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29 \example widgets/imageviewer
30 \title Image Viewer Example
32 The example shows how to combine QLabel and QScrollArea to
33 display an image. QLabel is typically used for displaying text,
34 but it can also display an image. QScrollArea provides a
35 scrolling view around another widget. If the child widget exceeds
36 the size of the frame, QScrollArea automatically provides scroll
39 The example demonstrates how QLabel's ability to scale its
40 contents (QLabel::scaledContents), and QScrollArea's ability to
41 automatically resize its contents (QScrollArea::widgetResizable),
42 can be used to implement zooming and scaling features. In
43 addition the example shows how to use QPainter to print an image.
45 \image imageviewer-example.png Screenshot of the Image Viewer example
47 With the Image Viewer application, the users can view an image of
48 their choice. The \uicontrol File menu gives the user the possibility
52 \li \uicontrol{Open...} - Open an image file
53 \li \uicontrol{Print...} - Print an image
54 \li \uicontrol{Exit} - Exit the application
57 Once an image is loaded, the \uicontrol View menu allows the users to:
60 \li \uicontrol{Zoom In} - Scale the image up by 25%
61 \li \uicontrol{Zoom Out} - Scale the image down by 25%
62 \li \uicontrol{Normal Size} - Show the image at its original size
63 \li \uicontrol{Fit to Window} - Stretch the image to occupy the entire window
66 In addition the \uicontrol Help menu provides the users with information
67 about the Image Viewer example in particular, and about Qt in
70 \section1 ImageViewer Class Definition
72 \snippet widgets/imageviewer/imageviewer.h 0
74 The \c ImageViewer class inherits from QMainWindow. We reimplement
75 the constructor, and create several private slots to facilitate
76 the menu entries. In addition we create four private functions.
78 We use \c createActions() and \c createMenus() when constructing
79 the \c ImageViewer widget. We use the \c updateActions() function
80 to update the menu entries when a new image is loaded, or when
81 the \uicontrol {Fit to Window} option is toggled. The zoom slots use \c
82 scaleImage() to perform the zooming. In turn, \c
83 scaleImage() uses \c adjustScrollBar() to preserve the focal point after
86 \section1 ImageViewer Class Implementation
88 \snippet widgets/imageviewer/imageviewer.cpp 0
90 In the constructor we first create the label and the scroll area.
92 We set \c {imageLabel}'s size policy to \l
93 {QSizePolicy::Ignored}{ignored}, making the users able to scale
94 the image to whatever size they want when the \uicontrol {Fit to Window}
95 option is turned on. Otherwise, the default size polizy (\l
96 {QSizePolicy::Preferred}{preferred}) will make scroll bars appear
97 when the scroll area becomes smaller than the label's minimum size
100 We ensure that the label will scale its contents to fill all
101 available space, to enable the image to scale properly when
102 zooming. If we omitted to set the \c {imageLabel}'s \l
103 {QLabel::scaledContents}{scaledContents} property, zooming in
104 would enlarge the QLabel, but leave the pixmap at
105 its original size, exposing the QLabel's background.
107 We make \c imageLabel the scroll area's child widget, and we make
108 \c scrollArea the central widget of the QMainWindow. At the end
109 we create the associated actions and menus, and customize the \c
110 {ImageViewer}'s appearance.
112 \snippet widgets/imageviewer/imageviewer.cpp 1
113 \snippet widgets/imageviewer/imageviewer.cpp 2
115 In the \c open() slot, we show a file dialog to the user. The
116 easiest way to create a QFileDialog is to use the static
117 convenience functions. QFileDialog::getOpenFileName() returns an
118 existing file selected by the user. If the user presses \uicontrol
119 Cancel, QFileDialog returns an empty string.
121 Unless the file name is a empty string, we check if the file's
122 format is an image format by constructing a QImage which tries to
123 load the image from the file. If the constructor returns a null
124 image, we use a QMessageBox to alert the user.
126 The QMessageBox class provides a modal dialog with a short
127 message, an icon, and some buttons. As with QFileDialog the
128 easiest way to create a QMessageBox is to use its static
129 convenience functions. QMessageBox provides a range of different
130 messages arranged along two axes: severity (question,
131 information, warning and critical) and complexity (the number of
132 necessary response buttons). In this particular example an
133 information message with an \uicontrol OK button (the default) is
134 sufficient, since the message is part of a normal operation.
136 \snippet widgets/imageviewer/imageviewer.cpp 3
137 \snippet widgets/imageviewer/imageviewer.cpp 4
139 If the format is supported, we display the image in \c imageLabel
140 by setting the label's \l {QLabel::pixmap}{pixmap}. Then we enable
141 the \uicontrol Print and \uicontrol {Fit to Window} menu entries and update
142 the rest of the view menu entries. The \uicontrol Open and \uicontrol Exit
143 entries are enabled by default.
145 If the \uicontrol {Fit to Window} option is turned off, the
146 QScrollArea::widgetResizable property is \c false and it is
147 our responsibility (not QScrollArea's) to give the QLabel a
148 reasonable size based on its contents. We call
149 \{QWidget::adjustSize()}{adjustSize()} to achieve this, which is
150 essentially the same as
152 \snippet doc/src/snippets/code/doc_src_examples_imageviewer.cpp 0
154 In the \c print() slot, we first make sure that an image has been
155 loaded into the application:
157 \snippet widgets/imageviewer/imageviewer.cpp 5
158 \snippet widgets/imageviewer/imageviewer.cpp 6
160 If the application is built in debug mode, the \c Q_ASSERT() macro
163 \snippet doc/src/snippets/code/doc_src_examples_imageviewer.cpp 1
165 In release mode, the macro simply disappear. The mode can be set
166 in the application's \c .pro file. One way to do so is to add an
167 option to \uicontrol qmake when building the application:
169 \snippet doc/src/snippets/code/doc_src_examples_imageviewer.qdoc 2
173 \snippet doc/src/snippets/code/doc_src_examples_imageviewer.qdoc 3
175 Another approach is to add this line directly to the \c .pro
178 \snippet widgets/imageviewer/imageviewer.cpp 7
179 \snippet widgets/imageviewer/imageviewer.cpp 8
181 Then we present a print dialog allowing the user to choose a
182 printer and to set a few options. We construct a painter with a
183 QPrinter as the paint device. We set the painter's window
184 and viewport in such a way that the image is as large as possible
185 on the paper, but without altering its
186 \l{Qt::KeepAspectRatio}{aspect ratio}.
188 In the end we draw the pixmap at position (0, 0).
190 \snippet widgets/imageviewer/imageviewer.cpp 9
191 \snippet widgets/imageviewer/imageviewer.cpp 10
193 We implement the zooming slots using the private \c scaleImage()
194 function. We set the scaling factors to 1.25 and 0.8,
195 respectively. These factor values ensure that a \uicontrol {Zoom In}
196 action and a \uicontrol {Zoom Out} action will cancel each other (since
197 1.25 * 0.8 == 1), and in that way the normal image size can be
198 restored using the zooming features.
200 The screenshots below show an image in its normal size, and the
201 same image after zooming in:
205 \li \inlineimage imageviewer-original_size.png
206 \li \inlineimage imageviewer-zoom_in_1.png
207 \li \inlineimage imageviewer-zoom_in_2.png
210 \snippet widgets/imageviewer/imageviewer.cpp 11
211 \snippet widgets/imageviewer/imageviewer.cpp 12
213 When zooming, we use the QLabel's ability to scale its contents.
214 Such scaling doesn't change the actual size hint of the contents.
215 And since the \l {QLabel::adjustSize()}{adjustSize()} function
216 use those size hint, the only thing we need to do to restore the
217 normal size of the currently displayed image is to call \c
218 adjustSize() and reset the scale factor to 1.0.
220 \snippet widgets/imageviewer/imageviewer.cpp 13
221 \snippet widgets/imageviewer/imageviewer.cpp 14
223 The \c fitToWindow() slot is called each time the user toggled
224 the \uicontrol {Fit to Window} option. If the slot is called to turn on
225 the option, we tell the scroll area to resize its child widget
226 with the QScrollArea::setWidgetResizable() function. Then we
227 disable the \uicontrol {Zoom In}, \uicontrol {Zoom Out} and \uicontrol {Normal
228 Size} menu entries using the private \c updateActions() function.
230 If the \l {QScrollArea::widgetResizable} property is set to \c
231 false (the default), the scroll area honors the size of its child
232 widget. If this property is set to \c true, the scroll area will
233 automatically resize the widget in order to avoid scroll bars
234 where they can be avoided, or to take advantage of extra space.
235 But the scroll area will honor the minimum size hint of its child
236 widget independent of the widget resizable property. So in this
237 example we set \c {imageLabel}'s size policy to \l
238 {QSizePolicy::Ignored}{ignored} in the constructor, to avoid that
239 scroll bars appear when the scroll area becomes smaller than the
240 label's minimum size hint.
242 The screenshots below shows an image in its normal size, and the
243 same image with the \uicontrol {Fit to window} option turned on.
244 Enlarging the window will stretch the image further, as shown in
245 the third screenshot.
249 \li \inlineimage imageviewer-original_size.png
250 \li \inlineimage imageviewer-fit_to_window_1.png
251 \li \inlineimage imageviewer-fit_to_window_2.png
254 If the slot is called to turn off the option, the
255 {QScrollArea::setWidgetResizable} property is set to \c false. We
256 also restore the image pixmap to its normal size by adjusting the
257 label's size to its content. And in the end we update the view
260 \snippet widgets/imageviewer/imageviewer.cpp 15
261 \snippet widgets/imageviewer/imageviewer.cpp 16
263 We implement the \c about() slot to create a message box
264 describing what the example is designed to show.
266 \snippet widgets/imageviewer/imageviewer.cpp 17
267 \snippet widgets/imageviewer/imageviewer.cpp 18
269 In the private \c createAction() function, we create the
270 actions providing the application features.
272 We assign a short-cut key to each action and connect them to the
273 appropriate slots. We only enable the \c openAct and \c exitAct at
274 the time of creation, the others are updated once an image has
275 been loaded into the application. In addition we make the \c
276 fitToWindowAct \l {QAction::checkable}{checkable}.
278 \snippet widgets/imageviewer/imageviewer.cpp 19
279 \snippet widgets/imageviewer/imageviewer.cpp 20
281 In the private \c createMenu() function, we add the previously
282 created actions to the \uicontrol File, \uicontrol View and \uicontrol Help menus.
284 The QMenu class provides a menu widget for use in menu bars,
285 context menus, and other popup menus. The QMenuBar class provides
286 a horizontal menu bar that consists of a list of pull-down menu
287 items. So at the end we put the menus in the \c {ImageViewer}'s
288 menu bar which we retrieve with the QMainWindow::menuBar()
291 \snippet widgets/imageviewer/imageviewer.cpp 21
292 \snippet widgets/imageviewer/imageviewer.cpp 22
294 The private \c updateActions() function enables or disables the
295 \uicontrol {Zoom In}, \uicontrol {Zoom Out} and \uicontrol {Normal Size} menu
296 entries depending on whether the \uicontrol {Fit to Window} option is
299 \snippet widgets/imageviewer/imageviewer.cpp 23
300 \snippet widgets/imageviewer/imageviewer.cpp 24
302 In \c scaleImage(), we use the \c factor parameter to calculate
303 the new scaling factor for the displayed image, and resize \c
304 imageLabel. Since we set the
305 \l{QLabel::scaledContents}{scaledContents} property to \c true in
306 the constructor, the call to QWidget::resize() will scale the
307 image displayed in the label. We also adjust the scroll bars to
308 preserve the focal point of the image.
310 At the end, if the scale factor is less than 33.3% or greater
311 than 300%, we disable the respective menu entry to prevent the
312 image pixmap from becoming too large, consuming too much
313 resources in the window system.
315 \snippet widgets/imageviewer/imageviewer.cpp 25
316 \snippet widgets/imageviewer/imageviewer.cpp 26
318 Whenever we zoom in or out, we need to adjust the scroll bars in
319 consequence. It would have been tempting to simply call
321 \snippet doc/src/snippets/code/doc_src_examples_imageviewer.cpp 4
323 but this would make the top-left corner the focal point, not the
324 center. Therefore we need to take into account the scroll bar
325 handle's size (the \l{QScrollBar::pageStep}{page step}).