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/*!

    \example widgets/scribble

    \title Scribble Example

    The Scribble example shows how to reimplement some of QWidget's

    event handlers to receive the events generated for the

    application's widgets.

    We reimplement the mouse event handlers to implement drawing, the

    paint event handler to update the application and the resize event

    handler to optimize the application's appearance. In addition we

    reimplement the close event handler to intercept the close events

    before terminating the application.

    The example also demonstrates how to use QPainter to draw an image

    in real time, as well as to repaint widgets.

    \image scribble-example.png Screenshot of the Scribble example

    With the Scribble application the users can draw an image.  The

    \gui File menu gives the users the possibility to open and edit an

    existing image file, save an image and exit the application. While

    drawing, the \gui Options menu allows the users to to choose the

    pen color and pen width, as well as clear the screen. In addition

    the \gui Help menu provides the users with information about the

    Scribble example in particular, and about Qt in general.

    The example consists of two classes:

    \list

    \o \c ScribbleArea is a custom widget that displays a QImage and

       allows to the user to draw on it.

    \o \c MainWindow provides a menu above the \c ScribbleArea.

    \endlist

    We will start by reviewing the \c ScribbleArea class, which

    contains the interesting, then we will take a look at the \c

    MainWindow class that uses it.

    \section1 ScribbleArea Class Definition

    \snippet examples/widgets/scribble/scribblearea.h 0

    The \c ScribbleArea class inherits from QWidget. We reimplement

    the \c mousePressEvent(), \c mouseMoveEvent() and \c

    mouseReleaseEvent() functions to implement the drawing. We

    reimplement the \c paintEvent() function to update the scribble

    area, and the \c resizeEvent() function to ensure that the QImage

    on which we draw is at least as large as the widget at any time.

    We need several public functions: \c openImage() loads an image

    from a file into the scribble area, allowing the user to edit the

    image; \c save() writes the currently displayed image to file; \c

    clearImage() slot clears the image displayed in the scribble

    area. We need the private \c drawLineTo() function to actually do

    the drawing, and \c resizeImage() to change the size of a

    QImage. The \c print() slot handles printing.

    We also need the following private variables:

    \list

    \o \c modified is \c true if there are unsaved

       changes to the image displayed in the scribble area.

    \o \c scribbling is \c true while the user is pressing

       the left mouse button within the scribble area.

    \o \c penWidth and \c penColor hold the currently

       set width and color for the pen used in the application.

    \o \c image stores the image drawn by the user.

    \o \c lastPoint holds the position of the cursor at the last

       mouse press or mouse move event.

    \endlist

    \section1 ScribbleArea Class Implementation

    \snippet examples/widgets/scribble/scribblearea.cpp 0

    In the constructor, we set the Qt::WA_StaticContents

    attribute for the widget, indicating that the widget contents are

    rooted to the top-left corner and don't change when the widget is

    resized. Qt uses this attribute to optimize paint events on

    resizes. This is purely an optimization and should only be used

    for widgets whose contents are static and rooted to the top-left

    corner.

    \snippet examples/widgets/scribble/scribblearea.cpp 1

    \snippet examples/widgets/scribble/scribblearea.cpp 2

    In the \c openImage() function, we load the given image. Then we

    resize the loaded QImage to be at least as large as the widget in

    both directions using the private \c resizeImage() function and

    we set the \c image member variable to be the loaded image. At

    the end, we call QWidget::update() to schedule a repaint.

    \snippet examples/widgets/scribble/scribblearea.cpp 3

    \snippet examples/widgets/scribble/scribblearea.cpp 4

    The \c saveImage() function creates a QImage object that covers

    only the visible section of the actual \c image and saves it using

    QImage::save(). If the image is successfully saved, we set the

    scribble area's \c modified variable to \c false, because there is

    no unsaved data.

    \snippet examples/widgets/scribble/scribblearea.cpp 5

    \snippet examples/widgets/scribble/scribblearea.cpp 6

    \codeline

    \snippet examples/widgets/scribble/scribblearea.cpp 7

    \snippet examples/widgets/scribble/scribblearea.cpp 8

    The \c setPenColor() and \c setPenWidth() functions set the

    current pen color and width. These values will be used for future

    drawing operations.

    \snippet examples/widgets/scribble/scribblearea.cpp 9

    \snippet examples/widgets/scribble/scribblearea.cpp 10

    The public \c clearImage() slot clears the image displayed in the

    scribble area. We simply fill the entire image with white, which

    corresponds to RGB value (255, 255, 255). As usual when we modify

    the image, we set \c modified to \c true and schedule a repaint.

    \snippet examples/widgets/scribble/scribblearea.cpp 11

    \snippet examples/widgets/scribble/scribblearea.cpp 12

    For mouse press and mouse release events, we use the

    QMouseEvent::button() function to find out which button caused

    the event. For mose move events, we use QMouseEvent::buttons()

    to find which buttons are currently held down (as an OR-combination).

    If the users press the left mouse button, we store the position

    of the mouse cursor in \c lastPoint. We also make a note that the

    user is currently scribbling. (The \c scribbling variable is

    necessary because we can't assume that a mouse move and mouse

    release event is always preceded by a mouse press event on the

    same widget.)

    If the user moves the mouse with the left button pressed down or

    releases the button, we call the private \c drawLineTo() function

    to draw.

    \snippet examples/widgets/scribble/scribblearea.cpp 13

    \snippet examples/widgets/scribble/scribblearea.cpp 14

    In the reimplementation of the \l

    {QWidget::paintEvent()}{paintEvent()} function, we simply create

    a QPainter for the scribble area, and draw the image.

    At this point, you might wonder why we don't just draw directly

    onto the widget instead of drawing in a QImage and copying the

    QImage onto screen in \c paintEvent(). There are at least three

    good reasons for this:

    \list

    \o The window system requires us to be able to redraw the widget

       \e{at any time}. For example, if the window is minimized and

       restored, the window system might have forgotten the contents

       of the widget and send us a paint event. In other words, we

       can't rely on the window system to remember our image.

    \o Qt normally doesn't allow us to paint outside of \c

       paintEvent(). In particular, we can't paint from the mouse

       event handlers. (This behavior can be changed using the

       Qt::WA_PaintOnScreen widget attribute, though.)

    \o If initialized properly, a QImage is guaranteed to use 8-bit

       for each color channel (red, green, blue, and alpha), whereas

       a QWidget might have a lower color depth, depending on the

       monitor configuration. This means that if we load a 24-bit or

       32-bit image and paint it onto a QWidget, then copy the

       QWidget into a QImage again, we might lose some information.

    \endlist

    \snippet examples/widgets/scribble/scribblearea.cpp 15

    \snippet examples/widgets/scribble/scribblearea.cpp 16

    When the user starts the Scribble application, a resize event is

    generated and an image is created and displayed in the scribble

    area. We make this initial image slightly larger than the

    application's main window and scribble area, to avoid always

    resizing the image when the user resizes the main window (which

    would be very inefficient). But when the main window becomes

    larger than this initial size, the image needs to be resized.

    \snippet examples/widgets/scribble/scribblearea.cpp 17

    \snippet examples/widgets/scribble/scribblearea.cpp 18

    In \c drawLineTo(), we draw a line from the point where the mouse

    was located when the last mouse press or mouse move occurred, we

    set \c modified to true, we generate a repaint event, and we

    update \c lastPoint so that next time \c drawLineTo() is called,

    we continue drawing from where we left.

    We could call the \c update() function with no parameter, but as

    an easy optimization we pass a QRect that specifies the rectangle

    inside the scribble are needs updating, to avoid a complete

    repaint of the widget.

    \snippet examples/widgets/scribble/scribblearea.cpp 19

    \snippet examples/widgets/scribble/scribblearea.cpp 20

    QImage has no nice API for resizing an image. There's a

    QImage::copy() function that could do the trick, but when used to

    expand an image, it fills the new areas with black, whereas we

    want white.

    So the trick is to create a brand new QImage with the right size,

    to fill it with white, and to draw the old image onto it using

    QPainter. The new image is given the QImage::Format_RGB32

    format, which means that each pixel is stored as 0xffRRGGBB

    (where RR, GG, and BB are the red, green and blue

    color channels, ff is the hexadecimal value 255).

    Printing is handled by the \c print() slot:

    \snippet examples/widgets/scribble/scribblearea.cpp 21

    We construct a high resolution QPrinter object for the required

    output format, using a QPrintDialog to ask the user to specify a

    page size and indicate how the output should be formatted on the page.

    If the dialog is accepted, we perform the task of printing to the paint

    device:

    \snippet examples/widgets/scribble/scribblearea.cpp 22

    Printing an image to a file in this way is simply a matter of

    painting onto the QPrinter. We scale the image to fit within the

    available space on the page before painting it onto the paint

    device.

    \section1 MainWindow Class Definition

    \snippet examples/widgets/scribble/mainwindow.h 0

    The \c MainWindow class inherits from QMainWindow. We reimplement

    the \l{QWidget::closeEvent()}{closeEvent()} handler from QWidget.

    The \c open(), \c save(), \c penColor() and \c penWidth()

    slots correspond to menu entries. In addition we create four

    private functions.

    We use the boolean \c maybeSave() function to check if there are

    any unsaved changes. If there are unsaved changes, we give the

    user the opportunity to save these changes. The function returns

    \c false if the user clicks \gui Cancel. We use the \c saveFile()

    function to let the user save the image currently displayed in

    the scribble area.

    \section1 MainWindow Class Implementation

    \snippet examples/widgets/scribble/mainwindow.cpp 0

    In the constructor, we create a scribble area which we make the

    central widget of the \c MainWindow widget. Then we create the

    associated actions and menus.

    \snippet examples/widgets/scribble/mainwindow.cpp 1

    \snippet examples/widgets/scribble/mainwindow.cpp 2

    Close events are sent to widgets that the users want to close,

    usually by clicking \gui{File|Exit} or by clicking the \gui X

    title bar button. By reimplementing the event handler, we can

    intercept attempts to close the application.

    In this example, we use the close event to ask the user to save

    any unsaved changes. The logic for that is located in the \c

    maybeSave() function. If \c maybeSave() returns true, there are

    no modifications or the users successfully saved them, and we

    accept the event. The application can then terminate normally. If

    \c maybeSave() returns false, the user clicked \gui Cancel, so we

    "ignore" the event, leaving the application unaffected by it.

    \snippet examples/widgets/scribble/mainwindow.cpp 3

    \snippet examples/widgets/scribble/mainwindow.cpp 4

    In the \c open() slot we first give the user the opportunity to

    save any modifications to the currently displayed image, before a

    new image is loaded into the scribble area. Then we ask the user

    to choose a file and we load the file in the \c ScribbleArea.

    \snippet examples/widgets/scribble/mainwindow.cpp 5

    \snippet examples/widgets/scribble/mainwindow.cpp 6

    The \c save() slot is called when the users choose the \gui {Save

    As} menu entry, and then choose an entry from the format menu. The

    first thing we need to do is to find out which action sent the

    signal using QObject::sender(). This function returns the sender

    as a QObject pointer. Since we know that the sender is an action

    object, we can safely cast the QObject. We could have used a

    C-style cast or a C++ \c static_cast<>(), but as a defensive

    programming technique we use a qobject_cast(). The advantage is

    that if the object has the wrong type, a null pointer is

    returned. Crashes due to null pointers are much easier to diagnose

    than crashes due to unsafe casts.

    Once we have the action, we extract the chosen format using

    QAction::data(). (When the actions are created, we use

    QAction::setData() to set our own custom data attached to the

    action, as a QVariant. More on this when we review \c

    createActions().)

    Now that we know the format, we call the private \c saveFile()

    function to save the currently displayed image.

    \snippet examples/widgets/scribble/mainwindow.cpp 7

    \snippet examples/widgets/scribble/mainwindow.cpp 8

    We use the \c penColor() slot to retrieve a new color from the

    user with a QColorDialog. If the user chooses a new color, we

    make it the scribble area's color.

    \snippet examples/widgets/scribble/mainwindow.cpp 9

    \snippet examples/widgets/scribble/mainwindow.cpp 10

    To retrieve a new pen width in the \c penWidth() slot, we use

    QInputDialog. The QInputDialog class provides a simple

    convenience dialog to get a single value from the user. We use

    the static QInputDialog::getInt() function, which combines a

    QLabel and a QSpinBox. The QSpinBox is initialized with the

    scribble area's pen width, allows a range from 1 to 50, a step of

    1 (meaning that the up and down arrow increment or decrement the

    value by 1).

    The boolean \c ok variable will be set to \c true if the user

    clicked \gui OK and to \c false if the user pressed \gui Cancel.

    \snippet examples/widgets/scribble/mainwindow.cpp 11

    \snippet examples/widgets/scribble/mainwindow.cpp 12

    We implement the \c about() slot to create a message box

    describing what the example is designed to show.

    \snippet examples/widgets/scribble/mainwindow.cpp 13

    \snippet examples/widgets/scribble/mainwindow.cpp 14

    In the \c createAction() function we create the actions

    representing the menu entries and connect them to the appropiate

    slots. In particular we create the actions found in the \gui

    {Save As} sub-menu. We use QImageWriter::supportedImageFormats()

    to get a list of the supported formats (as a QList<QByteArray>).

    Then we iterate through the list, creating an action for each

    format. We call QAction::setData() with the file format, so we

    can retrieve it later as QAction::data(). We could also have

    deduced the file format from the action's text, by truncating the

    "...", but that would have been inelegant.

    \snippet examples/widgets/scribble/mainwindow.cpp 15

    \snippet examples/widgets/scribble/mainwindow.cpp 16

    In the \c createMenu() function, we add the previously created

    format actions to the \c saveAsMenu. Then we add the rest of the

    actions as well as the \c saveAsMenu sub-menu to the \gui File,

    \gui Options and \gui Help menus.

    The QMenu class provides a menu widget for use in menu bars,

    context menus, and other popup menus. The QMenuBar class provides

    a horizontal menu bar with a list of pull-down \l{QMenu}s. At the

    end we put the \gui File and \gui Options menus in the \c

    {MainWindow}'s menu bar, which we retrieve using the

    QMainWindow::menuBar() function.

    \snippet examples/widgets/scribble/mainwindow.cpp 17

    \snippet examples/widgets/scribble/mainwindow.cpp 18

    In \c mayBeSave(), we check if there are any unsaved changes. If

    there are any, we use QMessageBox to give the user a warning that

    the image has been modified and the opportunity to save the

    modifications.

    As with QColorDialog and QFileDialog, the easiest way to create a

    QMessageBox is to use its static functions. QMessageBox provides

    a range of different messages arranged along two axes: severity

    (question, information, warning and critical) and complexity (the

    number of necessary response buttons). Here we use the \c

    warning() function sice the message is rather important.

    If the user chooses to save, we call the private \c saveFile()

    function. For simplicitly, we use PNG as the file format; the

    user can always press \gui Cancel and save the file using another

    format.

    The \c maybeSave() function returns \c false if the user clicks

    \gui Cancel; otherwise it returns \c true.

    \snippet examples/widgets/scribble/mainwindow.cpp 19

    \snippet examples/widgets/scribble/mainwindow.cpp 20

    In \c saveFile(), we pop up a file dialog with a file name

    suggestion. The static QFileDialog::getSaveFileName() function

    returns a file name selected by the user. The file does not have

    to exist.

*/