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The QApplication class manages the GUI application's control flow and main settings. More...
Inherits QCoreApplication.
The QApplication class manages the GUI application's control flow and main settings.
QApplication contains the main event loop, where all events from the window system and other sources are processed and dispatched. It also handles the application's initialization, finalization, and provides session management. In addition, QApplication handles most of the system-wide and application-wide settings.
For any GUI application using Qt, there is precisely one QApplication object, no matter whether the application has 0, 1, 2 or more windows at any given time. For non-GUI Qt applications, use QCoreApplication instead, as it does not depend on the QtGui library.
The QApplication object is accessible through the instance() function that returns a pointer equivalent to the global qApp pointer.
QApplication's main areas of responsibility are:
Since the QApplication object does so much initialization, it must be created before any other objects related to the user interface are created. QApplication also deals with common command line arguments. Hence, it is usually a good idea to create it before any interpretation or modification of argv is done in the application itself.
| Constant | Value | Description |
|---|---|---|
| QApplication.NormalColor | 0 | the default color allocation policy |
| QApplication.CustomColor | 1 | the same as NormalColor for X11; allocates colors to a palette on demand under Windows |
| QApplication.ManyColor | 2 | the right choice for applications that use thousands of colors |
See setColorSpec() for full details.
| Constant | Value | Description |
|---|---|---|
| QApplication.Tty | 0 | a console application |
| QApplication.GuiClient | 1 | a GUI client application |
| QApplication.GuiServer | 2 | a GUI server application (for Qt for Embedded Linux) |
Initializes the window system and constructs an application object with argc command line arguments in argv.
Warning: The data referred to by argc and argv must stay valid for the entire lifetime of the QApplication object. In addition, argc must be greater than zero and argv must contain at least one valid character string.
The global qApp pointer refers to this application object. Only one application object should be created.
This application object must be constructed before any paint devices (including widgets, pixmaps, bitmaps etc.).
Note: argc and argv might be changed as Qt removes command line arguments that it recognizes.
Qt debugging options (not available if Qt was compiled without the QT_DEBUG flag defined):
See Debugging Techniques for a more detailed explanation.
All Qt programs automatically support the following command line options:
The X11 version of Qt supports some traditional X11 command line options:
If QApplication fails to open the X11 display, it will terminate the process. This behavior is consistent with most X11 applications.
See also arguments().
Constructs an application object with argc command line arguments in argv. If GUIenabled is true, a GUI application is constructed, otherwise a non-GUI (console) application is created.
Warning: The data referred to by argc and argv must stay valid for the entire lifetime of the QApplication object. In addition, argc must be greater than zero and argv must contain at least one valid character string.
Set GUIenabled to false for programs without a graphical user interface that should be able to run without a window system.
On X11, the window system is initialized if GUIenabled is true. If GUIenabled is false, the application does not connect to the X server. On Windows and Mac OS, currently the window system is always initialized, regardless of the value of GUIenabled. This may change in future versions of Qt.
The following example shows how to create an application that uses a graphical interface when available.
int main(int argc, char **argv) { #ifdef Q_WS_X11 bool useGUI = getenv("DISPLAY") != 0; #else bool useGUI = true; #endif QApplication app(argc, argv, useGUI); if (useGUI) { // start GUI version ... } else { // start non-GUI version ... } return app.exec(); }
Constructs an application object with argc command line arguments in argv.
Warning: The data referred to by argc and argv must stay valid for the entire lifetime of the QApplication object. In addition, argc must be greater than zero and argv must contain at least one valid character string.
With Qt for Embedded Linux, passing QApplication.GuiServer for type makes this application the server (equivalent to running with the -qws option).
Creates an application, given an already open display display. If visual and colormap are non-zero, the application will use those values as the default Visual and Colormap contexts.
Warning: Qt only supports TrueColor visuals at depths higher than 8 bits-per-pixel.
This function is only available on X11.
Creates an application, given an already open display and using argc command line arguments in argv. If visual and colormap are non-zero, the application will use those values as the default Visual and Colormap contexts.
Warning: Qt only supports TrueColor visuals at depths higher than 8 bits-per-pixel.
This function is only available on X11.
This method is also a Qt slot with the C++ signature void aboutQt().
Displays a simple message box about Qt. The message includes the version number of Qt being used by the application.
This is useful for inclusion in the Help menu of an application, as shown in the Menus example.
This function is a convenience slot for QMessageBox.aboutQt().
Returns the active modal widget.
A modal widget is a special top-level widget which is a subclass of QDialog that specifies the modal parameter of the constructor as true. A modal widget must be closed before the user can continue with other parts of the program.
Modal widgets are organized in a stack. This function returns the active modal widget at the top of the stack.
See also activePopupWidget() and topLevelWidgets().
Returns the active popup widget.
A popup widget is a special top-level widget that sets the Qt.WType_Popup widget flag, e.g. the QMenu widget. When the application opens a popup widget, all events are sent to the popup. Normal widgets and modal widgets cannot be accessed before the popup widget is closed.
Only other popup widgets may be opened when a popup widget is shown. The popup widgets are organized in a stack. This function returns the active popup widget at the top of the stack.
See also activeModalWidget() and topLevelWidgets().
Returns the application top-level window that has the keyboard input focus, or 0 if no application window has the focus. There might be an activeWindow() even if there is no focusWidget(), for example if no widget in that window accepts key events.
See also setActiveWindow(), QWidget.setFocus(), QWidget.hasFocus(), and focusWidget().
Causes an alert to be shown for widget if the window is not the active window. The alert is shown for msec miliseconds. If msec is zero (the default), then the alert is shown indefinitely until the window becomes active again.
Currently this function does nothing on Qt for Embedded Linux.
On Mac OS X, this works more at the application level and will cause the application icon to bounce in the dock.
On Windows, this causes the window's taskbar entry to flash for a time. If msec is zero, the flashing will stop and the taskbar entry will turn a different color (currently orange).
On X11, this will cause the window to be marked as "demands attention", the window must not be hidden (i.e. not have hide() called on it, but be visible in some sort of way) in order for this to work.
This function was introduced in Qt 4.3.
Returns a list of all the widgets in the application.
The list is empty (QList.isEmpty()) if there are no widgets.
Note: Some of the widgets may be hidden.
Example:
void updateAllWidgets() { foreach (QWidget *widget, QApplication.allWidgets()) widget->update(); }
See also topLevelWidgets() and QWidget.isVisible().
Sounds the bell, using the default volume and sound. The function is not available in Qt for Embedded Linux.
Changes the currently active application override cursor to cursor.
This function has no effect if setOverrideCursor() was not called.
See also setOverrideCursor(), overrideCursor(), restoreOverrideCursor(), and QWidget.setCursor().
Returns a pointer to the application global clipboard.
Note: The QApplication object should already be constructed before accessing the clipboard.
This method is also a Qt slot with the C++ signature void closeAllWindows().
Closes all top-level windows.
This function is particularly useful for applications with many top-level windows. It could, for example, be connected to a Exit entry in the File menu:
exitAct = new QAction(tr("E&xit"), this);
exitAct->setShortcuts(QKeySequence.Quit);
exitAct->setStatusTip(tr("Exit the application"));
connect(exitAct, SIGNAL(triggered()), qApp, SLOT(closeAllWindows()));
The windows are closed in random order, until one window does not accept the close event. The application quits when the last window was successfully closed; this can be turned off by setting quitOnLastWindowClosed to false.
See also quitOnLastWindowClosed, lastWindowClosed(), QWidget.close(), QWidget.closeEvent(), lastWindowClosed(), quit(), topLevelWidgets(), and QWidget.isWindow().
Returns the color specification.
See also QApplication.setColorSpec().
This function deals with session management. It is invoked when the QSessionManager wants the application to commit all its data.
Usually this means saving all open files, after getting permission from the user. Furthermore you may want to provide a means by which the user can cancel the shutdown.
You should not exit the application within this function. Instead, the session manager may or may not do this afterwards, depending on the context.
Warning: Within this function, no user interaction is possible, unless you ask the manager for explicit permission. See QSessionManager.allowsInteraction() and QSessionManager.allowsErrorInteraction() for details and example usage.
The default implementation requests interaction and sends a close event to all visible top-level widgets. If any event was rejected, the shutdown is canceled.
See also isSessionRestored(), sessionId(), saveState(), and Session Management.
Returns the desktop widget (also called the root window).
The desktop may be composed of multiple screens, so it would be incorrect, for example, to attempt to center some widget in the desktop's geometry. QDesktopWidget has various functions for obtaining useful geometries upon the desktop, such as QDesktopWidget.screenGeometry() and QDesktopWidget.availableGeometry().
On X11, it is also possible to draw on the desktop.
Returns true if Qt is set to use the system's standard colors, fonts, etc.; otherwise returns false. The default is true.
See also setDesktopSettingsAware().
Reimplemented from QObject.event().
Enters the main event loop and waits until exit() is called, then returns the value that was set to exit() (which is 0 if exit() is called via quit()).
It is necessary to call this function to start event handling. The main event loop receives events from the window system and dispatches these to the application widgets.
Generally, no user interaction can take place before calling exec(). As a special case, modal widgets like QMessageBox can be used before calling exec(), because modal widgets call exec() to start a local event loop.
To make your application perform idle processing, i.e., executing a special function whenever there are no pending events, use a QTimer with 0 timeout. More advanced idle processing schemes can be achieved using processEvents().
We recommend that you connect clean-up code to the aboutToQuit() signal, instead of putting it in your application's main() function. This is because, on some platforms the QApplication.exec() call may not return. For example, on the Windows platform, when the user logs off, the system terminates the process after Qt closes all top-level windows. Hence, there is no guarantee that the application will have time to exit its event loop and execute code at the end of the main() function, after the QApplication.exec() call.
See also quitOnLastWindowClosed, quit(), exit(), processEvents(), and QCoreApplication.exec().
Returns the application widget that has the keyboard input focus, or 0 if no widget in this application has the focus.
See also QWidget.setFocus(), QWidget.hasFocus(), activeWindow(), and focusChanged().
Returns the default application font.
See also setFont(), fontMetrics(), and QWidget.font().
This is an overloaded function.
Returns the default font for the widget.
See also fontMetrics() and QWidget.setFont().
This is an overloaded function.
Returns the font for widgets of the given className.
See also setFont() and QWidget.font().
Returns display (screen) font metrics for the application font.
See also font(), setFont(), QWidget.fontMetrics(), and QPainter.fontMetrics().
Returns the QInputContext instance used by the application.
See also setInputContext().
Returns true if effect is enabled; otherwise returns false.
By default, Qt will try to use the desktop settings. To prevent this, call setDesktopSettingsAware(false).
Note: All effects are disabled on screens running at less than 16-bit color depth.
See also setEffectEnabled() and Qt.UIEffect.
Returns true if the application's layout direction is Qt.LeftToRight; otherwise returns false.
See also layoutDirection() and isRightToLeft().
Returns true if the application's layout direction is Qt.RightToLeft; otherwise returns false.
See also layoutDirection() and isLeftToRight().
Returns true if the application has been restored from an earlier session; otherwise returns false.
See also sessionId(), commitData(), and saveState().
Returns the current keyboard input direction.
This function was introduced in Qt 4.2.
Returns the current keyboard input locale.
This function was introduced in Qt 4.2.
Returns the current state of the modifier keys on the keyboard. The current state is updated sychronously as the event queue is emptied of events that will spontaneously change the keyboard state (QEvent.KeyPress and QEvent.KeyRelease events).
It should be noted this may not reflect the actual keys held on the input device at the time of calling but rather the modifiers as last reported in one of the above events. If no keys are being held Qt.NoModifier is returned.
See also mouseButtons() and queryKeyboardModifiers().
Returns the current state of the buttons on the mouse. The current state is updated syncronously as the event queue is emptied of events that will spontaneously change the mouse state (QEvent.MouseButtonPress and QEvent.MouseButtonRelease events).
It should be noted this may not reflect the actual buttons held on the input device at the time of calling but rather the mouse buttons as last reported in one of the above events. If no mouse buttons are being held Qt.NoButton is returned.
See also keyboardModifiers().
Reimplemented from QCoreApplication.notify().
Returns the active application override cursor.
This function returns 0 if no application cursor has been defined (i.e. the internal cursor stack is empty).
See also setOverrideCursor() and restoreOverrideCursor().
Returns the application palette.
See also setPalette() and QWidget.palette().
This is an overloaded function.
If a widget is passed, the default palette for the widget's class is returned. This may or may not be the application palette. In most cases there is no special palette for certain types of widgets, but one notable exception is the popup menu under Windows, if the user has defined a special background color for menus in the display settings.
See also setPalette() and QWidget.palette().
This is an overloaded function.
Returns the palette for widgets of the given className.
See also setPalette() and QWidget.palette().
Queries and returns the state of the modifier keys on the keyboard. Unlike keyboardModifiers, this method returns the actual keys held on the input device at the time of calling the method.
It does not rely on the keypress events having been received by this process, which makes it possible to check the modifiers while moving a window, for instance. Note that in most cases, you should use keyboardModifiers(), which is faster and more accurate since it contains the state of the modifiers as they were when the currently processed event was received.
This function was introduced in Qt 4.8.
See also keyboardModifiers().
Undoes the last setOverrideCursor().
If setOverrideCursor() has been called twice, calling restoreOverrideCursor() will activate the first cursor set. Calling this function a second time restores the original widgets' cursors.
See also setOverrideCursor() and overrideCursor().
This function deals with session management. It is invoked when the session manager wants the application to preserve its state for a future session.
For example, a text editor would create a temporary file that includes the current contents of its edit buffers, the location of the cursor and other aspects of the current editing session.
You should never exit the application within this function. Instead, the session manager may or may not do this afterwards, depending on the context. Futhermore, most session managers will very likely request a saved state immediately after the application has been started. This permits the session manager to learn about the application's restart policy.
Warning: Within this function, no user interaction is possible, unless you ask the manager for explicit permission. See QSessionManager.allowsInteraction() and QSessionManager.allowsErrorInteraction() for details.
See also isSessionRestored(), sessionId(), commitData(), and Session Management.
Returns the current session's identifier.
If the application has been restored from an earlier session, this identifier is the same as it was in that previous session. The session identifier is guaranteed to be unique both for different applications and for different instances of the same application.
See also isSessionRestored(), sessionKey(), commitData(), and saveState().
Returns the session key in the current session.
If the application has been restored from an earlier session, this key is the same as it was when the previous session ended.
The session key changes with every call of commitData() or saveState().
See also isSessionRestored(), sessionId(), commitData(), and saveState().
Sets the active window to the active widget in response to a system event. The function is called from the platform specific event handlers.
Warning: This function does not set the keyboard focus to the active widget. Call QWidget.activateWindow() instead.
It sets the activeWindow() and focusWidget() attributes and sends proper WindowActivate/WindowDeactivate and FocusIn/FocusOut events to all appropriate widgets. The window will then be painted in active state (e.g. cursors in line edits will blink), and it will have tool tips enabled.
See also activeWindow() and QWidget.activateWindow().
This method is also a Qt slot with the C++ signature void setAutoSipEnabled(const bool).
Sets the color specification for the application to spec.
The color specification controls how the application allocates colors when run on a display with a limited amount of colors, e.g. 8 bit / 256 color displays.
The color specification must be set before you create the QApplication object.
The options are:
On Windows, Qt creates a Windows palette, and fills it with a color cube.
Be aware that the CustomColor and ManyColor choices may lead to colormap flashing: The foreground application gets (most) of the available colors, while the background windows will look less attractive.
Example:
int main(int argc, char *argv[]) { QApplication.setColorSpec(QApplication.ManyColor); QApplication app(argc, argv); ... return app.exec(); }
See also colorSpec().
Sets whether Qt should use the system's standard colors, fonts, etc., to on. By default, this is true.
This function must be called before creating the QApplication object, like this:
int main(int argc, char *argv[]) { QApplication.setDesktopSettingsAware(false); QApplication app(argc, argv); ... return app.exec(); }
See also desktopSettingsAware().
Enables the UI effect effect if enable is true, otherwise the effect will not be used.
Note: All effects are disabled on screens running at less than 16-bit color depth.
See also isEffectEnabled(), Qt.UIEffect, and setDesktopSettingsAware().
Changes the default application font to font. If className is passed, the change applies only to classes that inherit className (as reported by QObject.inherits()).
On application start-up, the default font depends on the window system. It can vary depending on both the window system version and the locale. This function lets you override the default font; but overriding may be a bad idea because, for example, some locales need extra large fonts to support their special characters.
Warning: Do not use this function in conjunction with Qt Style Sheets. The font of an application can be customized using the "font" style sheet property. To set a bold font for all QPushButtons, set the application styleSheet() as "QPushButton { font: bold }"
See also font(), fontMetrics(), and QWidget.setFont().
Sets the default graphics backend to system, which will be used for on-screen widgets and QPixmaps. The available systems are "native", "raster" and "opengl".
There are several ways to set the graphics backend, in order of decreasing precedence:
If the highest precedence switch sets an invalid name, the error will be ignored and the default backend will be used.
Warning: This function is only effective before the QApplication constructor is called.
Note: The "opengl" option is currently experimental.
This function was introduced in Qt 4.5.
The QInputContext argument has it's ownership transferred to Qt.
This function replaces the QInputContext instance used by the application with inputContext.
Qt takes ownership of the given inputContext.
See also inputContext().
Sets the application override cursor to cursor.
Application override cursors are intended for showing the user that the application is in a special state, for example during an operation that might take some time.
This cursor will be displayed in all the application's widgets until restoreOverrideCursor() or another setOverrideCursor() is called.
Application cursors are stored on an internal stack. setOverrideCursor() pushes the cursor onto the stack, and restoreOverrideCursor() pops the active cursor off the stack. changeOverrideCursor() changes the curently active application override cursor.
Every setOverrideCursor() must eventually be followed by a corresponding restoreOverrideCursor(), otherwise the stack will never be emptied.
Example:
QApplication.setOverrideCursor(QCursor(Qt.WaitCursor)); calculateHugeMandelbrot(); // lunch time... QApplication.restoreOverrideCursor();
See also overrideCursor(), restoreOverrideCursor(), changeOverrideCursor(), and QWidget.setCursor().
Changes the default application palette to palette.
If className is passed, the change applies only to widgets that inherit className (as reported by QObject.inherits()). If className is left 0, the change affects all widgets, thus overriding any previously set class specific palettes.
The palette may be changed according to the current GUI style in QStyle.polish().
Warning: Do not use this function in conjunction with Qt Style Sheets. When using style sheets, the palette of a widget can be customized using the "color", "background-color", "selection-color", "selection-background-color" and "alternate-background-color".
Note: Some styles do not use the palette for all drawing, for instance, if they make use of native theme engines. This is the case for the Windows XP, Windows Vista, and Mac OS X styles.
See also QWidget.setPalette(), palette(), and QStyle.polish().
The QStyle argument has it's ownership transferred to Qt.
Sets the application's GUI style to style. Ownership of the style object is transferred to QApplication, so QApplication will delete the style object on application exit or when a new style is set and the old style is still the parent of the application object.
Example usage:
QApplication.setStyle(new QWindowsStyle);
When switching application styles, the color palette is set back to the initial colors or the system defaults. This is necessary since certain styles have to adapt the color palette to be fully style-guide compliant.
Setting the style before a palette has been set, i.e., before creating QApplication, will cause the application to use QStyle.standardPalette() for the palette.
Warning: Qt style sheets are currently not supported for custom QStyle subclasses. We plan to address this in some future release.
See also style(), QStyle, setPalette(), and desktopSettingsAware().
This is an overloaded function.
Requests a QStyle object for style from the QStyleFactory.
The string must be one of the QStyleFactory.keys(), typically one of "windows", "motif", "cde", "plastique", "windowsxp", or "macintosh". Style names are case insensitive.
Returns 0 if an unknown style is passed, otherwise the QStyle object returned is set as the application's GUI style.
Warning: To ensure that the application's style is set correctly, it is best to call this function before the QApplication constructor, if possible.
This method is also a Qt slot with the C++ signature void setStyleSheet(const QString&).
Returns the application's style object.
See also setStyle() and QStyle.
Synchronizes with the X server in the X11 implementation. This normally takes some time. Does nothing on other platforms.
Returns the top-level widget at the given point; returns 0 if there is no such widget.
This is an overloaded function.
Returns the top-level widget at the point (x, y); returns 0 if there is no such widget.
Returns a list of the top-level widgets (windows) in the application.
Note: Some of the top-level widgets may be hidden, for example a tooltip if no tooltip is currently shown.
Example:
void showAllHiddenTopLevelWidgets() { foreach (QWidget *widget, QApplication.topLevelWidgets()) { if (widget->isHidden()) widget->show(); } }
See also allWidgets(), QWidget.isWindow(), and QWidget.isHidden().
Returns the type of application (Tty, GuiClient, or GuiServer). The type is set when constructing the QApplication object.
Returns the widget at global screen position point, or 0 if there is no Qt widget there.
This function can be slow.
See also QCursor.pos(), QWidget.grabMouse(), and QWidget.grabKeyboard().
Warning: This virtual function is only implemented under X11.
If you create an application that inherits QApplication and reimplement this function, you get direct access to all X events that the are received from the X server. The events are passed in the event parameter.
Return true if you want to stop the event from being processed. Return false for normal event dispatching. The default implementation returns false.
It is only the directly addressed messages that are filtered. You must install an event filter directly on the event dispatcher, which is returned by QAbstractEventDispatcher.instance(), to handle system wide messages.
See also x11ProcessEvent().
This function does the core processing of individual X events, normally by dispatching Qt events to the right destination.
It returns 1 if the event was consumed by special handling, 0 if the event was consumed by normal handling, and -1 if the event was for an unrecognized widget.
See also x11EventFilter().
This is the default overload of this signal.
This signal deals with session management. It is emitted when the QSessionManager wants the application to commit all its data.
Usually this means saving all open files, after getting permission from the user. Furthermore you may want to provide a means by which the user can cancel the shutdown.
You should not exit the application within this signal. Instead, the session manager may or may not do this afterwards, depending on the context.
Warning: Within this signal, no user interaction is possible, unless you ask the manager for explicit permission. See QSessionManager.allowsInteraction() and QSessionManager.allowsErrorInteraction() for details and example usage.
Note: You should use Qt.DirectConnection when connecting to this signal.
This function was introduced in Qt 4.2.
See also isSessionRestored(), sessionId(), saveState(), and Session Management.
This is the default overload of this signal.
This signal is emitted when the widget that has keyboard focus changed from old to now, i.e., because the user pressed the tab-key, clicked into a widget or changed the active window. Both old and now can be the null-pointer.
The signal is emitted after both widget have been notified about the change through QFocusEvent.
This function was introduced in Qt 4.1.
See also QWidget.setFocus(), QWidget.clearFocus(), and Qt.FocusReason.
This is the default overload of this signal.
This signal is emitted when application fonts are loaded or removed.
This function was introduced in Qt 4.5.
See also QFontDatabase.addApplicationFont(), QFontDatabase.addApplicationFontFromData(), QFontDatabase.removeAllApplicationFonts(), and QFontDatabase.removeApplicationFont().
This is the default overload of this signal.
This signal is emitted from QApplication.exec() when the last visible primary window (i.e. window with no parent) with the Qt.WA_QuitOnClose attribute set is closed.
By default,
This feature can be turned off by setting quitOnLastWindowClosed to false.
See also QWidget.close().
This is the default overload of this signal.
This signal deals with session management. It is invoked when the session manager wants the application to preserve its state for a future session.
For example, a text editor would create a temporary file that includes the current contents of its edit buffers, the location of the cursor and other aspects of the current editing session.
You should never exit the application within this signal. Instead, the session manager may or may not do this afterwards, depending on the context. Futhermore, most session managers will very likely request a saved state immediately after the application has been started. This permits the session manager to learn about the application's restart policy.
Warning: Within this function, no user interaction is possible, unless you ask the manager for explicit permission. See QSessionManager.allowsInteraction() and QSessionManager.allowsErrorInteraction() for details.
Note: You should use Qt.DirectConnection when connecting to this signal.
This function was introduced in Qt 4.2.
See also isSessionRestored(), sessionId(), commitData(), and Session Management.
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