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29 \example desktop/screenshot
30 \title Screenshot Example
31 \ingroup examples-desktop
32 \brief The Screenshot example shows how to take a screenshot of the
35 \brief The Screenshot example shows how to take a screenshot of the
36 desktop using QApplication and QDesktopWidget. It also shows how
37 to use QTimer to provide a single-shot timer, and how to
38 reimplement the QWidget::resizeEvent() event handler to make sure
39 that an application resizes smoothly and without data loss.
41 \image screenshot-example.png
43 With the application the users can take a screenshot of their
44 desktop. They are provided with a couple of options:
47 \li Delaying the screenshot, giving them time to rearrange
49 \li Hiding the application's window while the screenshot is taken.
52 In addition the application allows the users to save their
53 screenshot if they want to.
55 \section1 Screenshot Class Definition
57 \snippet desktop/screenshot/screenshot.h 0
59 The \c Screenshot class inherits QWidget and is the application's
60 main widget. It displays the application options and a preview of
63 We reimplement the QWidget::resizeEvent() function to make sure
64 that the preview of the screenshot scales properly when the user
65 resizes the application widget. We also need several private slots
66 to facilitate the options:
69 \li The \c newScreenshot() slot prepares a new screenshot.
70 \li The \c saveScreenshot() slot saves the last screenshot.
71 \li The \c shootScreen() slot takes the screenshot.
72 \li The \c updateCheckBox() slot enables or disables the
73 \uicontrol {Hide This Window} option.
76 We also declare some private functions: We use the \c
77 createOptionsGroupBox(), \c createButtonsLayout() and \c
78 createButton() functions when we construct the widget. And we call
79 the private \c updateScreenshotLabel() function whenever a new
80 screenshot is taken or when a resize event changes the size of the
81 screenshot preview label.
83 In addition we need to store the screenshot's original pixmap. The
84 reason is that when we display the preview of the screenshot, we
85 need to scale its pixmap, storing the original we make sure that
86 no data are lost in that process.
88 \section1 Screenshot Class Implementation
90 \snippet desktop/screenshot/screenshot.cpp 0
92 In the constructor we first create the QLabel displaying the
95 We set the QLabel's size policy to be QSizePolicy::Expanding both
96 horizontally and vertically. This means that the QLabel's size
97 hint is a sensible size, but the widget can be shrunk and still be
98 useful. Also, the widget can make use of extra space, so it should
99 get as much space as possible. Then we make sure the QLabel is
100 aligned in the center of the \c Screenshot widget, and set its
103 We create the applications's buttons and the group box containing
104 the application's options, and put it all into a main
105 layout. Finally we take the initial screenshot, and set the initial
106 delay and the window title, before we resize the widget to a
109 \snippet desktop/screenshot/screenshot.cpp 1
111 The \c resizeEvent() function is reimplemented to receive the
112 resize events dispatched to the widget. The purpose is to scale
113 the preview screenshot pixmap without deformation of its content,
114 and also make sure that the application can be resized smoothly.
116 To achieve the first goal, we scale the screenshot pixmap using
117 Qt::KeepAspectRatio. We scale the pixmap to a rectangle as large
118 as possible inside the current size of the screenshot preview
119 label, preserving the aspect ratio. This means that if the user
120 resizes the application window in only one direction, the preview
121 screenshot keeps the same size.
123 To reach our second goal, we make sure that the preview screenshot
124 only is repainted (using the private \c updateScreenshotLabel()
125 function) when it actually changes its size.
127 \snippet desktop/screenshot/screenshot.cpp 2
129 The private \c newScreenshot() slot is called when the user
130 requests a new screenshot; but the slot only prepares a new
133 First we see if the \uicontrol {Hide This Window} option is checked, if
134 it is we hide the \c Screenshot widget. Then we disable the \uicontrol
135 {New Screenshot} button, to make sure the user only can request
136 one screenshot at a time.
138 We create a timer using the QTimer class which provides repetitive
139 and single-shot timers. We set the timer to time out only once,
140 using the static QTimer::singleShot() function. This function
141 calls the private \c shootScreen() slot after the time interval
142 specified by the \uicontrol {Screenshot Delay} option. It is \c
143 shootScreen() that actually performs the screenshot.
145 \snippet desktop/screenshot/screenshot.cpp 3
147 The \c saveScreenshot() slot is called when the user push the \uicontrol
148 Save button, and it presents a file dialog using the QFileDialog
151 QFileDialog enables a user to traverse the file system in order to
152 select one or many files or a directory. The easiest way to create
153 a QFileDialog is to use the convenience static
156 We define the default file format to be png, and we make the file
157 dialog's initial path the path the application is run from. We
158 create the file dialog using the static
159 QFileDialog::getSaveFileName() function which returns a file name
160 selected by the user. The file does not have to exist. If the file
161 name is valid, we use the QPixmap::save() function to save the
162 screenshot's original pixmap in that file.
164 \snippet desktop/screenshot/screenshot.cpp 4
166 The \c shootScreen() slot is called to take the screenshot. If the
167 user has chosen to delay the screenshot, we make the application
168 beep when the screenshot is taken using the static
169 QApplication::beep() function.
171 The QApplication class manages the GUI application's control flow
172 and main settings. It contains the main event loop, where all
173 events from the window system and other sources are processed and
176 \snippet desktop/screenshot/screenshot.cpp 5
178 Using the static function QApplication::primaryScreen(), we
179 obtain the QScreen object for the application's main screen.
181 We take the screenshot using the QScreen::grabWindow()
182 function. The function grabs the contents of the window passed as
183 an argument, makes a pixmap out of it and returns that pixmap.
184 The window id can be obtained with QWidget::winId() or QWindow::winId().
185 Here, however, we just pass 0 as the window id, indicating that we
186 want to grab the entire screen.
188 We update the screenshot preview label using the private \c
189 updateScreenshotLabel() function. Then we enable the \uicontrol {New
190 Screenshot} button, and finally we make the \c Screenshot widget
191 visible if it was hidden during the screenshot.
193 \snippet desktop/screenshot/screenshot.cpp 6
195 The \uicontrol {Hide This Window} option is enabled or disabled
196 depending on the delay of the screenshot. If there is no delay,
197 the application window cannot be hidden and the option's checkbox
200 The \c updateCheckBox() slot is called whenever the user changes
201 the delay using the \uicontrol {Screenshot Delay} option.
203 \snippet desktop/screenshot/screenshot.cpp 7
205 The private \c createOptionsGroupBox() function is called from the
208 First we create a group box that will contain all of the options'
209 widgets. Then we create a QSpinBox and a QLabel for the \uicontrol
210 {Screenshot Delay} option, and connect the spinbox to the \c
211 updateCheckBox() slot. Finally, we create a QCheckBox for the \uicontrol
212 {Hide This Window} option, add all the options' widgets to a
213 QGridLayout and install the layout on the group box.
215 Note that we don't have to specify any parents for the widgets
216 when we create them. The reason is that when we add a widget to a
217 layout and install the layout on another widget, the layout's
218 widgets are automatically reparented to the widget the layout is
221 \snippet desktop/screenshot/screenshot.cpp 8
223 The private \c createButtonsLayout() function is called from the
224 constructor. We create the application's buttons using the private
225 \c createButton() function, and add them to a QHBoxLayout.
227 \snippet desktop/screenshot/screenshot.cpp 9
229 The private \c createButton() function is called from the \c
230 createButtonsLayout() function. It simply creates a QPushButton
231 with the provided text, connects it to the provided receiver and
232 slot, and returns a pointer to the button.
234 \snippet desktop/screenshot/screenshot.cpp 10
236 The private \c updateScreenshotLabel() function is called whenever
237 the screenshot changes, or when a resize event changes the size of
238 the screenshot preview label. It updates the screenshot preview's
239 label using the QLabel::setPixmap() and QPixmap::scaled()
242 QPixmap::scaled() returns a copy of the given pixmap scaled to a
243 rectangle of the given size according to the given
244 Qt::AspectRatioMode and Qt::TransformationMode.
246 We scale the original pixmap to fit the current screenshot label's
247 size, preserving the aspect ratio and giving the resulting pixmap