Qt Application Programming with C++ - Part 1

Fundamentals of Qt
• The Story of Qt
• Developing a Hello World Application
• Hello World using Qt Creator
• Practical Tips for Developers

Objectives
• About the history of Qt
• About Qt's ecosystem
• A high-level overview of Qt
• How to create ﬁrst hello world program
• Build and run a program cross platform
• To use Qt Creator IDE
• Some practical tips for developing with Qt

Some history
• Qt Development Frameworks founded in 1994
• Trolltech acquired by Nokia in 2008
• Qt Commercial business acquired by Digia in 2011
• Qt business acquired by Digia from Nokia in 2012
• Trusted by over 6,500 companies worldwide
• Qt: a cross-platform application and UI
framework
• For desktop, mobile and embedded development
• Used by more than 350,000 commercial and open
source developers
• Backed by Qt consulting, support and training

Why Qt
• Write code once to target multiple platforms
• Produce compact, high-performance applications
• Focus on innovation, not infrastructure coding
• Choose the license that ﬁts you
• Commercial, LGPL or GPL
• Count on professional services, support and
training
• Take part in an active Qt ecosystem

Qt Commercial
• Improved development tools for increased
productivity and tangible cost savings
• Qt Commercial SDK
• All Qt Commercial libraries and tools
• Additional tools and components
• Qt Creator Embedded Target
• Deploy directly to embedded Linux target
• RTOS toolchain integration
• Visual Studio Add-On

Qt5
• Awesome graphics capabilities
• OpenGL as a standard feature of user interfaces
• Shader-based graphics effects in QtQuick 2
• New modular structure
• Qt Essential modules available on all platforms
• Add-on modules provide additional or platform-speciﬁc
functionality
• Developer productivity and ﬂexibility
• More web-like development with QtQuick 2, V8 JvaScript
engine, and Qt JSON DB
• Cross-platform portability
• Qt Platform Abstraction (QPA) replaces QWS and platform
ports

Qt Demo
• Let's have a look at the QtDemo Application
• Comes with every Qt installation
Technology Demo
Painting Demonstrations/Path Stroking
Widgets Demonstrations/Books
Widgets Demonstrations/TextEdit
Graphics View Demonstrations/40.000 Chips
OpenGL Demonstrations/Boxes
WebKit Demonstrations/Browser

“Hello world” in Qt
// main.cpp
#include <QtWidgets>
int main(int argc, char *argv[])
{
QApplication app(argc, argv);
QPushButton button("Hello world");
button.show();
return app.exec();
}
• Program consists of
• main.cpp - application code
• helloworld.pro - project ﬁle
•Hello world demo

Project File
helloworld.pro
• helloworld.pro file
• lists source and header files
• provides project configuration
# File: helloworld.pro
SOURCES = main.cpp
HEADERS += # No headers used
QT = core gui widgets
• Assignment to variables
• Possible operators =, +=, -=
• Qmake-tutorial

Using qmake
• qmake tool
• Creates cross-platform makeﬁles
• Build project using qmake
cd helloworld
qmake helloworld.pro # creates Makefile
Make # compiles and links
application
./helloworld # executes application
• Tip: qmake -project
• Creates default project ﬁle based on directory content
• Qmake manual
Qt Creator does it all for you

QtCreator IDE
• Advanced C++ code editor
• Integrated GUI layout and forms designer
• Project and build management tools
• Integrated, context-sensitive help system
• Visual debugger
• Rapid code navigation tools
• Supports multiple
platforms

Qt Creator Demo
"Hello World"
• Creation of an empty Qt Widget project
• Adding the main.cpp source ﬁle
• Writing of the Qt Hello World Code
• Running the application
• Debugging the application

C++ knowledge
• Objects and classes
• Declaring a class, inheritance, calling member
functions etc.
• Polymorphism
• That is virtual methods
• Operator overloading

• Reference Documentation
• All classes documented
• Contains tons of examples
• Collection of Howto's and Overviews
• A set of Tutorials for Learners
Qt Documentation

• Qt Modules
• QtCore, QtGui, QtWidgets, QtXml, QtSql,
QtNetwork, QtTest .. See documentation
• Enable Qt Module in qmake .pro file:
• QT += network
• Default: qmake projects use QtCore and QtGui
• Any Qt class has a header file.
#include <QLabel>
#include <QtWidgets/QLabel>
• Any Qt Module has a header file.
#include <QtGui>
Modules and
Include files

Module includes
#include <QtGui>
• Precompiled header and the compiler
• If not supported may add extra compile time
• If supported may speed up compilation
• Supported on: Windows, Mac OS X, Unix
Class includes
#include <QLabel>
• Reduce compilation time
• Use class includes (#include <QLabel>)
• Forward declarations (class QLabel;)
Place module includes before other includes.
Includes and
Compilation Time

• What is Qt
• Which code lines do you need for a minimal Qt
application?
• What is a .pro ﬁle?
• What is qmake, and when is it a good idea to use
it?
• What is a Qt module and how to enable it in your
project?
• How can you include a QLabel from the QtGui
module?
• Name places where you can ﬁnd answers about Qt
problems
Summary

Core classes
• Qt's Object Model
• QObject
• Qwidget
• Variants
• Properties
• String Handling
• Container Classes

Objectives
• What the Qt object model is
• The basics of the widget system in C++
• Which utility classes exists to help you in
C++

Core classes
• Qt's Object Model
• QObject
• QWidget
• Variants
• Properties
• String Handling
• Container Classes

QObject
• QObject is the heart of Qt's object model
• Include these features:
• Memory management
• Object properties
• Signals and slots
• Event handling
• QObject has no visual representation

Object tree
• QObjects organize themselves in object trees
• Based on parent-child relationship
• QObject(QObject *parent = 0)
• Parent adds object to list of children
• Parent owns children
• Widget Centric
• Used intensively with QtWidget
• Less so when using Qt/C++ from QML
Note: Parent-child relationship is NOT inheritance
Parent Children
QObjectQObject
QObjectQObject
QObjectQObject

Creating Objects
• On Heap - QObject with parent:
QTimer* timer = new QTimer(this);
• Parent takes ownership.
• Copy is disabled
• On Stack - value types
• QString, QStringList, Qcolor
• On Stack - QObject without parent:Exceptions
• QFile, Qapplication (Usually allocated on the stack)
• Top level QWidgets: QMainWindow
• Stack or Heap - QDialog - depending on
lifetime

QWidget
• Derived from QObject
• Adds visual representation
• Receives events
• e.g. mouse, keyboard events
• Paints itself on screen
• Using styles
• Base of UI objects
QObjectQObject
QFileQFileQWidgetQWidget
QTextEditQTextEditQPushButto
n
QPushButto
nQLabelQLabel

QWidget
• new QWidget(0)
• Widget with no parent="window"
• QWidget's children
• Positioned in parent's coordinate system
• Clipped by parent's boundaries
• QWidget parent
• Propagates state changes
• hides/shows children when it is hidden/shown itself
• enables/disables children when it is
enabled/disabled itself

Widgets contain
other widgets
• Container Widget
• Aggregates other child-widgets
• Use layouts for aggregation
• In this example: QHBoxLayout and
QVBoxLayout
• Note: Layouts are not widgets
• Layout Process
• Add widgets to layout
• Layouts may be nested
• Set layout on container widget

Container Widget
// container (window) widget creation
QWidget* container = new QWidget;
QLabel* label = new QLabel("Note:", container);
QTextEdit* edit = new QTextEdit(container);
QPushButton* clear = new QPushButton("Clear", container);
QPushButton* save = new QPushButton("Save", container);
// widget layout
QVBoxLayout* outer = new QVBoxLayout();
outer->addWidget(label);
outer->addWidget(edit);
QHBoxLayout* inner = new QHBoxLayout();
inner->addWidget(clear);
inner->addWidget(save);
container->setLayout(outer);
outer->addLayout(inner); // nesting layouts
Demo

QVariant
• QVariant
• Union for common Qt "value types" (copyable,
assignable)
• Supports implicit sharing (fast copying)
• Supports user types
• Use cases:
QVariant property(const char* name) const;
void setProperty(const char* name, const
QVariant &value);
class QAbstractItemModel {
virtual QVariant data( const QModelIndex&
index, int role );
}
• Qvariant Documentation

QVariant
• For QtCore types
QVariant variant(42);
int value = variant.toInt(); // read back
qDebug() << variant.typeName(); // int
• For non-core and custom types:
QVariant variant =
QVariant::fromValue(QColor(Qt::red));
QColor color = variant.value<QColor>(); //
read back
qDebug() << variant.typeName();
// "QColor"

Properties
• Direct access (Broken, due to private
headers)
QQuickRectangle* rectangle=
root->findChild<QQuickRectangle*>("myRect");
int height = rectangle->height();
• Generic property access:
QObject* rectangle = root-
>findChild<QObject*>("myRect");
int height = rectangle-
>property("height").value<int>();

Properties
• Q_Property is a macro:
Q_PROPERTY( type name READ getFunction [WRITE setFunction]
[RESET resetFunction] [NOTIFY notifySignal] [DESIGNABLE bool]
[SCRIPTABLE bool] [STORED bool] )
• Property access methods:
QVariant property(const char* name) const;
void setProperty(const char* name, const QVariant &value);
• If name is not declared as a Q_PROPERTY
• -> dynamic property
• Not accessible from Qt Quick.
Note:
• Q_OBJECT macro required for properties to work
• QMetaObject knows nothing about dynamic properties

Properties
• QMetaObject support property introspection
const QMetaObject *metaObject = object->metaObject();
const QString className = metaObject->className();
const int propertyCount = metaObject->propertyCount();
for ( int i=0; i<propertyCount; ++i ) {
const QMetaProperty metaProperty = metaObject-
>property(i);
const QString typeName = metaProperty.typeName()
const QString propertyName = metaProperty.name();
const QVariant value = object-
>property(metaProperty.name());
}
Demo

QString
Strings can be created in a number of ways:
• Conversion constructor and assignment operators:
QString str(“abc”);
str = “def”;
• From a number using a static function:
QString n = QString::number(1234);
• From a char pointer using the static functions:
QString text = QString::fromLatin1(“Hello Qt”);
QString text = QString::fromUtf8(inputText);
QString text = QString::fromLocal8Bit(cmdLineInput);
QString text = QStringLiteral(“Literal String”); (Assumed
to be UTF-8)
• From char pointer with translations:
QString text = tr(“Hello Qt”);

QString
• operator+ and operator+=
QString str = str1 + str2;
fileName += ".txt";
• simplified() // removes duplicate whitespace
• left(), mid(), right() // part of a string
• leftJustified(), rightJustified() // padded version
QString s = "apple";
QString t = s.leftJustified(8, '.');
// t == "apple..."

QString
Data can be extracted from strings.
• Numbers:
QString text = ...;
int value = text.toInt();
float value = text.toFloat();
• Strings:
QString text = ...;
QByteArray bytes = text.toLatin1();
QByteArray bytes = text.toUtf8();
QByteArray bytes = text.toLocal8Bit();

QString::arg()
int i = ...;
int total = ...;
QString fileName = ...;
QString status = tr("Processing file %1 of %2:
%3")
.arg(i).arg(total).arg(fileName);
double d = 12.34;
QString str = QString::fromLatin1("delta:
%1").arg(d,0,'E',3);
// str == "delta: 1.234E+01"
• Safer: arg(QString,...,QString) (``multi-arg()'').
• But: only works with QString arguments.

QString
• Obtaining raw character data from a QByteArray:
char *str = bytes.data();
const char *str = bytes.constData();
WARNING:
• Character data is only valid for the lifetime of the byte
array.
• Calling a non-const member of bytes also invalidates ptr.
• Either copy the character data or keep a copy of the
byte array.

QString
• length()
• endsWith() and startsWith()
• contains(), count()
• indexOf() and lastIndexOf()
Expression can be characters, strings, or regular
expressions

QString
• QString::split(), QStringList::join()
• QStringList::replaceInStrings()
• QStringList::filter()

QString
• QRegExp supports
• Regular expression matching
• Wildcard matching
• QString cap(int)
QStringList capturedTexts()
QRegExp rx(“^dd?$”); // match integers 0 to 99
rx.indexIn(“123”); // returns -1 (no match)
rx.indexIn(“-6”); // returns -1 (no match)
rx.indexIn(“6”); // returns 0 (matched as position 0)
• Regular expression documentation

Container Classes
General purpose template-based container classes
• QList<QString> - Sequence Container
• Other: QLinkedList, QStack , QQueue ...
• QMap<int, QString> - Associative Container
• Other: QHash, QSet, QMultiMap, QMultiHash
Qt's Container Classes compared to STL
• Lighter, safer, and easier to use than STL
containers
• If you prefer STL, feel free to continue using it.
• Methods exist that convert between Qt and STL
• e.g. you need to pass std::list to a Qt method

Container Classes
Using QList
QList<QString> list;
list << “one” << “two” << “three”;
QString item1 = list[1]; // “two”
for(int i=0; i<list.count(); i++) {
const QString &item2 = list.at(i);
}
int index = list.indexOf("two"); // returns 1
Using QMap
QMap<QString, int> map;
map[“Norway”] = 5; map[“Italy”] = 48;
int value = map[“France”]; // inserts key if not exists
if(map.contains(“Norway”)) {
int value2 = map.value(“Norway”); // recommended lookup
}

Complexity
How fast is a function when number of items grow
Sequential Container
Looku
p
Inser
t
Appen
d
Prepe
nd
QList O(1) O(n) O(1) O(1)
QVector O(1) O(n) O(1) O(n)
QLinkedL
ist
O(n) O(1) O(1) O(1)

Complexity
Associative Container
all complexities are amortized
Lookup Insert
Qmap
QHash
O(log(n))
O(1)
O(log(n))
O(1)

Classes
in Container
• Class must be an assignable data type
• Class is assignable, if:
class Contact {
public:
Contact() {} // default constructor
Contact(const Contact &other); // copy constructor
// assignment operator
Contact &operator=(const Contact &other);
};
• If copy constructor or assignment operator is not provided
• C++ will provide one (uses member copying)
• If no constructors provided
• Empty default constructor provided by C++

Container Keys
• Type K as key for QMap:
• bool K::operator<( const K& ) or
bool operator<( const K&, const K& )
bool Contact::operator<(const Contact& c);
bool operator<(const Contact& c1, const Contact&
c2);
• QMap Documentation
• Type K as key for QHash or QSet:
• bool K::operator==( const K& ) or bool operator==( const
K&, const K& )
• uint qHash( const K& )
• QHash Documentation

Iterators
• Allow reading a container's content
sequentially
• Java-style iterators: simple and easy to
use
• QListIterator<...>for read
• QMutableListIterator<...>for read-write
• STL-style iterators slightly more efficient
• QList::const_iterator for read
• QList::iterator() for read-write
• Same works for QSet, QMap, QHash, ...

Iterators
Java style
• Example QList iterator
list << "A" << "B" << "C" << "D“;
QListIterator<QString> it(list);
• Forward iteration
while(it.hasNext()) {
qDebug() << it.next(); // A B C D
}
• Backward iteration
it.toBack(); // position after the last item
while(it.hasPrevious()) {
qDebug() << it.previous(); // D C B A
}
• QListIterator Documentation

Modifying
During Iteration
• Use mutable versions of the iterators
• e.g. QMutableListIterator.
QList<int> list;
list << 1 << 2 << 3 << 4;
QMutableListIterator<int> i(list);
while (i.hasNext()) {
if (i.next() % 2 != 0)
i.remove();
}
// list now 2, 4
• remove() and setValue()
• Operate on items just jumped over using next()/previous()
• insert()
• Inserts item at current position in sequence
• previous() reveals just inserted item

QMap and QHash
• next() and previous()
• Return Item class with key() and value()
• Alternatively use key() and value() from iterator
QMap<QString, QString> map;
map[“Paris”] = “France”;
map[“Guatemala City”] = “Guatemala”;
map[“Mexico City”] = “Mexico”;
map[“Moscow”] = “Russia”;
QMutableMapIterator<QString, QString> i(map);
while (i.hasNext()) {
if (i.next().key().endsWith(“City”))
i.remove();
}
// map now "Paris", "Moscow“
• Demo

Iterators
STL-style
• Example QList iterator
list << "A" << "B" << "C" << "D";
QList<QString>::iterator i;
• Forward mutable iteration
for (i = list.begin(); i != list.end(); ++i) {
*i = (*i).toLower();
}
• Backward mutable iteration
i = list.end();
while (i != list.begin()) {
--i;
*i = (*i).toLower();
}
• QList<QString>::const_iterator for read-only

foreach
• It is a macro, feels like a keyword
foreach ( variable, container ) statement
foreach (const QString& str, list) {
if (str.isEmpty())
break;
qDebug() << str;
}
• Break and continue as normal
• Modifying the container while iterating
• results in container being copied
• iteration continues in unmodiﬁed version
• Not possible to modify item
• iterator variable is a const reference.

Qt algorithms
• qSort(begin, end) sort items in range
• qFind(begin, end, value) ﬁnd value
• qEqual(begin1, end1, begin2) checks two
ranges
• qCopy(begin1, end1, begin2) from one range to
another
• qCount(begin, end, value, n) occurrences of
value in range
• For parallel (ie. multi-threaded) algorithms
• QtConcurrent()
• QtAlgorithms Documentation

Examples
• Counting 1's in list
QList<int> list;
list << 1 << 2 << 3 << 1;
int count = 0;
qCount(list, 1, count); // count the 1's
qDebug() << count; // 2 (means 2 times 1)
• Copy list to vector
list << "one" << "two" << "three";
QVector<QString> vector(3);
qCopy(list.begin(), list.end(), vector.begin());
// vector: [ "one", "two", "three" ]

Examples
• Case insensitive sort
bool lessThan(const QString& s1, const QString&
s2) {
return s1.toLower() < s2.toLower();
}
// ...
list << "AlPha" << "beTA" << "gamma" << "DELTA";
qSort(list.begin(), list.end(), lessThan);
// list: [ "AlPha", "beTA", "DELTA", "gamma" ]

Implicit Sharing
If an object is copied, then its data is copied only when
the data of one of the objects is changed
• Shared class has a pointer to shared data block
• Shared data block = reference counter and actual data
• Assignment is a shallow copy
• Changing results into deep copy (detach)
QList<int> l1, l2; l1 << 1 << 2;
l2 = l1; // shallow-copy: l2 shares date with l1
l2 << 3; // deep-copy: change triggers detach from l1
Important to remember when inserting items into a
container, or when returning a container.

Summary
• Explain how object ownership works in Qt?
• What are the key responsibilities of QObject?
• What does it mean when a QWidget has no
parent?
• What is the purpose of the class QVariant?
• What do you need to do to support properties in
a class?
• Name the different ways to go through the
elements in a list, and
• Discuss advantages and disadvantages of each
method.

Widgets
• Common Widgets
• Layout Management
• Custom Widgets

Widgets
• Common Widgets
• Text widgets
• Value based widgets
• Organizer widgets
• Item based widgtes
• Layout Management
• Geometry management
• Advantages of layout managers
• Qt's layout managers
• Size policies
• Custom Widgets
• Rules for creating own widgets

Text Widgets
• QLabel
label = new QLabel("Text", parent);
• setPixmap( pixmap ) - as content
• QLineEdit
line = new QLineEdit(parent);
line->setText("Edit me");
line->setEchoMode(QLineEdit::Password);
connect(line, SIGNAL(textChanged(QString)) ...
connect(line, SIGNAL(editingFinished()) ...
• QTextEdit
edit = new QTextEdit(parent);
edit->setPlainText("Plain Text");
edit->append("<h1>Html Text</h1>");
connect(edit,SIGNAL(textChanged(QString)) …

Button widgets
• QAbstractButton
Abstract base class of buttons
• QPushButton
button = new QPushButton("Push Me", parent);
button->setIcon(QIcon("images/icon.png"));
connect(button, SIGNAL(clicked()) ...
setCheckable(bool) - toggle button
• QRadioButton
radio = new QRadioButton("Option 1", parent);
• QCheckBox
check = new QCheckBox("Choice 1", parent);

Value Widgets
• QSlider
slider = new QSlider(Qt::Horizontal, parent);
slider->setRange(0, 99);
slider->setValue(42);
connect(slider, SIGNAL(valueChanged(int)) ...
• QProgressBar
progress = new QProgressBar(parent);
progress->setRange(0, 99);
progress->setValue(42);
// format: %v for value; %p for percentage
progress->setFormat("%v (%p%)");
• QSpinBox
spin = new QSpinBox(parent);
spin->setRange(0, 99);
spin->setValue(42);
spin->setSuffix(" USD");
connect(spin,SIGNAL(valueChanged(int)) ...

Organizer Widgets
• QGroupBox
box = new QGroupBox("Your Options", parent);
// ... set layout and add widgets
setCheckable( bool ) - checkbox in title
• QTabWidget
tab = new QTabWidget(parent);
tab->addWidget(widget, icon, "Tab 1");
connect(tab, SIGNAL(currentChanged(int)) ...
• setCurrentWidget( widget )
• Displays page assoziated by widget
• setTabPosition( position )
• Deﬁnes where tabs are drawn
• setTabsClosable( bool )
• Adds close buttons

Item Widgets
• QComboBox
combo = new QComboBox(parent);
combo->addItem("Option 1", data);
connect(combo, SIGNAL(activated(int)) ...
QVariant data = combo->itemData(index);
• setCurrentIndex( index )
• QListWidget
list = new QListWidget(parent);
list->addItem("Item 1");
// ownership of items with list
item = new QListWidgetItem("Item 2", list);
item->setCheckState(Qt::Checked);
connect(list, SIGNAL(itemActivated(QListWidgetItem*)) .
• Other Item Widgets: QTableWidget, QTreeWidget

Other Widgets
• QToolBox
Column of tabbed widget items
• QDateEdit, QTimeEdit, QDateTimeEdit
Widget for editing date and times
• QCalendarWidget
Monthly calendar widget
• QToolButton
Quick-access button to commands
• QSplitter
Implements a splitter widget
• QStackedWidget
Stack of widgets
Only one widget visible at a time
• Widget Documentation

Layout widgets
Definition
Layout: Specifying the relations of elements to each
other instead of the absolute positions and sizes.
• Advantages:
• Works with different languages.
• Works with different dialog sizes.
• Works with different font sizes.
• Better to maintain.
• Disadvantage
• Need to design your layout ﬁrst.

Layout widgets
• Place and resize widgets (Without layouts)
• move()
• resize()
• setGeometry()
• Example:
QWidget *parent = new QWidget(...);
parent->resize(400,400);
QCheckBox *cb = new QCheckBox(parent);
cb->move(10, 10);

Layout widgets
• On layout managed widgets never call
setGeometry(), resize(), or move()
• Preferred
• Override
• sizeHint()
• minimumSizeHint()
• Or call
• setFixedSize()
• setMinimumSize()
• setMaximumSize()

Layout classes
• QHBoxLayout
• Lines up widgets horizontally
• QVBoxLayout
• Lines up widgets vertically
• QGridLayout
• Arranges the widgets in a grid
• QFormLayout
• Lines up a (label, widget) pairs in two columns.
• QStackedLayout
• Arranges widgets in a stack
• only topmost is visible

QHBoxLayout &
QVBoxLayout
• Lines up widgets horizontally or vertically
• Divides space into boxes
• Each managed widgets ﬁlls in one box
QWidget* window = new QWidget;
QPushButton* one = new QPushButton("One");
...
QHBoxLayout* layout = new QHBoxLayout;
layout->addWidget(one);
...
window->setLayout(layout);

QGridLayout
QWidget* window = new QWidget;
QGridLayout* layout = new QGridLayout;
layout->addWidget(one, 0, 0); // row:0, col:0
layout->addWidget(two, 0, 1); // row:0, col:1
// row:1, col:0, rowSpan:1, colSpan:2
layout->addWidget(three, 1, 0, 1, 2);
window->setLayout(layout)
Demo
• Additional
• setColumnMinimumWidth() (minimum width of column)
• setRowMinimumHeight() (minimum height of row)
No need to specify rows and columns before adding children.

QFormLayout
• A two-column layout
• Column 1 a label (as annotation)
• Column 2 a widget (as ﬁeld)
• Respects style guide of individual platforms.
QWidget* window = new QWidget();
...
QFormLayout* layout = new QFormLayout();
layout->addRow("One", one);
...
window->setLayout(layout)
• Form layout with clean looks and mac style

Hands-on
Lab 4 Contact form
• Objectives
• Template code

Layout Terms
• Stretch
• Relative resize factor
• QBoxLayout::addWidget(widget, stretch)
• QBoxLayout::addStretch(stretch)
• QGridLayout::setRowStretch(row, stretch)
• QGridLayout::setColumnStretch(col, stretch)
• Contents Margins
• Space reserved around the managed widgets.
• QLayout::setContentsMargins(l,t,r,b)
• Spacing
• Space reserved between widgets
• QBoxLayout::addSpacing(size)

Layout Terms
• Strut
• Limits perpendicular box dimension
• e.g. height for QHBoxLayout
• Only for box layouts
• Min, max and fixed sizes
• QWidget::setMinimumSize( QSize )
• QWidget::setMaximumSize( QSize )
• QWidget::setFixedSize( QSize )
• Individual width and height constraints also available
• Nested Layouts
• Allows ﬂexible layouts
• QLayout::addLayout(...)

Widgets
Size Policies
• QSizePolicy describes interest of widget in resizing
QSizePolicy policy = widget->sizePolicy();
policy.setHorizontalPolicy(QSizePolicy::Fixed);
widget->setSizePolicy(policy);
• One policy per direction (horizontal and vertical)
• Button-like widgets set size policy to the following:
• may stretch horizontally
• are ﬁxed vertically
• Similar to QLineEdit, QProgressBar, ...
• Widgets which provide scroll bars (e.g. QTextEdit)
• Can use additional space
• Work with less than sizeHint()
• sizeHint(): recommended size for widget

Widgets
Size Policies
Policy sizeHint() Widget
Fixed authoritative can not grow or shrink
Minimum minimal, sufficient can expand, no
advantage of being
larger
Maximum is maximum can shrink
Preferred is best can shrink, no
advantage of being
larger
Minimum
Expanding
is minimum can use extra space
Expanding sensible size can grow and shrink

Hands-on
• Lab 4: Layout buttons
• Objective
• Template code

Summary
• How do you change the minimum size of
a widget?
• Name the available layout managers.
• How do you specify stretch?
• When are you allowed to call resize and
move on a widget?

Custom Widget
Creation
• It's as easy as deriving from QWidget
class CustomWidget : public QWidget
{
public:
explicit CustomWidget(QWidget* parent=0);
}
• If you need custom Signal Slots
• add Q_OBJECT
• Use layouts to arrange widgets inside, or paint
the widget yourself.

Custom Widget
Base class and Event Handlers
• Do not reinvent the wheel
• Decide on a base class
• Often QWidget or QFrame
• Overload needed event handlers
• Often:
• QWidget::mousePressEvent(),
• QWidget::mouseReleaseEvent()
• If widget accepts keyboard input
• QWidget::keyPressEvent()
• If widget changes appearance on focus
• QWidget::focusInEvent(),
• QWidget::focusOutEvent()

Custom Widget
Drawing a Widget
• Decide on composite or draw approach?
• If composite: Use layouts to arrange other widgets
• If draw: implement paint event
• Reimplement QWidget::paintEvent() for drawing
• To draw widget's visual appearance
• Drawing often depends on internal states
• Decide which signals to emit
• Usually from within event handlers
• Especially mousePressEvent() or
mouseDoubleClickEvent()
• Decide carefully on types of signal parameters
• General types increase reusability
• Candidates are bool, int and const QString&

Custom Widget
Internal States and Subclassing
• Decide on publishing internal states
• Which internal states should be made publically
accessible?
• Implement accessor methods
• Decide which setter methods should be slots
• Candidates are methods with integral or common
parameters
• Decide on allowing subclassing
• If yes
• Decide which methods to make protected instead of
private
• Which methods to make virtual

Custom Widget
Widget Constructor
• Decide on parameters at construction time
• Enrich the constructor as necessary
• Or implement more than one constructor
• If a parameter is needed for widget to work
correctly
• User should be forced to pass it in the
constructor
• Keep the Qt convention with:
explicit Constructor(..., QWidget *parent
= 0)

Hands-on
• Lab 5: File chooser
• Objective
• Template code
• Lab 6: Compass widget
• Objective
• Template code

Object
communication
• Signals & Slots
• Event Handling

Objectives
• How objects communication
• Details of signals & slots
• Which variations for signal/slot connections exist
• How to create custom signals & slots
• What the role of the Qt event loop is
• How Qt handles events

• Between objects
Signals & Slots
• Between Qt and the application
Events
• Between Objects on threads
Signal & Slots + Events
• Between Applications
DBus, QSharedMemory
Object
communication

Callbacks
General Problem
How do you get from "the user clicks a button" to your
logic?
• Possible solutions
• Callbacks
• Based on function pointers
• Not type-safe
• Observer Pattern (Listener)
• Based on interface classes
• Needs listener registration
• Many interface classes
• Qt uses
• Signals and slots for high-level (semantic) callbacks
• Virtual methods for low-level (syntactic) events.

Connecting
signals and slots
Signal
emitted
slot implementedSignal/slot
connection
QObject::connect(slider,SI
GNAL(valueChanged),prog
ressbar, SLOT(setValue))
QObject::connect(slider,SI
GNAL(valueChanged),prog
ressbar, SLOT(setValue))
void
QProgressBar::setValue(in
t value)
{
mvalue = value
}
void
QProgressBar::setValue(in
t value)
{
mvalue = value
}
void
Qslider::mousePressEve
nt(…)
{
emit
valueChanged(value)
}
void
Qslider::mousePressEve
nt(…)
{
emit
valueChanged(value)
}
Demo

Connection
variants
• Qt 4 style:
connect(slider,SIGNAL(valueChanged(int)),
progressBar, SLOT(setValue(int)));
• Using function pointers (Qt5):
connect( slider, &QSlider::valueChanged,
progressBar, &QSpinBox::setValue );
Demo
• Using non-member function:
static void printValue(int value) {...}
connect( slider, &QSignal::valueChanged,
&printValue );
Demo

Custom slots
• File: myclass.h
class MyClass : public QObject
{
Q_OBJECT // marker for moc
// ...
public slots:
void setValue(int value); // a custom slot
};
• File: myclass.cpp
void MyClass::setValue(int value) {
// slot implementation
}
Demo

Custom signals
• File: myclass.h
class MyClass : public QObject
{
Q_OBJECT // marker for moc
// ...
signals:
void valueChanged(int value); // a custom signal
};
• File: myclass.cpp
// No implementation for a signal
• Sending a signal
emit valueChanged(value);
• Demo

Q_OBJECT - ﬂag
• Q_OBJECT
• Enhances QObject with meta-object information
• Required for signals
• Required for slots when using the Qt4 way
• moc creates meta-object information
• moc -o moc_myclass.cpp myclass.h
• c++ -c myclass.cpp; c++ -c moc_myclass.cpp
• c++ -o myapp moc_myclass.o myclass.o
• qmake takes care of mocing ﬁles for you

Variations of
Signal/Slot
Signal(s
)
Connect
to
Slot(s)
One  Many
Many  One
One  Another
signal
• Signal to Signal connection
connect(bt, SIGNAL(clicked()), this,
SIGNAL(okSignal()));
• Not allowed to name parameters
connect( m_slider,
SIGNAL( valueChanged( int value ) )
this, SLOT( setValue( int newValue ) ) )

Rules for
Signal/Slot
Can ignore arguments, but not create values from nothing.
Eg:
Signal Slot
rangeChanged(int,i
nt)



setRange(int,int)
setValue(int)
Update()
valueChanged(int)


X
setValue(int)
Update()
setRange(int,int)
textChanged(QStin
g)
X setValue(int)

Hands-on
• Lab 1: Select colour
• Objective
• Template code
• Lab 2: Slider
• Objective
• Template code

Event Processing
Qt is an event-driven UI toolkit
Qapplication::exec() runs the event loop
1.Generate Events
• by input devices: keyboard, mouse, etc.
• by Qt itself (e.g. timers)
2.Queue Events
• by event loop
3.Dispatch Events
• by QApplication to receiver: QObject
• Key events sent to widget with focus
• Mouse events sent to widget under cursor
4.Handle Events
• by QObject event handler methods

Event Processing
QPAQPA
QcoreApplicat
ion
QcoreApplicat
ion
Event loopEvent loop
QQuickVie
w
QQuickVie
w
QQuickMouseAr
ea
QQuickMouseAr
ea
Press
me
Press
me
QQuickMouseArea
QQuickView

Event Processing
QPAQPA
QcoreApplicat
ion
QcoreApplicat
ion
Event loopEvent loop
sendSpontaneousEvent
(QObject *, QEvent *)

Event Processing
QCoreApplicati
on
QCoreApplicati
on
QWindowQWindow
QQuickCanvasQQuickCanvas
QQuickViewQQuickView
1 event (QEvent *)
2 event (...)
3 mousePressEvent
(...)
MouseEvent

Event Processing
QWindowQWindow
QQuickCanvasQQuickCanvas
QQuickViewQQuickView
QQuickItemQQuickItem
QQuickMouseAr
ea
QQuickMouseAr
ea
mousePressEvent (QMouseEvent *)

Event Handling
• QObject::event(QEvent *event)
• Handles all events for this object
• Specialized event handlers for QWidget and
QQuickItem:
• mousePressEvent() for mouse clicks
• touchEvent() for key presses
• Accepting an Event
• event->accept() / event->ignore()
• Accepts or ignores the event
• Accepted is the default.
• Event propagation
• Happens if event is ignored
• Might be propagated to parent widget
Demo

Event Handling
• QCloseEvent delivered to top level widgets (windows)
• Accepting event allows window to close
• Ignoring event keeps window open
void MyWidget::closeEvent(QCloseEvent *event) {
if (maybeSave()) {
writeSettings();
event->accept(); // close window
} else {
event->ignore(); // keep window
}
}
Demo

Summary
• How do you connect a signal to a slot?
• How would you implement a slot?
• How would you emit a signal?
• Can you return a value from a slot?
• When do you need to run qmake?
• Where do you place the Q_OBJECT macro
and when do you need it?
• What is the purpose of the event loop
• How does an event make it from the device
to an object in Qt?

Multithreading
• Most GUI applications have a single thread of
execution in which the event loop is running
• However, if the user invokes a time
consuming operation the interface freezes.
We can work around this in different ways:
• Using the QApplication::processEvent()
during long tasks to make sure events (key,
window, etc.) are delivered and the UI stays
responsive.
• Using threads to perform the long running tasks.
Qt has a number of options for this.

Multithreading
Technologies
• QThread: Low-Level API with
Optional Event Loops
• QThreadPool and QRunnable:
Reusing Threads
• QtConcurrent: Using a High-level API
• WorkerScript: Threading in QML

QThread
• QThread is the central class in Qt to
run code in a different thread
• It's a QObject subclass
• Not copiable/moveable
• Has signals to notify when the thread
starts/ﬁnishes
• It is meant to manage a thread

QThread
usage
• To create a new thread executing some code, subclass QThread
and reimplement run()
• Then create an instance of the subclass and call start()
• Threads have priorities that you can specify as an optional
parameter to start(), or change with setPriority()
• The thread will stop running when (some time after) returning
from run()
• QThread::isRunning() and QThread::isFinished() provide
information about the execution of the thread
• You can also connect to the QThread::started() and
QThread::finished() signals
• A thread can stop its execution temporarily by calling one of the
QThread::sleep() functions
• Generally a bad idea, being event driven (or polling) is much much
Better
• You can wait for a QThread to ﬁnish by calling wait() on it
• Optionally passing a maximum number of milliseconds to wait

QThread caveats
From a non-main thread you cannot:
• Perform any GUI operation
• Including, but not limited to: using any QWidget / Qt
Quick / Qpixmap APIs
• Using QImage, QPainter, etc. (i.e. "client side") is OK
• Using OpenGL may be OK: check at runtime
QOpenGLContext::supportsThreadedOpenGL()
• Call Q(Core|Gui)Application::exec()
• Be sure to always destroy all the QObjects living in
secondary threads before destroying the
corresponding QThread object
• Do not ever block the GUI thread

QThread usage
• There are two basic strategies of
running code in a separate thread
with QThread:
• Without an event loop
• With an event loop

QThread usage
without an event loop
• Subclass QThread and override
QThread::run()
• Create an instance and start the
new thread via QThread::start()
• Demo

QThread usage
with an event loop
• An event loop is necessary when dealing with timers,
networking, queued connections, and so on.
• Qt supports per-thread event loops:
• Each thread-local event loop delivers events for the QObjects
living in that thread.
QThread::exec(
)
QThread::exec(
)
QApplication::exec(
)
QApplication::exec(
)
Obj
1
Obj
1 Obj 2Obj 2
Obj
3
Obj
3
Obj
1
Obj
1 Obj 2Obj 2
Obj 3Obj 3 Obj 4Obj 4
Thread
1
Main
Thread
QThread::exec(
)
QThread::exec(
)
Obj
1
Obj
1 Obj 2Obj 2
Obj
3
Obj
3
Thread
2

QThread usage
with an event loop
• We can start a thread-local event loop by calling QThread::exec() from within
run():
1 class MyThread : public QThread {
2 private:
3 void run() override {
4 auto socket = new QTcpSocket;
5 socket->connectToHost(...);
67 exec(); // run the event loop
89 // cleanup
10 }
11}; Demo
• QThread::quit() or QThread::exit() will quit the event loop
• We can also use QEventLoop
• Or manual calls to QCoreApplication::processEvents()
• The default implementation of QThread::run() actually calls QThread::exec()
• This allows us to run code in other threads without sub classing QThread:

QtConcurrent
• QtConcurrent is a namespace that provides
higher-level classes and algorithms for writing
concurrent software.
• Using QtConcurrent's functional
map/ﬁlter/reduce algorithms, which apply
functions in parallel to each item in a container.
• You can write a program that automatically
takes advantage of the system's multiple cores
by distributing the processing across the
threads managed by the thread pool.

QtConcurrent
• Qt Concurrent supports several STL-
compatible container and iterator
types, but works best with Qt
containers that have random-access
iterators, such as QList or Qvector
• Demo

QThreadPool and
QRunnable
• Creating and destroying threads frequently can
be expensive.
• To avoid the cost of thread creation, a thread
pool can be used.
• A thread pool is a place where threads can be
parked and fetched.
• We derive a class from QRunnable. The code we
want to run in another thread needs to be
placed in the reimplemented QRunnable::run()
method.
• Demo

Synchronization
• Any concurrent access to shared
resources must not result in a data
race
• Two conditions for this to happen:
1. At least one of the accesses is a write
2. The accesses are not atomic and no
access happens before the other

Synchronization
Qt has a complete set of cross-platform, low-
level APIs for dealing with synchronization:
• QMutex is a mutex class (recursive and non-
recursive)
• QSemaphore is a semaphore
• QWaitCondition is a condition variable
• QReadWriteLock is a shared mutex
• QAtomicInt is an atomic int
• QAtomicPointer<T> is an atomic pointer to T
• Demo’

Thread safety in
Qt
A function is:
• Thread safe: if it's safe for it to be invoked at the
same time, from multiple threads, on the same data,
without synchronization
• Reentrant: if it's safe for it to be invoked at the
same time, from multiple threads, on different data;
otherwise it requires external synchronization
• Non-reentrant (thread unsafe): if it cannot be
invoked from more than one thread at all
For classes, the above deﬁnitions apply to non-static
member functions when invoked on the same instance.

Examples
• Thread safe:
• QMutex
• QObject::connect()
• QCoreApplication::postEvent()
• Reentrant:
• QString
• QVector
• QImage
• value classes in general
• Non-reentrant:
• QWidget (including all of its subclasses)
• QQuickItem
• QPixmap
• in general, GUI classes are usable only from the main thread

QtMultimedia
• Qt Multimedia is an essential
module that provides a rich set of
QML types and C++ classes to
handle multimedia content.
• It also provides necessary APIs to
access the camera and radio
functionality.

Features
• Access raw audio devices for input and output
• Play low latency sound effects
• Play media files in playlists (such as compressed audio
or video files)
• Record audio and compress it
• Tune and listen to radio stations
• Use a camera, including viewfinder, image capture, and
movie recording
• Play 3D positional audio with Qt Audio Engine
• Decode audio media files into memory for processing
• Accessing video frames or audio buffers as they are
played or recorded

Audio
• Qt Multimedia offers a range of audio classes, covering both
low and high level approaches to audio input, output and
processing.
• For playing media or audio ﬁles that are not simple,
uncompressed audio, you can use the QMediaPlayer C++
class.
• The QMediaPlayer class and associated QML types are also
capable of playing video, if required.
• The compressed audio formats supported does depend on the
operating system environment, and also what media plugins
the user may have installed.
• For recording audio to a ﬁle, the QAudioRecorder class allows
you to compress audio data from an input device and record it.
• Demo

Video
• We can use the QMediaPlayer class
to decode a video ﬁle, and display it
using QVideoWidget,
QGraphicsVideoItem, or a custom
class.
• Demo

Objectives
• Painting on Widgets
• Color Handling
• Painting Operations
• Style Sheets

Objectives
• Painting
• You paint with a painter on a paint device during a
paint event
• Qt widgets know how to paint themselves
• Often widgets look like we want
• Painting allows device independent 2D visualization
• Allows to draw pie charts, line charts and many
more
• StyleSheets
• Fine grained control over the look and feel
• Easily applied using style sheets in CSS format

Module
Objectives
Covers techniques for general 2D graphics and styling applications.
• Painting
• Painting infrastructure
• Painting on widget
• Color Handling
• Deﬁne and use colors
• Pens, Brushes, Palettes
• Shapes
• Drawing shapes
• Transformation
• 2D transformations of a coordinate system
• Style Sheets
• How to make small customizations
• How to apply a theme to a widget or application

Painting &
Styling
• Painting on Widgets
• Color Handling
• Painting Operations
• Style Sheets

QPainter
• Paints on paint devices (QPaintDevice)
• QPaintDevice implemented by
• On-Screen: QWidget
• Off-Screen: QImage, QPixmap
• And others ...
• Provides drawing functions
• Lines, shapes, text or pixmaps
• Controls
• Rendering quality
• Clipping
• Composition modes

Painting
on Widgets
• Override paintEvent(QPaintEvent*)
void CustomWidget::paintEvent(QPaintEvent *) {
QPainter painter(this);
painter.drawRect(0,0,100,200); // x,y,w,h
}
• Schedule painting
• update(): schedules paint event
• repaint(): repaints directly
• Qt handles double-buffering
• To enable ﬁlling background:
• QWidget::setAutoFillBackground(true)

Coordinate System
• Controlled by QPainter
• Origin: Top-Left
• Rendering
• Logical - mathematical
• Aliased - right and below
• Anti-aliased – smoothing
• Rendering quality switch
• QPainter::setRenderHint()

Geometry Classes
• QSize(w,h)
• scale, transpose
• QPoint(x,y)
• QLine(point1, point2)
• translate, dx, dy
• QRect(point, size)
• adjust, move
• translate, scale, center
QSize size(100,100);
QPoint point(0,0);
QRect rect(point, size);
rect.adjust(10,10,-10,-10);
QPoint center = rect.center();

Color Values
• Using different color models:
• QColor(255,0,0) // RGB
• QColor::fromHsv(h,s,v) // HSV
• QColor::fromCmyk(c,m,y,k) // CMYK
• Deﬁning colors:
QColor(255,0,0); // red in RGB
QColor(255,0,0, 63); // red 25% opaque (75% transparent)
QColor("#FF0000"); // red in web-notation
QColor("red"); // by svg-name
Qt::red; // predefined Qt global colors
• Many powerful helpers for manipulating colors
QColor("black").lighter(150); // a shade of gray
• QColor always refers to device color space

QPen
• A pen (QPen) consists of:
• a color or brush
• a width
• a style (e.g. NoPen or SolidLine)
• a cap style (i.e. line endings)
• a join style (connection of lines)
• Activate with QPainter::setPen().
QPen pen = painter.pen();
pen.setBrush(Qt::red);
pen.setWidth(3);
painter.setPen(pen);
// draw a rectangle with 3 pixel width red outline
painter.drawRect(0,0,100,100);

The Outline
Rule
The outline equals the size plus half the pen
width on each side.
•For a pen of width 1:
QPen pen(Qt::red, 1); // width = 1
float hpw = pen.widthF()/2; // half-pen width
QRectF rect(x,y,width,height);
QRectF outline = rect.adjusted(-hpw, -hpw, hpw, hpw);
•Due to integer rounding on a non-antialiased
grid, the outline is shifted by 0.5 pixel towards
the bottom right.
•Demo

QBrush
• QBrush defines fill pattern of shapes
• Brush configuration
• setColor(color)
• setStyle(Qt::BrushStyle)
• NoBrush, SolidPattern, ...
• QBrush(gradient) // QGradient's
• setTexture(pixmap)
• Brush with solid red fill
painter.setPen(Qt::red);
painter.setBrush(QBrush(Qt::yellow,
Qt::SolidPattern));
painter.drawRect(rect);

Gradient ﬁlls
• Gradients used with QBrush
• Gradient types
• QLinearGradient
• QConicalGradient
• QRadialGradient
• Gradient from P1(0,0) to P2(100,100)
QLinearGradient gradient(0, 0, 100, 100);
// position, color: position from 0..1
gradient.setColorAt(0, Qt::red);
gradient.setColorAt(0.5, Qt::green);
gradient.setColorAt(1, Qt::blue);
painter.setBrush(gradient);
// draws rectangle, filled with brush
painter.drawRect(0, 0, 100, 100 );
• Demo

Brush on QPen
• Possible to set a brush on a pen
• Strokes generated will be ﬁlled with the brush
• Demo

Color Themes
and Palettes
• To support widgets color theming
• setColor(blue) not recommended
• Colors needs to be managed
• QPalette manages colors
• Consist of color groups
• enum QPalette::ColorGroup
• Resemble widget states
• QPalette::Active
• Used for window with keyboard focus
• QPalette::Inactive
• Used for other windows
• QPalette::Disabled
• Used for disabled widgets

Drawing Figures
• Painter conﬁguration
• pen width: 2
• pen color: red
• font size: 10
• brush color: yellow
• brush style: solid
• Demo

Drawing Text
• QPainter::drawText(rect, flags, text)
painter.drawText(rect, Qt::AlignCenter,
tr("Qt"));
painter.drawRect(rect);
• QFontMetrics
• calculate size of strings
QFont font("times", 24);
QFontMetrics fm(font);
int pixelsWide = fm.width("Width of this text?");
int pixelsHeight = fm.height();

Transformation
• Manipulating the coordinate system
• translate(x,y)
• scale(sx,sy)
• rotate(a)
• shear(sh,
• Demo

Transform
and Center
• scale(sx, sy)
• scales around QPoint(0,0)
• Same applies to all transform operations
• Scale around center?
painter.drawRect(r);
painter.translate(r.center());
painter.scale(sx,sy);
painter.translate(-r.center());
// draw center-scaled rect
painter.drawRect(r);

QPainterPath
• Container for painting operations
• Enables reuse of shapes
QPainterPath path;
path.addRect(20, 20, 60, 60);
path.moveTo(0, 0);
path.cubicTo(99, 0, 50, 50, 99, 99);
path.cubicTo(0, 99, 50, 50, 0, 0);
painter.drawPath(path);
• Path information
controlPointRect() - rect containing all points
contains() - test if given shape is inside path
intersects() - test given shape intersects path
• Demo

Hands-on
• Lab 7: Pie chart
• Objectives
• Template code

Qt Style Sheets
• Mechanism to customize appearance of
widgets
• Additional to subclassing QStyle
• Inspired by HTML CSS
• Textual speciﬁcations of styles
• Applying Style Sheets
• QApplication::setStyleSheet(sheet)
• On whole application
• QWidget::setStyleSheet(sheet)
• On a speciﬁc widget (incl. child widgets)
• Demo

CSS Rules
CSS Rule
selector { property : value; property : value }
• Selector: speciﬁes the widgets
• Property/value pairs: specify properties to change.
QPushButton {color:red; background-color:white}
• Examples of stylable elements
• Colors, fonts, pen style, alignment.
• Background images.
• Position and size of sub controls.
• Border and padding of the widget itself.
• Reference of stylable elements
• stylesheet-reference

The Box Model
• Every widget treated as box
• Four concentric rectangles
• Margin, Border, Padding, Content
• Customizing QPushButton
QPushButton {
border-width: 2px;
border-radius: 10px;
padding: 6px;
// ...
}
• By default, margin, border-width, and padding are 0

Selector Types
• *{ } // Universal selector
• All widgets
• QPushButton { } // Type Selector
• All instances of class
• QPushButton { } // Class Selector
• All instances of class, but not subclasses
• QPushButton#objectName // ID Selector
• All Instances of class with objectName
• QDialog QPushButton { } // Descendant Selector
• All instances of QPushButton which are child of QDialog
• QDialog > QPushButton { } // Direct Child Selector
• All instances of QPushButton which are direct child of QDialog
• QPushButton[enabled="true"] // Property Selector
• All instances of class which match property

Selector Details
• Property Selector
• If property changes it is required to re-set style sheet
• Combining Selectors
• QLineEdit, QComboBox, QPushButton { color: red }
• Pseudo-States
• Restrict selector based on widget's state
• Example: QPushButton:hover {color:red}
• Demo
• Selecting Subcontrols
• Access subcontrols of complex widgets
• as QComboBox, QSpinBox, ...
• QComboBox::drop-down { image: url(dropdown.png) }
• Subcontrols positioned relative to other elements
• Change using subcontrol-origin and subcontrol-position

Cascading
Effective style sheet obtained by merging
1. Widgets's ancestor (parent, grandparent, etc.)
2. Application stylesheet
• On conﬂict: widget own style sheet preferred
qApp->setStyleSheet("QPushButton { color:
white }");
button->setStyleSheet("* { color: blue }");
• Style on button forces button to have blue
text
• In spite of more speciﬁc application rule
• Demo

Selector Specifity
• Conflict: When rules on same level specify same property
• Specificity of selectors apply
QPushButton:hover { color: white }
QPushButton { color: red }
• Selectors with pseudo-states are more specific
• Calculating selector's specificity
• a Count number of ID attributes in selector
• b Count number of property specifications
• c Count number of class names
• Concatenate numbers a-b-c. Highest score wins.
• If rules scores equal, use last declared rule
QPushButton {} /* a=0 b=0 c=1 -> specificity = 1 */
QPushButton#ok {} /* a=1 b=0 c=1 -> specificity = 101 */
• Demo

Hands-on
• Try this demo code and
• Investigate style sheet
• Modify style sheet
• Remove style sheetand implement your own

Objectives
• Main Windows
• Settings
• Resources
• Deploying Qt Applications

Objectives
We will create an application to show
fundamental concepts
• Main Window: How a typical main window is
structured
• Settings: Store/Restore application settings
• Resources: Adding icons and other ﬁles to your
application
• Deployment: Distributing your application

Application
creation
• Main Windows
• Settings
• Resources
• Deploying Qt Applications

Application
Ingredients
• Main window with
• Menu bar
• Tool bar, Status bar
• Central widget
• Often a dock window
• Settings (saving state)
• Resources (e.g icons)
• Translation
• Load/Save documents
Not a complete list

Main Window
• QMainWindow: main application window
• Has own layout
• Central Widget
• QMenuBar
• QToolBar
• QDockWidget
• QStatusBar
• Central Widget
• QMainWindow::setCentralWidget( widget )
• Just any widget object

QAction
Action is an abstract user interface command
• Emits signal triggered on execution
• Connected slot performs action
• Added to menus, toolbar, key shortcuts
• Each performs same way
• Regardless of user interface used
void MainWindow::setupActions() {
QAction* action = new QAction(tr("Open ..."), this);
action->setIcon(QIcon(":/images/open.png"));
action->setShortcut(QKeySequence::Open);
action->setStatusTip(tr("Open file"));
connect(action, SIGNAL(triggered()), this, SLOT(onOpen()));
menu->addAction(action);
toolbar->addAction(action);
• Qaction Documentation

QAction
capabilities
• setEnabled(bool)
• Enables disables actions
• In menu and toolbars, etc...
• setCheckable(bool)
• Switches checkable state (on/off)
• setChecked(bool) toggles checked state
• setData(QVariant)
• Stores data with the action
• Documentation
• QAction

Menu Bar
• QMenuBar: a horizontal menu bar
• QMenu: represents a menu
• indicates action state
• QAction: menu items added to QMenu
void MainWindow::setupMenuBar() {
QMenuBar* bar = menuBar();
QMenu* menu = bar->addMenu(tr("&File"));
menu->addAction(action);
menu->addSeparator();
QMenu* subMenu = menu->addMenu(tr("Sub Menu"));
...

QToolBar
• Movable panel ...
• Contains set of controls
• Can be horizontal or vertical
• QMainWindow::addToolbar( toolbar )
• Adds toolbar to main window
• QMainWindow::addToolBarBreak()
• Adds section splitter
• QToolBar::addAction( action )
• Adds action to toolbar
• QToolBar::addWidget(widget)
• Adds widget to toolbar
void MainWindow::setupToolBar() {
QToolBar* bar = addToolBar(tr("File"));
bar->addAction(action);
bar->addSeparator();
bar->addWidget(new QLineEdit(tr("Find ...")));
...

QToolButton
• Quick-access button to commands or options
• Used when adding action to QToolBar
• Can be used instead QPushButton
• Different visual appearance!
• Advantage: allows to attach action
QToolButton* button = new QToolButton(this);
button->setDefaultAction(action);
// Can have a menu
button->setMenu(menu);
// Shows menu indicator on button
button->setPopupMode(QToolButton::MenuButtonPopup);
// Control over text + icon placements
button->setToolButtonStyle(Qt::ToolButtonTextUnderIcon);

QStatusBar
Horizontal bar
Suitable for presenting status information
• showMessage( message, timeout )
• Displays temporary message for speciﬁed milli-seconds
• clearMessage()
• Removes any temporary message
• addWidget() or addPermanentWidget()
• Normal, permanent messages displayed by widget
void MainWindow::createStatusBar() {
QStatusBar* bar = statusBar();
bar->showMessage(tr("Ready"));
bar->addWidget(new QLabel(tr("Label on StatusBar")));

QDockWidget
• Window docked into main window
• Qt::DockWidgetArea enum
• Left, Right, Top, Bottom dock areas
• QMainWindow::setCorner(corner,area)
• Sets area to occupy speciﬁed corner
• QMainWindow::setDockOptions(options)
• Speciﬁes docking behavior (animated, nested, tabbed, ...)
void MainWindow::createDockWidget() {
QDockWidget *dock = new QDockWidget(tr("Title"), this);
dock->setAllowedAreas(Qt::LeftDockWidgetArea);
QListWidget *widget = new QListWidget(dock);
dock->setWidget(widget);
addDockWidget(Qt::LeftDockWidgetArea, dock);

QMenu
and Context Menus
• Launch via event handler
void MyWidget::contextMenuEvent(event) {
m_contextMenu->exec(event->globalPos());
• or signal customContextMenuRequested()
• Connect to signal to show context menu
• Or via QWidget::actions() list
• QWidget::addAction(action)
• setContextMenuPolicy(Qt::ActionsContextMen
u)
• Displays QWidget::actions() as context menu

Hands-on
• Lab 8: Text editor
• Objectives
• Template code

QSettings
• Configure QSettings
QCoreApplication::setOrganizationName("MyCompany");
QCoreApplication::setOrganizationDomain( "mycompany.com" );
QCoreApplication::setApplicationName("My Application");
• Typical usage
QSettings settings;
settings.setValue("group/value", 68);
int value = settings.value("group/value").toInt();
• Values are stored as QVariant
• Keys form hierarchies using '/'
• or use beginGroup(prefix) / endGroup()
• value() excepts default value
• settings.value("group/value", 68).toInt()
• If value not found and default not specified
Invalid QVariant() returned

Restoring State
• Store geometry of application
void MainWindow::writeSettings() {
QSettings settings;
settings.setValue("MainWindow/size", size());
settings.setValue("MainWindow/pos", pos());
}
• Restore geometry of application
void MainWindow::readSettings() {
QSettings settings;
settings.beginGroup("MainWindow");
resize(settings.value("size", QSize(400, 400)).toSize());
move(settings.value("pos", QPoint(200, 200)).toPoint());
settings.endGroup();
}

Resource System
• Platform-independent mechanism for storing binary
files
• Not limited to images
• Resource files stored in application's executable
• Useful if application requires files
• E.g. icons, translation files, sounds
• Don't risk of losing files, easier deployment
• Docs

Using Resources
• Resources specified in .qrc file
<!DOCTYPE RCC><RCC version="1.0">
<qresource>
<file>images/copy.png</file>
<file>images/cut.png</file>
</qresource>
</RCC>
• Can be created using QtCreator
• Resources are accessible with ':' prefix
• Example: ":/images/cut.png"
• Simply use resource path instead of file name
• QIcon(":/images/cut.png")
• To compile resource, edit .pro file
• RESOURCES += application.qrc
• qmake produces make rules to generate binary file

Hands-On
• Use your previous text editor, to use Qt resource
system for icons
• Tip: You can use Qt Creator to create QRC ﬁles

Ways of Deploying
• Static Linking
• Results in stand-alone executable
• +Only few files to deploy
• -Executables are large
• -No flexibility
• -You cannot deploy plugins
• Shared Libraries
• +Can deploy plugins
• +Qt libs shared between applications
• +Smaller, more flexible executables
• -More files to deploy
• Qt is by default compiled as shared library
• If Qt is pre-installed on system
• Use shared libraries approach
• Reference Documentation

Deployment
• Shared Library Version
• If Qt is not a system library
• Need to redistribute Qt libs with application
• Minimal deployment
• Libraries used by application
• Plugins used by Qt
• Ensure Qt libraries use correct path to ﬁnd Qt plugins
• Static Linkage Version
• Build Qt statically
• $QTDIR/conﬁgure -static <your other options>
• Specify required options (e.g. sql drivers)
• Link application against Qt
• Check that application runs stand-alone
• Copy application to machine without Qt and run it

Dialogs
and Designer
• Dialogs
• Common Dialogs
• Qt Designer

Dialogs
and Designer
• Custom Dialogs
• Modality
• Inheriting QDialog
• Dialog buttons
• Predefined Dialogs
• File, color, input and font dialogs
• Message boxes
• Progress dialogs
• Wizard dialogs
• Qt Designer
• Design UI Forms
• Using forms in your code
• Dynamic form loading

QDialog
• Base class of dialog window widgets
• General Dialogs can have 2 modes
• Modal dialog
• Remains in foreground, until closed
• Blocks input to remaining application
• Example: Conﬁguration dialog
• Modeless dialog
• Operates independently in application
• Example: Find/Search dialog
• Modal dialog example
MyDialog dialog(this);
dialog.setMyInput(text);
if(dialog.exec() == Dialog::Accepted) {
// exec blocks until user closes dialog

Modeless Dialog
• Use show()
• Displays dialog
• Returns control to caller
void EditorWindow::find() {
if (!m_findDialog) {
m_findDialog = new FindDialog(this);
connect(m_findDialog, SIGNAL(findNext()),
this, SLOT(onFindNext()));
}
m_findDialog->show(); // returns immediately
m_findDialog->raise(); // on top of other windows
m_findDialog->activateWindow(); // keyboard focus
}

Custom Dialogs
• Inherit from QDialog
• Create and layout widgets
• Use QDialogButtonBox for dialog buttons
• Connect buttons to accept()/reject()
• Override accept()/reject()
MyDialog::MyDialog(QWidget *parent) : QDialog(parent) {
m_label = new QLabel(tr("Input Text"), this);
m_edit = new QLineEdit(this);
m_box = new QDialogButtonBox( QDialogButtonBox::Ok|
QDialogButtonBox::Cancel, this);
connect(m_box, SIGNAL(accepted()), this, SLOT(accept()));
connect(m_box, SIGNAL(rejected()), this, SLOT(reject()));
... // layout widgets
}
void MyDialog::accept() { // customize close behaviour
if(isDataValid()) { QDialog::accept() }
}

Deletion and
Extension
• Deletion of dialogs
• No need to keep dialogs around forever
• Call QObject::deleteLater()
• Or setAttribute(Qt::WA_DeleteOnClose)
• Or override closeEvent()
• Dialogs with extensions:
• QWidget::show()/hide() used on extension
m_more = new QPushButton(tr("&More"));
m_more->setCheckable(true);
m_extension = new QWidget(this);
// add your widgets to extension
m_extension->hide();
connect(m_more, SIGNAL(toggled(bool)),
m_extension, SLOT(setVisible(bool)));
• Example

QFileDialog
• Allow users to select files or directories
• Asking for a file name
QString fileName =
QFileDialog::getOpenFileName(this, tr("Open File"));
if(!fileName.isNull()) {
// do something useful
}
• QFileDialog::getOpenFileNames()
• Returns one or more selected existing files
• QFileDialog::getSaveFileName()
• Returns a file name. File does not have to exist.
• QFileDialog::getExistingDirectory()
• Returns an existing directory.
• setFilter("Image Files (*.png *.jpg *.bmp)")
• Displays files matching the patterns

QMessageBox
• Provides a modal dialog for ...
• informing the user
• asking a question and receiving an answer
• Typical usage, questioning a user
QMessageBox::StandardButton ret =
QMessageBox::question(parent, title, text);
if(ret == QMessageBox::Ok) {
// do something useful
}
• Very ﬂexible in appearance
• Reference documentation
• Other convenience methods
• QMessageBox::information(...)
• QMessageBox::warning(...)
• QMessageBox::critical(...)
• QMessageBox::about(...)

QProgressDialog
• Provides feedback on the progress of a slow operation
QProgressDialog dialog("Copy", "Abort", 0, count, this);
dialog.setWindowModality(Qt::WindowModal);
for (int i = 0; i < count; i++) {
dialog.setValue(i);
if (dialog.wasCanceled()) { break; }
//... copy one file
}
dialog.setValue(count); // ensure set to maximum
• Initialize with setValue(0)
• Otherwise estimation of duration will not work
• When operation progresses, check for cancel
• QProgressDialog::wasCanceled()
• Or connect to QProgressDialog::canceled()
• To stay reactive call QApplication::processEvents()
• See Documentation

QErrorMessage
• Similar to QMessageBox with checkbox
• Asks if message shall be displayed again
m_error = new QErrorMessage(this);
m_error->showMessage(message, type);
• Messages will be queued
• QErrorMessage::qtHandler()
• installs an error handler for debugging
• Shows qDebug(), qWarning() and qFatal() messages in
QErrorMessage box

Other Common
Dialogs
• Asking for Input - QInputDialog
• QInputDialog::getText(...)
• QInputDialog::getInt(...)
• QInputDialog::getDouble(...)
• QInputDialog::getItem(...)
• Selecting Color - QColorDialog
• QColorDialog::getColor(...)
• Selecting Font - QFontDialog
• QFontDialog::getFont(...)
• Example

Qwizard
Guiding the user
• Input dialog
• Consisting of sequence of pages
• Purpose: Guide user through process
• Page by page
• Supports
• Linear and non-linear wizards
• Registering and using ﬁelds
• Access to pages by ID
• Page initialization and cleanup
• Title, sub-title
• Logo, banner, watermark, background
• Documentation
• Each page is a QWizardPage
• QWizard::addPage()
• Adds page to wizard
• example

Hands-on
• Lab 9: Dialog
• Objectives
• Template code

Summary
• When would you use a modal dialog, and when would
you use a non-modal dialog?
• When should you call exec() and when should you
call show()?
• Can you bring up a modal dialog, when a modal
dialog is already active?
• When do you need to keep widgets as instance
variables?
• What is the problem with this code:
QDialog *dialog = new QDialog(parent);
QCheckBox *box = new QCheckBox(dialog);

Qt Designer
• Design UI forms visually
• Visual Editor for
• Signal/slot connections
• Actions
• Tab handling
• Buddy widgets
• Widget properties
• Integration of custom widgets
• Resource ﬁles

Designer Views
Object Inspector
Displays hierarchy of objects on form
Property Editor
Displays properties of selected object
Widget Box
Provides selection of widgets, layouts

Editing Modes
• Widget Editing
• Change appearance of form
• Add layouts
• Edit properties of widgets
• Signal and Slots Editing
• Connect widgets together with signals & slots
• Buddy Editing
• Assign buddy widgets to label
• Buddy widgets help keyboard focus handling correctly
• Tab Order Editing
• Set order for widgets to receive the keyboard focus

UI Form Files
• Form stored in .ui ﬁle
• format is XML
• uic tool generates code
• From myform.ui
• to ui_myform.h
// ui_mainwindow.h
class Ui_MainWindow {
public:
QLineEdit *fileName;
... // simplified code
void setupUi(QWidget *) { /* setup widgets */ }
};
• Form ui ﬁle in project (.pro)
FORMS += mainwindow.ui

Form Wizards
• Add New... "Designer Form"
• or "Designer Form Class" (for C++ integration)

Naming Widgets
1. Place widgets on form
2. Edit objectName property
• objectName deﬁnes member name in generated code

Form layout
QFormLayout: Suitable for most input forms

Top-Level Layout
• First layout child widgets
• Finally select empty space and set top-level
layout

Preview Mode
Check that widget is nicely resizable

Code Integration
// orderform.h
class Ui_OrderForm;
class OrderForm : public QDialog {
private:
Ui_OrderForm *ui; // pointer to UI object
};
• "Your Widget" derives from appropriate base class
• *ui member encapsulate UI class
• Makes header independent of designer generated
code

Code Integration
// orderform.cpp
#include "ui_orderform.h"
OrderForm::OrderForm(QWidget *parent)
: QDialog(parent), ui(new Ui_OrderForm) {
ui->setupUi(this);
}
OrderForm::~OrderForm() {
delete ui; ui=0;
}
• Default behavior in Qt Creator

Signals and Slots
• Widgets are available as public members
• ui->ﬁleName->setText("image.png")
• Name based on widgets object name
• You can set up signals & slots traditionally...
• connect(ui->okButton, SIGNAL(clicked()), ...
• Auto-connection facility for custom slots
• Automatically connect signals to slots in your code
• Based on object name and signal
• void on_objectName_signal(parameters);
• Example: on_okButton_clicked() slot
• Automatic connections
• Qt Creator: right-click on widget and "Go To Slot"
• Generates a slot using auto-connected name

Loading .ui files
• Forms can be processed at runtime
• Produces dynamically generated user interfaces
• Disadvantages
• Slower, harder to maintain
• Risk: .ui file not available at runtime
• Loading .ui file
QUiLoader loader;
QFile file("forms/textfinder.ui");
file.open(QFile::ReadOnly);
QWidget *formWidget = loader.load(&file, this);
• Locate objects in form
ui_okButton = qFindChild<QPushButton*>(this,
"okButton");

Hands-on
• Lab 10: Order form
• Objectives
• Template code

Qt Application Programming with C++ - Part 1

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