QGuiApplication Class

Member Function Documentation

QGuiApplication::QGuiApplication(int &argc, char **argv)

Initializes the window system and constructs an application object with argc command line arguments in argv.

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 pixmaps, bitmaps etc.).

Supported Command Line Options

All Qt programs automatically support a set of command-line options that allow modifying the way Qt will interact with the windowing system. Some of the options are also accessible via environment variables, which are the preferred form if the application can launch GUI sub-processes or other applications (environment variables will be inherited by child processes). When in doubt, use the environment variables.

The options currently supported are the following:

• -platform platformName[:options], specifies the Qt Platform Abstraction (QPA) plugin.

  Overrides the QT_QPA_PLATFORM environment variable.

• -platformpluginpath path, specifies the path to platform plugins.

  Overrides the QT_QPA_PLATFORM_PLUGIN_PATH environment variable.

• -platformtheme platformTheme, specifies the platform theme.

  Overrides the QT_QPA_PLATFORMTHEME environment variable.

• -plugin plugin, specifies additional plugins to load. The argument may appear multiple times.

  Concatenated with the plugins in the QT_QPA_GENERIC_PLUGINS environment variable.

• -qmljsdebugger=, activates the QML/JS debugger with a specified port. The value must be of format port:1234[,block], where block is optional and will make the application wait until a debugger connects to it.
• -qwindowgeometry geometry, specifies window geometry for the main window using the X11-syntax. For example: -qwindowgeometry 100x100+50+50
• -qwindowicon, sets the default window icon
• -qwindowtitle, sets the title of the first window
• -reverse, sets the application's layout direction to Qt::RightToLeft. This option is intended to aid debugging and should not be used in production. The default value is automatically detected from the user's locale (see also QLocale::textDirection()).
• -session session, restores the application from an earlier session.

The following standard command line options are available for X11:

• -display hostname:screen_number, switches displays on X11.

  Overrides the DISPLAY environment variable.

• -geometry geometry, same as -qwindowgeometry.

Platform-Specific Arguments

You can specify platform-specific arguments for the -platform option. Place them after the platform plugin name following a colon as a comma-separated list. For example, -platform windows:dialogs=xp,fontengine=freetype.

The following parameters are available for -platform windows:

• altgr, detect the key AltGr found on some keyboards as Qt::GroupSwitchModifier (since Qt 5.12).
• darkmode=[1|2] controls how Qt responds to the activation of the Dark Mode for applications introduced in Windows 10 1903 (since Qt 5.15).

  A value of 1 causes Qt to switch the window borders to black when Dark Mode for applications is activated and no High Contrast Theme is in use. This is intended for applications that implement their own theming.

  A value of 2 will in addition cause the Windows Vista style to be deactivated and switch to the Windows style using a simplified palette in dark mode. This is currently experimental pending the introduction of new style that properly adapts to dark mode.

• dialogs=[xp|none], xp uses XP-style native dialogs and none disables them.
• dpiawareness=[0|1|2] Sets the DPI awareness of the process (see High DPI Displays, since Qt 5.4).
• fontengine=freetype, uses the FreeType font engine.
• menus=[native|none], controls the use of native menus.

  Native menus are implemented using Win32 API and are simpler than QMenu-based menus in for example that they do allow for placing widgets on them or changing properties like fonts and do not provide hover signals. They are mainly intended for Qt Quick. By default, they will be used if the application is not an instance of QApplication or for Qt Quick Controls 2 applications (since Qt 5.10).

• nocolorfonts Turn off DirectWrite Color fonts (since Qt 5.8).
• nodirectwrite Turn off DirectWrite fonts (since Qt 5.8).
• nomousefromtouch Ignores mouse events synthesized from touch events by the operating system.
• nowmpointer Switches from Pointer Input Messages handling to legacy mouse handling (since Qt 5.12).
• reverse Activates Right-to-left mode (experimental). Windows title bars will be shown accordingly in Right-to-left locales (since Qt 5.13).
• tabletabsoluterange=<value> Sets a value for mouse mode detection of WinTab tablets (Legacy, since Qt 5.3).

The following parameter is available for -platform cocoa (on macOS):

• fontengine=freetype, uses the FreeType font engine.

For more information about the platform-specific arguments available for embedded Linux platforms, see Qt for Embedded Linux.

See also arguments() and QGuiApplication::platformName.

[signal] void QGuiApplication::applicationStateChanged(Qt::ApplicationState state)

This signal is emitted when the state of the application changes.

This function was introduced in Qt 5.2.

See also applicationState().

[signal] void QGuiApplication::commitDataRequest(QSessionManager &manager)

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.

This function was introduced in Qt 4.2.

See also setFallbackSessionManagementEnabled(), isSessionRestored(), sessionId(), saveStateRequest(), and Session Management.

[signal] void QGuiApplication::focusObjectChanged(QObject *focusObject)

This signal is emitted when final receiver of events tied to focus is changed. focusObject is the new receiver.

See also focusObject().

[signal] void QGuiApplication::focusWindowChanged(QWindow *focusWindow)

This signal is emitted when the focused window changes. focusWindow is the new focused window.

See also focusWindow().

[signal] void QGuiApplication::fontChanged(const QFont &font)

This signal is emitted when the font of the application changes.

This function was introduced in Qt 5.11.

See also font().

[signal] void QGuiApplication::fontDatabaseChanged()

This signal is emitted when application fonts are loaded or removed.

See also QFontDatabase::addApplicationFont(), QFontDatabase::addApplicationFontFromData(), QFontDatabase::removeAllApplicationFonts(), and QFontDatabase::removeApplicationFont().

[signal] void QGuiApplication::lastWindowClosed()

This signal is emitted from exec() when the last visible primary window (i.e. window with no parent) is closed.

By default, QGuiApplication quits after this signal is emitted. This feature can be turned off by setting quitOnLastWindowClosed to false.

See also QWindow::close() and QWindow::isTopLevel().

[signal] void QGuiApplication::paletteChanged(const QPalette &palette)

This signal is emitted when the palette of the application changes.

This function was introduced in Qt 5.4.

See also palette().

[signal] void QGuiApplication::saveStateRequest(QSessionManager &manager)

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. Furthermore, 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.

This function was introduced in Qt 4.2.

See also isSessionRestored(), sessionId(), commitDataRequest(), and Session Management.

[signal] void QGuiApplication::screenAdded(QScreen *screen)

This signal is emitted whenever a new screen screen has been added to the system.

See also screens(), primaryScreen, and screenRemoved().

[signal] void QGuiApplication::screenRemoved(QScreen *screen)

This signal is emitted whenever a screen is removed from the system. It provides an opportunity to manage the windows on the screen before Qt falls back to moving them to the primary screen.

This function was introduced in Qt 5.4.

See also screens(), screenAdded(), QObject::destroyed(), and QWindow::setScreen().

[virtual] QGuiApplication::~QGuiApplication()

Destructs the application.

[static] QWindowList QGuiApplication::allWindows()

Returns a list of all the windows in the application.

The list is empty if there are no windows.

See also topLevelWindows().

[static] Qt::ApplicationState QGuiApplication::applicationState()

Returns the current state of the application.

You can react to application state changes to perform actions such as stopping/resuming CPU-intensive tasks, freeing/loading resources or saving/restoring application data.

This function was introduced in Qt 5.2.

[static] void QGuiApplication::changeOverrideCursor(const QCursor &cursor)

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().

[static] QClipboard *QGuiApplication::clipboard()

Returns the object for interacting with the clipboard.

[static] bool QGuiApplication::desktopSettingsAware()

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().

qreal QGuiApplication::devicePixelRatio() const

Returns the highest screen device pixel ratio found on the system. This is the ratio between physical pixels and device-independent pixels.

Use this function only when you don't know which window you are targeting. If you do know the target window, use QWindow::devicePixelRatio() instead.

See also QWindow::devicePixelRatio().

[override virtual protected] bool QGuiApplication::event(QEvent *e)

Reimplements: QCoreApplication::event(QEvent *e).

[static] int QGuiApplication::exec()

Enters the main event loop and waits until exit() is called, and 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().

To make your application perform idle processing, e.g., 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.

See also quitOnLastWindowClosed, quit(), exit(), processEvents(), and QCoreApplication::exec().

[static] QObject *QGuiApplication::focusObject()

Returns the QObject in currently active window that will be final receiver of events tied to focus, such as key events.

[static] QWindow *QGuiApplication::focusWindow()

Returns the QWindow that receives events tied to focus, such as key events.

[static] QFont QGuiApplication::font()

Returns the default application font.

See also setFont().

[static] Qt::HighDpiScaleFactorRoundingPolicy QGuiApplication::highDpiScaleFactorRoundingPolicy()

Returns the high-DPI scale factor rounding policy.

This function was introduced in Qt 5.14.

See also setHighDpiScaleFactorRoundingPolicy().

[static] QInputMethod *QGuiApplication::inputMethod()

returns the input method.

The input method returns properties about the state and position of the virtual keyboard. It also provides information about the position of the current focused input element.

See also QInputMethod.

[static] bool QGuiApplication::isFallbackSessionManagementEnabled()

Returns whether QGuiApplication will use fallback session management.

The default is true.

If this is true and the session manager allows user interaction, QGuiApplication will try to close toplevel windows after commitDataRequest() has been emitted. If a window cannot be closed, session shutdown will be canceled and the application will keep running.

Fallback session management only benefits applications that have an "are you sure you want to close this window?" feature or other logic that prevents closing a toplevel window depending on certain conditions, and that do nothing to explicitly implement session management. In applications that do implement session management using the proper session management API, fallback session management interferes and may break session management logic.

This function was introduced in Qt 5.6.

See also setFallbackSessionManagementEnabled(), QSessionManager::allowsInteraction(), saveStateRequest(), commitDataRequest(), and Session Management.

[static] bool QGuiApplication::isLeftToRight()

Returns true if the application's layout direction is Qt::LeftToRight; otherwise returns false.

See also layoutDirection() and isRightToLeft().

[static] bool QGuiApplication::isRightToLeft()

Returns true if the application's layout direction is Qt::RightToLeft; otherwise returns false.

See also layoutDirection() and isLeftToRight().

bool QGuiApplication::isSavingSession() const

Returns true if the application is currently saving the session; otherwise returns false.

This is true when commitDataRequest() and saveStateRequest() are emitted, but also when the windows are closed afterwards by session management.

This function was introduced in Qt 5.0.

See also sessionId(), commitDataRequest(), and saveStateRequest().

bool QGuiApplication::isSessionRestored() const

Returns true if the application has been restored from an earlier session; otherwise returns false.

See also sessionId(), commitDataRequest(), and saveStateRequest().

[static] Qt::KeyboardModifiers QGuiApplication::keyboardModifiers()

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().

[static] QWindow *QGuiApplication::modalWindow()

Returns the most recently shown modal window. If no modal windows are visible, this function returns zero.

A modal window is a window which has its modality property set to Qt::WindowModal or Qt::ApplicationModal. A modal window must be closed before the user can continue with other parts of the program.

Modal window are organized in a stack. This function returns the modal window at the top of the stack.

See also Qt::WindowModality and QWindow::setModality().

[static] Qt::MouseButtons QGuiApplication::mouseButtons()

Returns the current state of the buttons on the mouse. The current state is updated synchronously 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().

[override virtual] bool QGuiApplication::notify(QObject *object, QEvent *event)

Reimplements: QCoreApplication::notify(QObject *receiver, QEvent *event).

[static] QCursor *QGuiApplication::overrideCursor()

Returns the active application override cursor.

This function returns nullptr if no application cursor has been defined (i.e. the internal cursor stack is empty).

See also setOverrideCursor() and restoreOverrideCursor().

[static] QPalette QGuiApplication::palette()

Returns the current application palette.

Roles that have not been explicitly set will reflect the system's platform theme.

See also setPalette().

[static] QFunctionPointer QGuiApplication::platformFunction(const QByteArray &function)

Returns a function pointer from the platformplugin matching function

[static] QPlatformNativeInterface *QGuiApplication::platformNativeInterface()

Returns the platform's native interface, for platform specific functionality.

[static] Qt::KeyboardModifiers QGuiApplication::queryKeyboardModifiers()

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.

See also keyboardModifiers().

[static] void QGuiApplication::restoreOverrideCursor()

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().

[static] QScreen *QGuiApplication::screenAt(const QPoint &point)

Returns the screen at point, or nullptr if outside of any screen.

The point is in relation to the virtualGeometry() of each set of virtual siblings. If the point maps to more than one set of virtual siblings the first match is returned. If you wish to search only the virtual desktop siblings of a known screen (for example siblings of the screen of your application window QWidget::windowHandle()->screen()), use QScreen::virtualSiblingAt().

This function was introduced in Qt 5.10.

[static] QList<QScreen *> QGuiApplication::screens()

Returns a list of all the screens associated with the windowing system the application is connected to.

QString QGuiApplication::sessionId() const

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(), commitDataRequest(), and saveStateRequest().

QString QGuiApplication::sessionKey() const

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 every time the session is saved. If the shutdown process is cancelled, another session key will be used when shutting down again.

See also isSessionRestored(), sessionId(), commitDataRequest(), and saveStateRequest().

[static] void QGuiApplication::setDesktopSettingsAware(bool on)

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 QGuiApplication object, like this:

 int main(int argc, char *argv[])
 {
     QApplication::setDesktopSettingsAware(false);
     QApplication app(argc, argv);
     ...
     return app.exec();
 }

See also desktopSettingsAware().

[static] void QGuiApplication::setFallbackSessionManagementEnabled(bool enabled)

Sets whether QGuiApplication will use fallback session management to enabled.

This function was introduced in Qt 5.6.

See also isFallbackSessionManagementEnabled().

[static] void QGuiApplication::setFont(const QFont &font)

Changes the default application font to font.

See also font().

[static] void QGuiApplication::setHighDpiScaleFactorRoundingPolicy(Qt::HighDpiScaleFactorRoundingPolicy policy)

Sets the high-DPI scale factor rounding policy for the application. The policy decides how non-integer scale factors (such as Windows 150%) are handled, for applications that have AA_EnableHighDpiScaling enabled.

The two principal options are whether fractional scale factors should be rounded to an integer or not. Keeping the scale factor as-is will make the user interface size match the OS setting exactly, but may cause painting errors, for example with the Windows style.

If rounding is wanted, then which type of rounding should be decided next. Mathematically correct rounding is supported but may not give the best visual results: Consider if you want to render 1.5x as 1x ("small UI") or as 2x ("large UI"). See the Qt::HighDpiScaleFactorRoundingPolicy enum for a complete list of all options.

This function must be called before creating the application object, and can be overridden by setting the QT_SCALE_FACTOR_ROUNDING_POLICY environment variable. The QGuiApplication::highDpiScaleFactorRoundingPolicy() accessor will reflect the environment, if set.

The default value is Qt::HighDpiScaleFactorRoundingPolicy::Round. On Qt for Android the default is Qt::HighDpiScaleFactorRoundingPolicy::PassThrough, which preserves historical behavior from earlier Qt versions.

This function was introduced in Qt 5.14.

See also highDpiScaleFactorRoundingPolicy().

[static] void QGuiApplication::setOverrideCursor(const QCursor &cursor)

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:

 QGuiApplication::setOverrideCursor(QCursor(Qt::WaitCursor));
 calculateHugeMandelbrot();              // lunch time...
 QGuiApplication::restoreOverrideCursor();

See also overrideCursor(), restoreOverrideCursor(), changeOverrideCursor(), and QWidget::setCursor().

[static] void QGuiApplication::setPalette(const QPalette &pal)

Changes the application palette to pal.

The color roles from this palette are combined with the system's platform theme to form the application's final palette.

See also palette().

[static] QStyleHints *QGuiApplication::styleHints()

Returns the application's style hints.

The style hints encapsulate a set of platform dependent properties such as double click intervals, full width selection and others.

The hints can be used to integrate tighter with the underlying platform.

See also QStyleHints.

[static] void QGuiApplication::sync()

Function that can be used to sync Qt state with the Window Systems state.

This function will first empty Qts events by calling QCoreApplication::processEvents(), then the platform plugin will sync up with the windowsystem, and finally Qts events will be delived by another call to QCoreApplication::processEvents();

This function is timeconsuming and its use is discouraged.

This function was introduced in Qt 5.2.

[static] QWindow *QGuiApplication::topLevelAt(const QPoint &pos)

Returns the top level window at the given position pos, if any.

[static] QWindowList QGuiApplication::topLevelWindows()

Returns a list of the top-level windows in the application.

See also allWindows().