Update copyright headers

[qt:qt.git] / doc / src / getting-started / gettingstartedqt.qdoc

/****************************************************************************
**
** Copyright (C) 2015 The Qt Company Ltd.
** Contact: http://www.qt.io/licensing/
**
** This file is part of the documentation of the Qt Toolkit.
**
** $QT_BEGIN_LICENSE:FDL$
** Commercial License Usage
** Licensees holding valid commercial Qt licenses may use this file in
** accordance with the commercial license agreement provided with the
** Software or, alternatively, in accordance with the terms contained in
** a written agreement between you and The Qt Company. For licensing terms
** and conditions see http://www.qt.io/terms-conditions. For further
** information use the contact form at http://www.qt.io/contact-us.
**
** GNU Free Documentation License Usage
** Alternatively, this file may be used under the terms of the GNU Free
** Documentation License version 1.3 as published by the Free Software
** Foundation and appearing in the file included in the packaging of
** this file.  Please review the following information to ensure
** the GNU Free Documentation License version 1.3 requirements
** will be met: http://www.gnu.org/copyleft/fdl.html.
** $QT_END_LICENSE$
**
****************************************************************************/

/*!
    \page gettingstartedqt.html

    \title Getting Started Programming with Qt
        \ingroup gettingStarted

    Welcome to the world of Qt--the cross-platform GUI toolkit. In
    this getting started guide, we teach basic Qt knowledge by
    implementing a simple Notepad application. After reading this
    guide, you should be ready to delve into our overviews and API
    documentation, and find the information you need for the
    application you are developing.

    The code for this tutorial is available in \c
    {examples/tutorials/gettingStarted/gsQt} under your Qt
    installation. If you are using the Qt SDK, you will find it in
    \c{Examples/4.7/tutorials/gettingStarted/gsQt} (change \c{4.7} if
    you are using a later Qt version).

    \section1 Hello Notepad

    In this first example, we simply create and show a text edit in a
    window frame on the desktop. This represents the simplest possible
    Qt program that has a GUI.

    \image gs1.png

    Here is the code:

    \code
 1        #include <QApplication>
 2        #include <QTextEdit>
 3
 4        int main(int argv, char **args)
 5        {
 6            QApplication app(argv, args);
 7
 8            QTextEdit textEdit;
 9            textEdit.show();
10
11            return app.exec();
12        }
    \endcode

    Let us go through the code line by line. In the first two lines, we
    include the header files for QApplication and QTextEdit, which are
    the two classes that we need for this example. All Qt classes have
    a header file named after them.

    Line 6 creates a QApplication object. This object manages
    application-wide resources and is necessary to run any Qt program
    that has a GUI. It needs \c argv and \c args because Qt accepts a
    few command line arguments.

    Line 8 creates a QTextEdit object. A text edit is a visual element
    in the GUI. In Qt, we call such elements widgets. Examples of
    other widgets are scroll bars, labels, and radio buttons. A widget
    can also be a container for other widgets; a dialog or a main
    application window, for example.  

    Line 9 shows the text edit on the screen in its own window frame.
    Since widgets also function as containers (for instance a
    QMainWindow, which has toolbars, menus, a status bar, and a few
    other widgets), it is possible to show a single widget in its own
    window. Widgets are not visible by default; the function
    \l{QWidget::}{show()} makes the widget visible.

    Line 11 makes the QApplication enter its event loop. When a Qt
    application is running, events are generated and sent to the
    widgets of the application. Examples of events are mouse presses
    and key strokes. When you type text in the text edit widget, it
    receives key pressed events and responds by drawing the text
    typed.

    To run the application, open a command prompt, and enter the
    directory in which you have the \c .cpp file of the program. The
    following shell commands build the program.

    \code
        qmake -project
        qmake
        make
    \endcode

    This will leave an executable in the \c part1 directory (note that
    on Windows, you may have to use \c nmake instead of \c make. Also,
    the executable will be placed in part1\\debug or part1\\release
    (these directories are created when you run \c make). \c qmake is
    Qt's build tool, which takes a configuration file. \c qmake
    generates this for us when given the \c{-project} argument. Given
    the configuration file (suffixed .pro), \c qmake produces a \c
    make file that will build the program for you. We will look into
    writing our own \c .pro files later.

    \section2 Learn More

    \table
        \header
            \o About
            \o Here
        \row
            \o Widgets and Window Geometry
            \o \l{Window and Dialog Widgets}
        \row
            \o Events and event handling
            \o \l{The Event System}
    \endtable

    \section1 Adding a Quit Button

    In a real application, you will normally need more than one
    widget. We will now introduce a QPushButton beneath the text edit.
    The button will exit the Notepad application when pushed (i.e.,
    clicked on with the mouse).

    \image gs2.png

    Let us take a look at the code.

    \code
 1      #include <QtGui>
 2 
 3      int main(int argv, char **args)
 4      {
 5          QApplication app(argv, args);
 6 
 7          QTextEdit *textEdit = new QTextEdit;
 8          QPushButton *quitButton = new QPushButton("&Quit");
 9 
10          QObject::connect(quitButton, SIGNAL(clicked()), qApp, SLOT(quit()));
11 
12          QVBoxLayout *layout = new QVBoxLayout;
13          layout->addWidget(textEdit);
14          layout->addWidget(quitButton);
15 
16          QWidget window;
17          window.setLayout(layout);
18 
19          window.show();
20 
21          return app.exec();
22      }
    \endcode

    Line 1 includes QtGui, which contains all of Qt's GUI classes.

    Line 10 uses Qt's Signals and Slots mechanism to make the
    application exit when the \gui {Quit button} is pushed. A slot is
    a function that can be invoked at runtime using its name (as a
    literal string). A signal is a function that when called will
    invoke slots registered with it; we call that to connect the slot
    to the signal and to emit the signal.

    \l{QApplication::}{quit()} is a slot of QApplication that exits
    the application. \l{QPushButton::}{clicked()} is a signal that
    QPushButton emits when it is pushed. The static
    QObject::connect() function takes care of connecting the slot to
    the signal. SIGNAL() and SLOT() are two macros that take the
    function signatures of the signal and slot to connect. We also
    need to give pointers to the objects that should send and receive
    the signal.

    Line 12 creates a QVBoxLayout. As mentioned, widgets can contain
    other widgets. It is possible to set the bounds (the location and
    size) of child widgets directly, but it is usually easier to use a
    layout. A layout manages the bounds of a widget's children.
    QVBoxLayout, for instance, places the children in a vertical row.

    Line 13 and 14 adds the text edit and button to the layout. In
    line 17, we set the layout on a widget. 

    \section2 Learn More

    \table
        \header
            \o About
            \o Here
         \row
            \o Signals and slots
            \o \l{Signals & Slots}
        \row
            \o Layouts
            \o \l{Layout Management},
               \l{Widgets and Layouts},
               \l{Layout Examples}
        \row 
            \o The widgets that come with Qt
            \o \l{Qt Widget Gallery},
               \l{Widget Examples}
    \endtable

    \section1 Subclassing QWidget

    When the user wants to quit an application, you might want to
    pop-up a dialog that asks whether he/she really wants to quit. In
    this example, we subclass QWidget, and add a slot that we connect
    to the \gui {Quit button}.

    \image gs3.png

    Let us look at the code:

    \code
 5        class Notepad : public QWidget
 6        {
 7            Q_OBJECT
 8
 9        public:
10            Notepad();
11
12        private slots:
13            void quit();
14
15        private:
16            QTextEdit *textEdit;
17            QPushButton *quitButton;
18        };
    \endcode

    The \c Q_OBJECT macro must be first in the class definition, and
    declares our class as a \c QObject (Naturally, it must also
    inherit from QObject). A \l{QObject} adds several abilities to a
    normal C++ class. Notably, the class name and slot names can be
    queried at run-time. It is also possible to query a slot's
    parameter types and invoke it.

    Line 13 declares the slot \c quit(). This is easy using the \c
    slots macro. The \c quit() slot can now be connected to signals.
    We will do that later.

    Instead of setting up the GUI and connecting the slot in the \c
    main() function, we now use \c{Notepad}'s constructor. 

    \code
20        Notepad::Notepad()
21        {
22            textEdit = new QTextEdit;
23            quitButton = new QPushButton(tr("Quit"));
24
25            connect(quitButton, SIGNAL(clicked()), this, SLOT(quit()));
26
27            QVBoxLayout *layout = new QVBoxLayout;
28            layout->addWidget(textEdit);
29            layout->addWidget(quitButton);
30
31            setLayout(layout);
32
33            setWindowTitle(tr("Notepad"));
34        }
    \endcode

    As you saw in the class definition, we use pointers to our \l
    {QObject}s (\c textEdit and \c quitButton). As a rule, you should
    always allocate \l{QObject}s on the heap and never copy them.

    We now use the function \l{QObject::}{tr()} around our user
    visible strings. This function is necessary when you want to
    provide your application in more than one language (e.g.  English
    and Chinese). We will not go into details here, but you can follow
    the \c {Qt Linguist} link from the learn more table.

    Here is the \c quit() slot:

    \code
75      void Notepad::quit()
76      {
77          QMessageBox messageBox;
78          messageBox.setWindowTitle(tr("Notepad"));
79          messageBox.setText(tr("Do you really want to quit?"));
80          messageBox.setStandardButtons(QMessageBox::Yes | QMessageBox::No);
81          messageBox.setDefaultButton(QMessageBox::No);
82          if (messageBox.exec() == QMessageBox::Yes)
83              qApp->quit();
84      }
    \endcode

    We use the QMessageBox class to display a dialog that asks the
    user whether he/she really wants to quit.

    \section2 Learn More

    \table
        \header
            \o About
            \o Here
        \row
            \o tr() and internationalization
            \o \l{Qt Linguist Manual},
               \l{Writing Source Code for Translation},
               \l{Hello tr() Example},
               \l{Internationalization with Qt}
        \row
            \o QObjects and the Qt Object model (This is essential to understand Qt)
            \o \l{Object Model}
        \row
            \o qmake and the Qt build system
            \o \l{qmake Manual}
    \endtable

    \section2 Creating a .pro file

    For this example, we write our own \c .pro file instead of
    using \c qmake's \c -project option.

    \code
 1        HEADERS =  notepad.h
 2        SOURCES =  notepad.cpp \
 3                   main.cpp
    \endcode

    The following shell commands build the example.

    \code
        qmake
        make
    \endcode

    \section1 Using a QMainWindow

    Many applications will benefit from using a QMainWindow, which has
    its own layout to which you can add a menu bar, dock widgets, tool
    bars, and a status bar. QMainWindow has a center area that can be
    occupied by any kind of widget. In our case, we will place our
    text edit there.

    \image gs4.png

    Let us look at the new \c Notepad class definition.

    \code
 2        #include <QtGui>
 3 
 4        class Notepad : public QMainWindow
 5        {
 6            Q_OBJECT
 7 
 8        public:
 9            Notepad();
10
11        private slots:
12            void open();
13            void save();
14            void quit();
15
16        private:
17            QTextEdit *textEdit;
18
19            QAction *openAction;
20            QAction *saveAction;
21            QAction *exitAction;
22
23            QMenu *fileMenu;
24        };
    \endcode

    We include two more slots that can save and open a document. We
    will implement these in the next section.

    Often, in a main window, the same slot should be invoked by
    several widgets. Examples are menu items and buttons on a tool
    bar. To make this easier, Qt provides QAction, which can be given
    to several widgets, and be connected to a slot. For instance, both
    QMenu and QToolBar can create menu items and tool buttons from the
    same \l{QAction}s. We will see how this works shortly.

    As before, we use the \c {Notepad}s constructor to set up the
    GUI.

    \code
25        Notepad::Notepad()
26        {
27            openAction = new QAction(tr("&Open"), this);
28            saveAction = new QAction(tr("&Save"), this);
29            exitAction = new QAction(tr("E&xit"), this);
30
31            connect(openAction, SIGNAL(triggered()), this, SLOT(open()));
32            connect(saveAction, SIGNAL(triggered()), this, SLOT(save()));
33            connect(exitAction, SIGNAL(triggered()), qApp, SLOT(quit()));
34
35            fileMenu = menuBar()->addMenu(tr("&File"));
36            fileMenu->addAction(openAction);
37            fileMenu->addAction(saveAction);
38            fileMenu->addSeparator();
39            fileMenu->addAction(exitAction);
40
41            textEdit = new QTextEdit;
42            setCentralWidget(textEdit);
43
44            setWindowTitle(tr("Notepad"));
45        }
    \endcode

    \l{QAction}s are created with the text that should appear on the
    widgets that we add them to (in our case, menu items). If we also
    wanted to add them to a tool bar, we could have given
    \l{QIcon}{icons} to the actions.

    When a menu item is clicked now, the item will trigger the action,
    and the respective slot will be invoked.

    \section2 Learn More

    \table
        \header
            \o About
            \o Here
        \row
            \o Main windows and main window classes
            \o \l{Application Main Window},
               \l{Main Window Examples}
        \row
            \o MDI applications
            \o QMdiArea,
               \l{MDI Example}
    \endtable

    \section1 Saving and Loading

    In this example, we will implement the functionality of the \c
    open() and \c save() slots that we added in the previous example.

    \image gs5.png

    We will start with the \c open() slot:

    \code
48    void Notepad::open()
49    {
50        QString fileName = QFileDialog::getOpenFileName(this, tr("Open File"), "",
51            tr("Text Files (*.txt);;C++ Files (*.cpp *.h)"));
52
53        if (fileName != "") {
54            QFile file(fileName);
55            if (!file.open(QIODevice::ReadOnly)) {
56                QMessageBox::critical(this, tr("Error"), tr("Could not open file"));
57                return;
58            }
59            QTextStream in(&file);
60            textEdit->setText(in.readAll());
61            file.close();
62        }
63    }
    \endcode

    The first step is asking the user for the name of the file to
    open. Qt comes with QFileDialog, which is a dialog from which the
    user can select a file. The image above shows the dialog on
    Kubuntu. The static \l{QFileDialog::}{getOpenFileName()} function
    displays a modal file dialog. It returns the file path of the file
    selected, or an empty string if the user canceled the dialog. 

    If we have a file name, we try to open the file with
    \l{QIODevice::}{open()}, which returns true if the file could be
    opened. We will not go into error handling here, but you can follow
    the links from the learn more section. If the file could not be
    opened, we use QMessageBox to display a dialog with an error
    message (see the QMessageBox class description for further
    details).

    Actually reading in the data is trivial using the QTextStream
    class, which wraps the QFile object. The
    \l{QTextStream::}{readAll()} function returns the contents of the
    file as a QString. The contents can then be displayed in the text
    edit. We then \l{QIODevice::}{close()} the file to return the file
    descriptor back to the operating system.

    Now, let us move on to the the \c save() slot.

    \code
65    void Notepad::save()
66    {
67        QString fileName = QFileDialog::getSaveFileName(this, tr("Save File"), "",
68        tr("Text Files (*.txt);;C++ Files (*.cpp *.h)"));
69
70        if (fileName != "") {
71            QFile file(fileName);
72            if (!file.open(QIODevice::WriteOnly)) {
73                // error message
74            } else {
75                QTextStream stream(&file);
76                stream << textEdit->toPlainText();
77                stream.flush();
78                file.close();
79            }
80        }
81    }
    \endcode

    When we write the contents of the text edit to the file, we use
    the QTextStream class again. QTextStream can also write
    \l{QString}s to the file with the << operator.

    \section2 Learn More

    \table
        \header
            \o About
            \o Here
        \row
            \o Files and I/O devices
            \o QFile, QIODevice
    \endtable

    \omit
    \section1 Moving On

    This may not be true for the first release. 
    The Qt documentation comes with three getting started guides. You
    have come to the end of the first, which concerns itself with
    basic Qt concepts. We also have guides covering intermediate and
    advanced topics. They are found here: You may also have noticed that the learn more sections in
    this guide frequently linked to them.
    Basic Qt Architecture
    \endomit
*/