QApplication Class

 QSize MyWidget::sizeHint() const
 {
     return QSize(80, 25).expandedTo(QApplication::globalStrut());
 }

 if ((startPos - currentPos).manhattanLength() >=
         QApplication::startDragDistance())
     startTheDrag();

Member Function Documentation

QApplication::QApplication(int & argc, char ** argv, int _internal = ApplicationFlags)

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.

All Qt programs automatically support the following command line options:

• -style= style, sets the application GUI style. Possible values are motif, windows, and platinum. If you compiled Qt with additional styles or have additional styles as plugins these will be available to the -style command line option.
• -style style, is the same as listed above.
• -stylesheet= stylesheet, sets the application styleSheet. The value must be a path to a file that contains the Style Sheet.

  Note: Relative URLs in the Style Sheet file are relative to the Style Sheet file's path.

• -stylesheet stylesheet, is the same as listed above.
• -session= session, restores the application from an earlier session.
• -session session, is the same as listed above.
• -widgetcount, prints debug message at the end about number of widgets left undestroyed and maximum number of widgets existed at the same time
• -reverse, sets the application's layout direction to Qt::RightToLeft
• -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.

See also arguments().

QApplication::QApplication(int & argc, char ** argv, Type type, int _internal = ApplicationFlags)

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.

QApplication::~QApplication() [virtual]

Cleans up any window system resources that were allocated by this application. Sets the global variable qApp to 0.

void QApplication::aboutQt() [static slot]

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().

QWidget * QApplication::activeModalWidget() [static]

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().

QWidget * QApplication::activePopupWidget() [static]

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().

QWidget * QApplication::activeWindow() [static]

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().

void QApplication::alert(QWidget * widget, int msec = 0) [static]

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.

QWidgetList QApplication::allWidgets() [static]

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().

void QApplication::beep() [static]

Sounds the bell, using the default volume and sound. The function is not available in Qt for Embedded Linux.

void QApplication::closeAllWindows() [static slot]

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().

int QApplication::colorSpec() [static]

Returns the color specification.

See also QApplication::setColorSpec().

void QApplication::commitData(QSessionManager & manager) [virtual]

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.

void QApplication::commitDataRequest(QSessionManager & manager) [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.

QDesktopWidget * QApplication::desktop() [static]

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.

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

Reimplemented from QObject::event().

int QApplication::exec() [static]

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().

void QApplication::focusChanged(QWidget * old, QWidget * now) [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.

QWidget * QApplication::focusWidget() [static]

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().

QFont QApplication::font() [static]

Returns the default application font.

See also setFont(), fontMetrics(), and QWidget::font().

QFont QApplication::font(const QWidget * widget) [static]

This is an overloaded function.

Returns the default font for the widget.

See also fontMetrics() and QWidget::setFont().

QFont QApplication::font(const char * className) [static]

This is an overloaded function.

Returns the font for widgets of the given className.

See also setFont() and QWidget::font().

QFontMetrics QApplication::fontMetrics() [static]

Returns display (screen) font metrics for the application font.

See also font(), setFont(), QWidget::fontMetrics(), and QPainter::fontMetrics().

bool QApplication::isEffectEnabled(Qt::UIEffect effect) [static]

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.

bool QApplication::isSessionRestored() const

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

See also sessionId(), commitData(), and saveState().

bool QApplication::keypadNavigationEnabled() [static]

This function is deprecated.

Returns true if Qt is set to use keypad navigation; otherwise returns false. The default value is false.

This feature is available in Qt for Embedded Linux, and Windows CE only.

Note: On Windows CE this feature is disabled by default for touch device mkspecs. To enable keypad navigation, build Qt with QT_KEYPAD_NAVIGATION defined.

See also navigationMode().

Qt::NavigationMode QApplication::navigationMode() [static]

Returns what kind of focus navigation Qt is using.

This feature is available in Qt for Embedded Linux, and Windows CE only.

Note: On Windows CE this feature is disabled by default for touch device mkspecs. To enable keypad navigation, build Qt with QT_KEYPAD_NAVIGATION defined.

This function was introduced in Qt 4.6.

See also setNavigationMode() and keypadNavigationEnabled().

bool QApplication::notify(QObject * receiver, QEvent * e) [virtual]

Reimplemented from QCoreApplication::notify().

QPalette QApplication::palette(const QWidget * widget) [static]

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().

QPalette QApplication::palette(const char * className) [static]

This is an overloaded function.

Returns the palette for widgets of the given className.

See also setPalette() and QWidget::palette().

void QApplication::saveState(QSessionManager & manager) [virtual]

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.

void QApplication::saveStateRequest(QSessionManager & manager) [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.

QString QApplication::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(), commitData(), and saveState().

QString QApplication::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 with every call of commitData() or saveState().

See also isSessionRestored(), sessionId(), commitData(), and saveState().

void QApplication::setActiveWindow(QWidget * active) [static]

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().

void QApplication::setColorSpec(int spec) [static]

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:

• QApplication::NormalColor. This is the default color allocation strategy. Use this option if your application uses buttons, menus, texts and pixmaps with few colors. With this option, the application uses system global colors. This works fine for most applications under X11, but on the Windows platform, it may cause dithering of non-standard colors.
• QApplication::CustomColor. Use this option if your application needs a small number of custom colors. On X11, this option is the same as NormalColor. On Windows, Qt creates a Windows palette, and allocates colors to it on demand.
• QApplication::ManyColor. Use this option if your application is very color hungry, e.g., it requires thousands of colors.
  Under X11 the effect is:
  • For 256-color displays which have at best a 256 color true color visual, the default visual is used, and colors are allocated from a color cube. The color cube is the 6x6x6 (216 color) "Web palette" (the red, green, and blue components always have one of the following values: 0x00, 0x33, 0x66, 0x99, 0xCC, or 0xFF), but the number of colors can be changed by the -ncols option. The user can force the application to use the true color visual with the -visual option.
  • For 256-color displays which have a true color visual with more than 256 colors, use that visual. Silicon Graphics X servers this feature, for example. They provide an 8 bit visual by default but can deliver true color when asked.

  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().

void QApplication::setEffectEnabled(Qt::UIEffect effect, bool enable = true) [static]

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().

void QApplication::setFont(const QFont & font, const char * className = 0) [static]

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().

void QApplication::setNavigationMode(Qt::NavigationMode mode) [static]

Sets the kind of focus navigation Qt should use to mode.

This feature is available in Qt for Embedded Linux, and Windows CE only.

Note: On Windows CE this feature is disabled by default for touch device mkspecs. To enable keypad navigation, build Qt with QT_KEYPAD_NAVIGATION defined.

This function was introduced in Qt 4.6.

See also navigationMode() and keypadNavigationEnabled().

void QApplication::setPalette(const QPalette & palette, const char * className = 0) [static]

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().

void QApplication::setStyle(QStyle * style) [static]

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 se, 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().

QStyle * QApplication::setStyle(const QString & style) [static]

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.

QStyle * QApplication::style() [static]

Returns the application's style object.

See also setStyle() and QStyle.

QWidget * QApplication::topLevelAt(const QPoint & point) [static]

Returns the top-level widget at the given point; returns 0 if there is no such widget.

QWidget * QApplication::topLevelAt(int x, int y) [static]

This is an overloaded function.

Returns the top-level widget at the point (x, y); returns 0 if there is no such widget.

QWidgetList QApplication::topLevelWidgets() [static]

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().

Type QApplication::type() [static]

Returns the type of application (Tty, GuiClient, or GuiServer). The type is set when constructing the QApplication object.

QWidget * QApplication::widgetAt(const QPoint & point) [static]

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().

QWidget * QApplication::widgetAt(int x, int y) [static]

This is an overloaded function.

Returns the widget at global screen position (x, y), or 0 if there is no Qt widget there.