Part 1 - Designing the User Interface¶
Describes how to code the user interface of the Address Book Example. This first part covers the design of the basic graphical user interface (GUI) for our address book application.
The first step in creating a GUI program is to design the user interface. Here the our goal is to set up the labels and input fields to implement a basic address book. The figure below is a screenshot of the expected output.
We require two
QLabel
objects,nameLabel
andaddressLabel
, as well as two input fields, aQLineEdit
object,nameLine
, and aQTextEdit
object,addressText
, to enable the user to enter a contact’s name and address. The widgets used and their positions are shown in the figure below.There are three files used to implement this address book:
addressbook.h
- the definition file for theAddressBook
class,
addressbook.cpp
- the implementation file for theAddressBook
class, and
main.cpp
- the file containing amain()
function, with an instance ofAddressBook
.
Qt Programming - Subclassing¶
When writing Qt programs, we usually subclass Qt objects to add functionality. This is one of the essential concepts behind creating custom widgets or collections of standard widgets. Subclassing to extend or change the behavior of a widget has the following advantages:
We can write implementations of virtual or pure virtual functions to obtain exactly what we need, falling back on the base class’s implementation when necessary.
It allows us to encapsulate parts of the user interface within a class, so that the other parts of the application don’t need to know about the individual widgets in the user interface.
The subclass can be used to create multiple custom widgets in the same application or library, and the code for the subclass can be reused in other projects.
Since Qt does not provide a specific address book widget, we subclass a standard Qt widget class and add features to it. The
AddressBook
class we create in this tutorial can be reused in situations where a basic address book widget is needed.
Defining the AddressBook Class¶
The
tutorials/addressbook/part1/addressbook.h
file is used to define theAddressBook
class.We start by defining
AddressBook
as aQWidget
subclass and declaring a constructor. We also use theQ_OBJECT
macro to indicate that the class uses internationalization and Qt’s signals and slots features, even if we do not use all of these features at this stage.class AddressBook : public QWidget { Q_OBJECT public: AddressBook(QWidget *parent = nullptr); private: QLineEdit *nameLine; QTextEdit *addressText; };The class holds declarations of
nameLine
andaddressText
, the private instances ofQLineEdit
andQTextEdit
mentioned earlier. The data stored innameLine
andaddressText
will be needed for many of the address book functions.We don’t include declarations of the
QLabel
objects we will use because we will not need to reference them once they have been created. The way Qt tracks the ownership of objects is explained in the next section.The
Q_OBJECT
macro itself implements some of the more advanced features of Qt. For now, it is useful to think of theQ_OBJECT
macro as a shortcut which allows us to use thetr()
andconnect()
functions.We have now completed the
addressbook.h
file and we move on to implement the correspondingaddressbook.cpp
file.
Implementing the AddressBook Class¶
The constructor of
AddressBook
accepts aQWidget
parameter,parent
. By convention, we pass this parameter to the base class’s constructor. This concept of ownership, where a parent can have one or more children, is useful for grouping widgets in Qt. For example, if you delete a parent, all of its children will be deleted as well.AddressBook::AddressBook(QWidget *parent) : QWidget(parent) { QLabel *nameLabel = new QLabel(tr("Name:")); nameLine = new QLineEdit; QLabel *addressLabel = new QLabel(tr("Address:")); addressText = new QTextEdit;In this constructor, the
QLabel
objectsnameLabel
andaddressLabel
are instantiated, as well asnameLine
andaddressText
. Thetr()
function returns a translated version of the string, if there is one available. Otherwise it returns the string itself. This function marks itsQString
parameter as one that should be translated into other languages. It should be used wherever a translatable string appears.When programming with Qt, it is useful to know how layouts work. Qt provides three main layout classes:
QHBoxLayout
,QVBoxLayout
andQGridLayout
to handle the positioning of widgets.We use a
QGridLayout
to position our labels and input fields in a structured manner.QGridLayout
divides the available space into a grid and places widgets in the cells we specify with row and column numbers. The diagram above shows the layout cells and the position of our widgets, and we specify this arrangement using the following code:QGridLayout *mainLayout = new QGridLayout; mainLayout->addWidget(nameLabel, 0, 0); mainLayout->addWidget(nameLine, 0, 1); mainLayout->addWidget(addressLabel, 1, 0, Qt::AlignTop); mainLayout->addWidget(addressText, 1, 1);Notice that
addressLabel
is positioned usingAlignTop
as an additional argument. This is to make sure it is not vertically centered in cell (1,0). For a basic overview on Qt Layouts, refer to the Layout Management documentation.In order to install the layout object onto the widget, we have to invoke the widget’s
setLayout()
function:setLayout(mainLayout); setWindowTitle(tr("Simple Address Book")); }Lastly, we set the widget’s title to “Simple Address Book”.
Running the Application¶
A separate file,
main.cpp
, is used for themain()
function. Within this function, we instantiate aQApplication
object,app
.QApplication
is responsible for various application-wide resources, such as the default font and cursor, and for running an event loop. Hence, there is always oneQApplication
object in every GUI application using Qt.int main(int argc, char *argv[]) { QApplication app(argc, argv); AddressBook addressBook; addressBook.show(); return app.exec(); }We construct a new
AddressBook
widget on the stack and invoke itsshow()
function to display it. However, the widget will not be shown until the application’s event loop is started. We start the event loop by calling the application’sexec()
function; the result returned by this function is used as the return value from themain()
function. At this point, it becomes apparent why we instanciatedAddressBook
on the stack: It will now go out of scope. Therefore,AddressBook
and all its child widgets will be deleted, thus preventing memory leaks.
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