where one can anchor widgets together in Graphics View.
PySide2.QtWidgets.QGraphicsAnchor
The QGraphicsAnchor class represents an anchor between two items in a QGraphicsAnchorLayout.
PySide2.QtWidgets.QBoxLayout
The QBoxLayout class lines up child widgets horizontally or vertically.
PySide2.QtWidgets.QHBoxLayout
The QHBoxLayout class lines up widgets horizontally.
PySide2.QtWidgets.QVBoxLayout
The QVBoxLayout class lines up widgets vertically.
PySide2.QtWidgets.QFormLayout
The QFormLayout class manages forms of input widgets and their associated labels.
PySide2.QtWidgets.QGridLayout
The QGridLayout class lays out widgets in a grid.
PySide2.QtWidgets.QLayout
The QLayout class is the base class of geometry managers.
PySide2.QtWidgets.QLayoutItem
The QLayoutItem class provides an abstract item that a QLayout manipulates.
PySide2.QtWidgets.QSpacerItem
The QSpacerItem class provides blank space in a layout.
PySide2.QtWidgets.QWidgetItem
The QWidgetItem class is a layout item that represents a widget.
PySide2.QtWidgets.QSizePolicy
The QSizePolicy class is a layout attribute describing horizontal and vertical resizing policy.
PySide2.QtWidgets.QStackedLayout
The QStackedLayout class provides a stack of widgets where only one widget is visible at a time.
PySide2.QtWidgets.QButtonGroup
The QButtonGroup class provides a container to organize groups of button widgets.
PySide2.QtWidgets.QGroupBox
The QGroupBox widget provides a group box frame with a title.
PySide2.QtWidgets.QStackedWidget
The QStackedWidget class provides a stack of widgets where only one widget is visible at a time.
Horizontal, Vertical, Grid, and Form Layouts¶
The easiest way to give your widgets a good layout is to use the built-in layout managers:
QHBoxLayout
,QVBoxLayout
,QGridLayout
, andQFormLayout
. These classes inherit fromQLayout
, which in turn derives fromQObject
(notQWidget
). 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
QVBoxLayout
lays out widgets in a vertical column, from top to bottom.A
QGridLayout
lays out widgets in a two-dimensional grid. Widgets can occupy multiple cells.A
QFormLayout
lays out widgets in a 2-column descriptive label- field style.
Laying Out Widgets in Code¶
The following code creates a
QHBoxLayout
that manages the geometry of fiveQPushButtons
, as shown on the first screenshot above:window = QWidget() button1 = QPushButton("One") button2 = QPushButton("Two") button3 = QPushButton("Three") button4 = QPushButton("Four") button5 = QPushButton("Five") layout = QHBoxLayout() layout.addWidget(button1) layout.addWidget(button2) layout.addWidget(button3) layout.addWidget(button4) layout.addWidget(button5) window.setLayout(layout) window.show()The code for
QVBoxLayout
is identical, except the line where the layout is created. The code forQGridLayout
is a bit different, because we need to specify the row and column position of the child widget:window = QWidget() button1 = QPushButton("One") button2 = QPushButton("Two") button3 = QPushButton("Three") button4 = QPushButton("Four") button5 = QPushButton("Five") layout = 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 toaddWidget()
.
QFormLayout
will add two widgets on a row, commonly aQLabel
and aQLineEdit
to create forms. Adding aQLabel
and aQLineEdit
on the same row will set theQLineEdit
as theQLabel
‘s buddy. The following code will use theQFormLayout
to place threeQPushButtons
and a correspondingQLineEdit
on a row.QWidget *window = new QWidget; QPushButton *button1 = new QPushButton("One"); QLineEdit *lineEdit1 = new QLineEdit(); QPushButton *button2 = new QPushButton("Two"); QLineEdit *lineEdit2 = new QLineEdit(); QPushButton *button3 = new QPushButton("Three"); QLineEdit *lineEdit3 = new QLineEdit(); QFormLayout *layout = new QFormLayout(window); layout->addRow(button1, lineEdit1); layout->addRow(button2, lineEdit2); layout->addRow(button3, lineEdit3); window->show();
Tips for Using Layouts¶
When you use a layout, you do not need to pass a parent when constructing the child widgets. The layout will automatically reparent the widgets (using
setParent()
) so that they are children of the widget on which the layout is installed.Note
Widgets in a layout are children of the widget on which the layout is installed, not of the layout itself. Widgets can only have other widgets as parent, not layouts.
You can nest layouts using
addLayout()
on a layout; the inner layout then becomes a child of the layout it is inserted into.
Adding Widgets to a Layout¶
When you add widgets to a layout, the layout process works as follows:
All the widgets will initially be allocated an amount of space in accordance with their
sizePolicy()
andsizeHint()
.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).
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
Expanding
size policy first.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 stretch factor is their determining factor.)
Any widgets that are allocated more space than their maximum size are allocated the maximum size space they require. (Widgets do not have to have a maximum size in which case the stretch 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
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: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.
Custom Widgets in Layouts¶
When you make your own widget class, you should also communicate its layout properties. If the widget uses one of Qt’s layouts, this is already taken care of. If the widget does not have any child widgets, or uses a manual layout, you can change the behavior of the widget using any or all of the following mechanisms:
Reimplement
sizeHint()
to return the preferred size of the widget.Reimplement
minimumSizeHint()
to return the smallest size the widget can have.Call
setSizePolicy()
to specify the space requirements of the widget.Call
updateGeometry()
whenever the size hint, minimum size hint or size policy changes. This will cause a layout recalculation. Multiple consecutive calls toupdateGeometry()
will only cause one layout recalculation.If the preferred height of your widget depends on its actual width (e.g., a label with automatic word-breaking), set the
height-for-width
flag in the widget’ssize policy
and reimplementheightForWidth()
.Even if you implement
heightForWidth()
, it is still a good idea to provide a reasonable sizeHint().For further guidance when implementing these functions, see the Qt Quarterly article Trading Height for Width .
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()
andminimumSizeHint()
functions.In some cases, it is relevant when a layout is added to a widget. When you set the widget of a
QDockWidget
or aQScrollArea
(withsetWidget()
andsetWidget()
), 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
resizeEvent()
to calculate the required distribution of sizes and callsetGeometry()
on each child.The widget will get an event of type
LayoutRequest
when the layout needs to be recalculated. Reimplementevent()
to handleLayoutRequest
events.
How to Write A Custom Layout Manager¶
An alternative to manual layout is to write your own layout manager by subclassing
QLayout
. The Border Layout and Flow Layout examples show how to do this.Here we present an example in detail. The
CardLayout
class 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 byspacing()
.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
QLayoutItem
. We will use aQVector
in this example.
addItem()
, how to add items to the layout.
setGeometry()
, how to perform the layout.
sizeHint()
, the preferred size of the layout.
itemAt()
, how to iterate over the layout.
takeAt()
, how to remove items from the layout.In most cases, you will also implement
minimumSize()
.
The Header File (
card.h
)¶
#ifndef CARD_H #define CARD_H #include <QtWidgets> #include <QVector> class CardLayout : public QLayout { public: CardLayout(int spacing): QLayout() { setSpacing(spacing); } CardLayout(int spacing, QWidget *parent): QLayout(parent) { setSpacing(spacing); } ~CardLayout(); void addItem(QLayoutItem *item) override; QSize sizeHint() const override; QSize minimumSize() const override; int count() const override; QLayoutItem *itemAt(int) const override; QLayoutItem *takeAt(int) override; void setGeometry(const QRect &rect) override; private: QVector<QLayoutItem*> m_items; }; #endif
The Implementation File (
card.cpp
)¶
//#include "card.h"First we define
count()
to fetch the number of items in the list.int CardLayout::count() const { // QVector::size() returns the number of QLayoutItems in m_items return m_items.size(); }Then we define two functions that iterate over the layout:
itemAt()
andtakeAt()
. 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.QLayoutItem *CardLayout::itemAt(int idx) const { // QVector::value() performs index checking, and returns nullptr if we are // outside the valid range return m_items.value(idx); } QLayoutItem *CardLayout::takeAt(int idx) { // QVector::take does not do index checking return idx >= 0 && idx < m_items.size() ? m_items.takeAt(idx) : 0; }
addItem()
implements the default placement strategy for layout items. This function must be implemented. It is used by QLayout::add(), by theQLayout
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 ofaddItem()
,addWidget()
, andaddLayout()
.void CardLayout::addItem(QLayoutItem *item) { m_items.append(item); }The layout takes over responsibility of the items added. Since
QLayoutItem
does not inheritQObject
, we must delete the items manually. In the destructor, we remove each item from the list usingtakeAt()
, and then delete it.CardLayout::~CardLayout() { QLayoutItem *item; while ((item = takeAt(0))) delete item; }The
setGeometry()
function actually performs the layout. The rectangle supplied as an argument does not includemargin()
. If relevant, usespacing()
as the distance between items.void CardLayout::setGeometry(const QRect &r) { QLayout::setGeometry(r); if (m_items.size() == 0) return; int w = r.width() - (m_items.count() - 1) * spacing(); int h = r.height() - (m_items.count() - 1) * spacing(); int i = 0; while (i < m_items.size()) { QLayoutItem *o = m_items.at(i); QRect geom(r.x() + i * spacing(), r.y() + i * spacing(), w, h); o->setGeometry(geom); ++i; } }
sizeHint()
andminimumSize()
are normally very similar in implementation. The sizes returned by both functions should includespacing()
, but notmargin()
.QSize CardLayout::sizeHint() const { QSize s(0, 0); int n = m_items.count(); if (n > 0) s = QSize(100, 70); //start with a nice default size int i = 0; while (i < n) { QLayoutItem *o = m_items.at(i); s = s.expandedTo(o->sizeHint()); ++i; } return s + n * QSize(spacing(), spacing()); } QSize CardLayout::minimumSize() const { QSize s(0, 0); int n = m_items.count(); int i = 0; while (i < n) { QLayoutItem *o = m_items.at(i); s = s.expandedTo(o->minimumSize()); ++i; } return s + n * QSize(spacing(), spacing()); }
Further Notes¶
This custom layout does not handle height for width.
We ignore
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
invalidate()
to mark the cached data is dirty.Calling
sizeHint()
, etc. may be expensive. So, you should store the value in a local variable if you need it again later within in the same function.You should not call
setGeometry()
twice on the same item in the same function. This call can be very expensive if the item has several child widgets, because the layout manager must do a complete layout every time. Instead, calculate the geometry and then set it. (This does not only apply to layouts, you should do the same if you implement your own resizeEvent(), for example.)
Layout Examples¶
Many Qt Widgets examples already use layouts, however, several examples exist to showcase various layouts.
An introduction to GUI programming, showing how to put together a simple yet fully-functioning application.
Shows how to arrange child widgets along a border.
The example shows how to use signals and slots to implement the functionality of a calculator widget, and how to use QGridLayout to place child widgets in a grid.
The Calendar Widget example shows use of QCalendarWidget.
This example shows how to create a Qt plugin.
Shows how to arrange widgets for different window sizes.
Shows how composition modes work in QPainter.
The Menus example demonstrates how menus can be used in a main window application.
The Simple Tree Model example shows how to use a hierarchical model with Qt’s standard view classes.
This example shows Qt’s ability to combine the animation framework and the state machine framework to create a game.
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