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    41 

    42 /*!

    43     \example widgets/scribble

    44     \title Scribble Example

    45 

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

    47     event handlers to receive the events generated for the

    48     application's widgets.

    49 

    50     We reimplement the mouse event handlers to implement drawing, the

    51     paint event handler to update the application and the resize event

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

    53     reimplement the close event handler to intercept the close events

    54     before terminating the application.

    55 

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

    57     in real time, as well as to repaint widgets.

    58 

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

    60 

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

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

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

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

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

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

    67     Scribble example in particular, and about Qt in general.

    68 

    69     The example consists of two classes:

    70 

    71     \list

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

    73        allows to the user to draw on it.

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

    75     \endlist

    76 

    77     We will start by reviewing the \c ScribbleArea class. Then we will

    78     review the \c MainWindow class, which uses \c ScribbleArea.

    79 

    80     \section1 ScribbleArea Class Definition

    81 

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

    83 

    84     The \c ScribbleArea class inherits from QWidget. We reimplement

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

    86     mouseReleaseEvent() functions to implement the drawing. We

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

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

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

    90 

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

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

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

    94     clearImage() slot clears the image displayed in the scribble

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

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

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

    98 

    99     We also need the following private variables:

   100 

   101     \list

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

   103        changes to the image displayed in the scribble area.

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

   105        the left mouse button within the scribble area.

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

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

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

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

   110        mouse press or mouse move event.

   111     \endlist

   112 

   113     \section1 ScribbleArea Class Implementation

   114 

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

   116 

   117     In the constructor, we set the Qt::WA_StaticContents

   118     attribute for the widget, indicating that the widget contents are

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

   120     resized. Qt uses this attribute to optimize paint events on

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

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

   123     corner.

   124 

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

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

   127 

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

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

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

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

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

   133 

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

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

   136 

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

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

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

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

   141     no unsaved data.

   142 

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

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

   145     \codeline

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

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

   148 

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

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

   151     drawing operations.

   152 

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

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

   155 

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

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

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

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

   160 

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

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

   163 

   164     For mouse press and mouse release events, we use the

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

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

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

   168 

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

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

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

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

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

   174     same widget.)

   175 

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

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

   178     to draw.

   179 

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

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

   182 

   183     In the reimplementation of the \l

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

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

   186 

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

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

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

   190     good reasons for this:

   191 

   192     \list

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

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

   195        restored, the window system might have forgotten the contents

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

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

   198 

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

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

   201        event handlers. (This behavior can be changed using the

   202        Qt::WA_PaintOnScreen widget attribute, though.)

   203 

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

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

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

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

   208        32-bit image and paint it onto a QWidget, then copy the

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

   210     \endlist

   211 

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

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

   214 

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

   216     generated and an image is created and displayed in the scribble

   217     area. We make this initial image slightly larger than the

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

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

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

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

   222 

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

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

   225 

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

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

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

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

   230     we continue drawing from where we left.

   231 

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

   233     an easy optimization we pass a QRect that specifies the rectangle

   234     inside the scribble are needs updating, to avoid a complete

   235     repaint of the widget.

   236 

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

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

   239 

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

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

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

   243     want white.

   244 

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

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

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

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

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

   250     color channels, ff is the hexadecimal value 255).

   251 

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

   253 

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

   255 

   256     We construct a high resolution QPrinter object for the required

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

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

   259 

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

   261     device:

   262 

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

   264 

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

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

   267     available space on the page before painting it onto the paint

   268     device.

   269 

   270     \section1 MainWindow Class Definition

   271 

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

   273 

   274     The \c MainWindow class inherits from QMainWindow. We reimplement

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

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

   277     slots correspond to menu entries. In addition we create four

   278     private functions.

   279 

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

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

   282     user the opportunity to save these changes. The function returns

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

   284     function to let the user save the image currently displayed in

   285     the scribble area.

   286 

   287     \section1 MainWindow Class Implementation

   288 

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

   290 

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

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

   293     associated actions and menus.

   294 

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

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

   297 

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

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

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

   301     intercept attempts to close the application.

   302 

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

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

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

   306     no modifications or the users successfully saved them, and we

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

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

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

   310 

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

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

   313 

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

   315     save any modifications to the currently displayed image, before a

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

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

   318 

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

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

   321 

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

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

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

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

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

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

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

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

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

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

   332     than crashes due to unsafe casts.

   333 

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

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

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

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

   338     createActions().)

   339 

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

   341     function to save the currently displayed image.

   342 

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

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

   345 

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

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

   348     make it the scribble area's color.

   349 

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

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

   352 

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

   354     QInputDialog. The QInputDialog class provides a simple

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

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

   357     QLabel and a QSpinBox. The QSpinBox is initialized with the

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

   359     1 (meaning that the up and down arrow increment or decrement the

   360     value by 1).

   361 

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

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

   364 

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

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

   367 

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

   369     describing what the example is designed to show.

   370 

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

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

   373 

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

   375     representing the menu entries and connect them to the appropiate

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

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

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

   379 

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

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

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

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

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

   385 

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

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

   388 

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

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

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

   392     \gui Options and \gui Help menus.

   393 

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

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

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

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

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

   399     QMainWindow::menuBar() function.

   400 

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

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

   403 

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

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

   406     the image has been modified and the opportunity to save the

   407     modifications.

   408 

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

   410     QMessageBox is to use its static functions. QMessageBox provides

   411     a range of different messages arranged along two axes: severity

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

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

   414     warning() function sice the message is rather important.

   415 

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

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

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

   419     format.

   420 

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

   422     \gui Cancel; otherwise it returns \c true.

   423 

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

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

   426 

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

   428     suggestion. The static QFileDialog::getSaveFileName() function

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

   430     to exist.

   431 */