QGuiApplication Class

The QGuiApplication class manages the GUI application's control flow and main settings. More...

Header: #include <QGuiApplication>
qmake: QT += gui
Since: Qt 5.0
Inherits: QCoreApplication

Properties

Public Functions

QGuiApplication(int &argc, char **argv)
virtual ~QGuiApplication()
qreal devicePixelRatio() const
bool isSavingSession() const
bool isSessionRestored() const
QString sessionId() const
QString sessionKey() const

Reimplemented Public Functions

virtual bool notify(QObject *object, QEvent *event) override

Signals

void applicationDisplayNameChanged()
void applicationStateChanged(Qt::ApplicationState state)
void commitDataRequest(QSessionManager &sessionManager)
void focusObjectChanged(QObject *focusObject)
void focusWindowChanged(QWindow *focusWindow)
void fontChanged(const QFont &font)
void fontDatabaseChanged()
void lastWindowClosed()
void layoutDirectionChanged(Qt::LayoutDirection direction)
void paletteChanged(const QPalette &pal)
void primaryScreenChanged(QScreen *screen)
void saveStateRequest(QSessionManager &sessionManager)
void screenAdded(QScreen *screen)
void screenRemoved(QScreen *screen)

Static Public Members

QWindowList allWindows()
QString applicationDisplayName()
Qt::ApplicationState applicationState()
void changeOverrideCursor(const QCursor &cursor)
QClipboard *clipboard()
QString desktopFileName()
bool desktopSettingsAware()
int exec()
QObject *focusObject()
QWindow *focusWindow()
QFont font()
QInputMethod *inputMethod()
bool isFallbackSessionManagementEnabled()
bool isLeftToRight()
bool isRightToLeft()
Qt::KeyboardModifiers keyboardModifiers()
Qt::LayoutDirection layoutDirection()
QWindow *modalWindow()
Qt::MouseButtons mouseButtons()
QCursor *overrideCursor()
QPalette palette()
QFunctionPointer platformFunction(const QByteArray &function)
QString platformName()
QPlatformNativeInterface *platformNativeInterface()
QScreen *primaryScreen()
Qt::KeyboardModifiers queryKeyboardModifiers()
bool quitOnLastWindowClosed()
void restoreOverrideCursor()
QScreen *screenAt(const QPoint &point)
QList<QScreen *> screens()
void setApplicationDisplayName(const QString &name)
void setDesktopFileName(const QString &name)
void setDesktopSettingsAware(bool on)
void setFallbackSessionManagementEnabled(bool enabled)
void setFont(const QFont &font)
void setLayoutDirection(Qt::LayoutDirection direction)
void setOverrideCursor(const QCursor &)
void setPalette(const QPalette &pal)
void setQuitOnLastWindowClosed(bool quit)
void setWindowIcon(const QIcon &icon)
QStyleHints *styleHints()
void sync()
QWindow *topLevelAt(const QPoint &pos)
QWindowList topLevelWindows()
QIcon windowIcon()

Reimplemented Protected Functions

virtual bool event(QEvent *e) override

Macros

Additional Inherited Members

Detailed Description

The QGuiApplication class manages the GUI application's control flow and main settings.

QGuiApplication 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 and finalization, and provides session management. In addition, QGuiApplication handles most of the system-wide and application-wide settings.

For any GUI application using Qt, there is precisely one QGuiApplication 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 Qt GUI module. For QWidget based Qt applications, use QApplication instead, as it provides some functionality needed for creating QWidget instances.

The QGuiApplication object is accessible through the instance() function, which returns a pointer equivalent to the global qApp pointer.

QGuiApplication's main areas of responsibility are:

  • It initializes the application with the user's desktop settings, such as palette(), font() and styleHints(). It keeps track of these properties in case the user changes the desktop globally, for example, through some kind of control panel.
  • It performs event handling, meaning that it receives events from the underlying window system and dispatches them to the relevant widgets. You can send your own events to windows by using sendEvent() and postEvent().
  • It parses common command line arguments and sets its internal state accordingly. See the constructor documentation below for more details.
  • It provides localization of strings that are visible to the user via translate().
  • It provides some magical objects like the clipboard().
  • It knows about the application's windows. You can ask which window is at a certain position using topLevelAt(), get a list of topLevelWindows(), etc.
  • It manages the application's mouse cursor handling, see setOverrideCursor()
  • It provides support for sophisticated session management. This makes it possible for applications to terminate gracefully when the user logs out, to cancel a shutdown process if termination isn't possible and even to preserve the entire application's state for a future session. See isSessionRestored(), sessionId() and commitDataRequest() and saveStateRequest() for details.

Since the QGuiApplication object does so much initialization, it must be created before any other objects related to the user interface are created. QGuiApplication 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.

Groups of functions
System settingsdesktopSettingsAware(), setDesktopSettingsAware(), styleHints(), palette(), setPalette(), font(), setFont().
Event handlingexec(), processEvents(), exit(), quit(). sendEvent(), postEvent(), sendPostedEvents(), removePostedEvents(), hasPendingEvents(), notify().
WindowsallWindows(), topLevelWindows(), focusWindow(), clipboard(), topLevelAt().
Advanced cursor handlingoverrideCursor(), setOverrideCursor(), restoreOverrideCursor().
Session managementisSessionRestored(), sessionId(), commitDataRequest(), saveStateRequest().
MiscellaneousstartingUp(), closingDown().

See also QCoreApplication, QAbstractEventDispatcher, and QEventLoop.

Property Documentation

applicationDisplayName : QString

This property holds the user-visible name of this application

This name is shown to the user, for instance in window titles. It can be translated, if necessary.

If not set, the application display name defaults to the application name.

This property was introduced in Qt 5.0.

Access functions:

QString applicationDisplayName()
void setApplicationDisplayName(const QString &name)

Notifier signal:

void applicationDisplayNameChanged()

See also applicationName.

desktopFileName : QString

This property holds the base name of the desktop entry for this application

This is the file name, without the full path, of the desktop entry that represents this application according to the freedesktop desktop entry specification.

This property gives a precise indication of what desktop entry represents the application and it is needed by the windowing system to retrieve such information without resorting to imprecise heuristics.

The latest version of the freedesktop desktop entry specification can be obtained here.

This property was introduced in Qt 5.7.

Access functions:

QString desktopFileName()
void setDesktopFileName(const QString &name)

layoutDirection : Qt::LayoutDirection

This property holds the default layout direction for this application

On system start-up, the default layout direction depends on the application's language.

The notifier signal was introduced in Qt 5.4.

Access functions:

Qt::LayoutDirection layoutDirection()
void setLayoutDirection(Qt::LayoutDirection direction)

Notifier signal:

void layoutDirectionChanged(Qt::LayoutDirection direction)

See also QWidget::layoutDirection, isLeftToRight(), and isRightToLeft().

platformName : const QString

This property holds the name of the underlying platform plugin.

The QPA platform plugins are located in qtbase\src\plugins\platforms. At the time of writing, the following platform plugin names are supported:

  • android
  • cocoa is a platform plugin for macOS.
  • directfb
  • eglfs is a platform plugin for running Qt5 applications on top of EGL and OpenGL ES 2.0 without an actual windowing system (like X11 or Wayland). For more information, see EGLFS.
  • ios (also used for tvOS)
  • kms is an experimental platform plugin using kernel modesetting and DRM (Direct Rendering Manager).
  • linuxfb writes directly to the framebuffer. For more information, see LinuxFB.
  • minimal is provided as an examples for developers who want to write their own platform plugins. However, you can use the plugin to run GUI applications in environments without a GUI, such as servers.
  • minimalegl is an example plugin.
  • offscreen
  • openwfd
  • qnx
  • windows
  • wayland is a platform plugin for modern Linux desktops and some embedded systems.
  • xcb is the X11 plugin used on regular desktop Linux platforms.

For more information about the platform plugins for embedded Linux devices, see Qt for Embedded Linux.

Access functions:

QString platformName()

primaryScreen : QScreen* const

This property holds the primary (or default) screen of the application.

This will be the screen where QWindows are initially shown, unless otherwise specified.

The primaryScreenChanged signal was introduced in Qt 5.6.

Access functions:

QScreen *primaryScreen()

Notifier signal:

void primaryScreenChanged(QScreen *screen)

See also screens().

quitOnLastWindowClosed : bool

This property holds whether the application implicitly quits when the last window is closed.

The default is true.

If this property is true, the applications quits when the last visible primary window (i.e. window with no parent) is closed.

Access functions:

bool quitOnLastWindowClosed()
void setQuitOnLastWindowClosed(bool quit)

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

windowIcon : QIcon

This property holds the default window icon

Access functions:

QIcon windowIcon()
void setWindowIcon(const QIcon &icon)

See also QWindow::setIcon() and Setting the Application Icon.

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.

Warning: The data referred to by argc and argv must stay valid for the entire lifetime of the QGuiApplication 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 pixmaps, bitmaps etc.).

Note: argc and argv might be changed as Qt removes command line arguments that it recognizes.

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:

  • dialogs=[xp|none], xp uses XP-style native dialogs and none disables them.
  • 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.

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.

[virtual] QGuiApplication::~QGuiApplication()

Destructs the application.

[static] QWindowList QGuiApplication::allWindows()

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

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

[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.

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

[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)

Reimplemented from QCoreApplication::event().

[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.

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

[static] QWindow *QGuiApplication::focusWindow()

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

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

[static] QFont QGuiApplication::font()

Returns the default application font.

See also setFont().

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

[signal] void QGuiApplication::fontDatabaseChanged()

[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.

Warning: If all windows are closed due to fallback session management and quitOnLastWindowClosed() is true, the application will quit before it is explicitly instructed to quit through the platform's session management protocol. That violation of protocol may prevent the platform session manager from saving application state.

This function was introduced in Qt 5.6.

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

[static] bool QGuiApplication::isLeftToRight()

[static] bool QGuiApplication::isRightToLeft()

bool QGuiApplication::isSavingSession() const

bool QGuiApplication::isSessionRestored() const

[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().

[signal] void QGuiApplication::lastWindowClosed()

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 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().

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

Reimplemented from QCoreApplication::notify().

[static] QCursor *QGuiApplication::overrideCursor()

See also setOverrideCursor().

[static] QPalette QGuiApplication::palette()

Returns the default application palette.

See also setPalette().

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

[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()

[static] void QGuiApplication::restoreOverrideCursor()

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

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

[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.

This function was introduced in Qt 5.10.

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

[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

QString QGuiApplication::sessionKey() const

[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::setOverrideCursor(const QCursor &)

See also overrideCursor().

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

Changes the default application palette to pal.

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()

Macro Documentation

qGuiApp

A global pointer referring to the unique application object. Only valid for use when that object is a QGuiApplication.

See also QCoreApplication::instance() and qApp.