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The QApplication class manages the GUI application's control flow and main settings. More...
#include <QApplication>
Inherits QCoreApplication.
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The QApplication class manages the GUI application's control flow and main settings.
It 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. It also handles most system-wide and application-wide settings.
For any GUI application that uses Qt, there is precisely one QApplication object, no matter whether the application has 0, 1, 2 or more windows at any time. For non-GUI Qt applications, use QCoreApplication instead, which doesn't depend on the QtGui library.
The QApplication object is accessible through the instance() function which return 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.
Since it also deals with common command line arguments, it is usually a good idea to create it before any interpretation or modification of argv is done in the application itself.
See also QCoreApplication, QAbstractEventDispatcher, QEventLoop, and QSettings.
| 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 Qtopia Core) |
This property holds the text cursor's flash (blink) time in milliseconds.
The flash time is the time required to display, invert and restore the caret display. Usually the text cursor is displayed for half the cursor flash time, then hidden for the same amount of time, but this may vary.
The default value on X11 is 1000 milliseconds. On Windows, the control panel value is used. Widgets should not cache this value since it may be changed at any time by the user changing the global desktop settings.
Note that on Microsoft Windows, setting this property sets the cursor flash time for all applications.
Access functions:
This property holds the time limit in milliseconds that distinguishes a double click from two consecutive mouse clicks.
The default value on X11 is 400 milliseconds. On Windows and Mac OS X, the operating system's value is used.
On Microsoft Windows, calling this function sets the double click interval for all applications.
Access functions:
This property holds the minimum size that any GUI element that the user can interact with should have.
For example no button should be resized to be smaller than the global strut size. The strut size should be considered when reimplementing GUI controls that may be used on touch-screens or similar I/O devices.
Example:
QSize MyWidget::sizeHint() const
{
return QSize(80, 25).expandedTo(QApplication::globalStrut());
}
Access functions:
This property holds the time limit in milliseconds that distinguishes a key press from two consecutive key presses.
The default value on X11 is 400 milliseconds. On Windows and Mac OS X, the operating system's value is used.
Access functions:
This property holds the default layout direction for this application.
On system start-up, the default layout direction depends on the application's language.
Access functions:
See also QWidget::layoutDirection, isLeftToRight(), and isRightToLeft().
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) with the Qt::WA_QuitOnClose attribute set is closed. By default this attribute is set for all widgets except transient windows such as splash screens, tool windows, and popup menus.
Access functions:
See also quit() and QWidget::close().
If you support drag and drop in your application, and want to start a drag and drop operation after the user has moved the cursor a certain distance with a button held down, you should use this property's value as the minimum distance required.
For example, if the mouse position of the click is stored in startPos and the current position (e.g. in the mouse move event) is currentPos, you can find out if a drag should be started with code like this:
if ((startPos - currentPos).manhattanLength() >=
QApplication::startDragDistance())
startTheDrag();
Qt uses this value internally, e.g. in QFileDialog.
The default value is 4 pixels.
Access functions:
See also startDragTime(), QPoint::manhattanLength(), and Drag and Drop.
This property holds the time in milliseconds that a mouse button must be held down before a drag and drop operation will begin.
If you support drag and drop in your application, and want to start a drag and drop operation after the user has held down a mouse button for a certain amount of time, you should use this property's value as the delay.
Qt also uses this delay internally, e.g. in QTextEdit and QLineEdit, for starting a drag.
The default value is 500 ms.
Access functions:
See also startDragDistance() and Drag and Drop.
This property holds the number of lines to scroll when the mouse wheel is rotated.
If this number exceeds the number of visible lines in a certain widget, the widget should interpret the scroll operation as a single "page up" or "page down" operation instead.
Access functions:
This property holds the default window icon.
Access functions:
See also QWidget::setWindowIcon() and Setting the Application Icon.
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 widgets, pixmaps, bitmaps etc.).
Note that argc and argv might be changed. Qt removes command line arguments that it recognizes. The original argc and argv can be accessed later with qApp->argc() and qApp->argv(). The documentation for argv() contains a detailed description of how to process command line arguments.
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 also supports some traditional X11 command line options:
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.
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 Macintosh, 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.
For Qtopia Core, passing QApplication::GuiServer for type makes this application the server (equivalent to running with the -qws option).
Create an application, given an already open display display. If visual and colormap are non-zero, the application will use those as the default Visual and Colormap contexts.
Warning: Qt only supports TrueColor visuals at depths higher than 8 bits-per-pixel.
This is available only on X11.
Create 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 as the default Visual and Colormap contexts.
Warning: Qt only supports TrueColor visuals at depths higher than 8 bits-per-pixel.
This is available only on X11.
Cleans up any window system resources that were allocated by this application. Sets the global variable qApp to 0.
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. See the examples/menu/menu.cpp 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. Note that 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().
Returns a list of all the widgets in the application.
The list is empty (QList::isEmpty()) if there are no widgets.
Note that 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 Qtopia Core.
Changes the currently active application override cursor to cursor.
This function has no effect if setOverrideCursor() wasn't called.
See also setOverrideCursor(), overrideCursor(), restoreOverrideCursor(), and QWidget::setCursor().
Returns a pointer to the application global clipboard.
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->setShortcut(tr("Ctrl+Q"));
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.
Note that 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 session manager 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 widget is useful for obtaining the size of the screen. It may also be possible to draw on the desktop. We recommend against assuming that it's possible to draw on the desktop, since this does not work on all operating systems.
Example:
QDesktopWidget *desktop = QApplication::desktop();
int width = desktop->width();
int height = desktop->height();
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().
Enters the main event loop and waits until exit() is called or the main widget is destroyed, and 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 speaking, 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().
See also quitOnLastWindowClosed, quit(), exit(), processEvents(), and QCoreApplication::exec().
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. Note that 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.
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 font for the widget, or the default application font if widget is 0.
See also setFont(), fontMetrics(), 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. Call setDesktopSettingsAware(false) to prevent this.
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 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().
See also setKeypadNavigationEnabled().
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 be turned off by setting quitOnLastWindowClosed to false.
See also QWidget::close().
Warning: This virtual function is only implemented under Mac OS X.
If you create an application that inherits QApplication and reimplement this function, you get direct access to all Carbon Events that are received from Mac OS X with this function being called with the caller and the event.
Return true if you want to stop the event from being processed. Return false for normal event dispatching. The default implementation returns false.
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::MousePress and QEvent::MouseRelease events).
It should be noted this may not reflect the actual buttons held on theinput 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().
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 member function, provided for convenience.
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 isn't a 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 member function, provided for convenience.
Returns the palette for widgets of the given className.
See also setPalette() and QWidget::palette().
Warning:
Return the QWSDecoration used for decorating windows.
This method is non-portable. It is only available in Qtopia Core.
See also QDecoration.
This virtual function is only implemented under Qtopia Core.
If you create an application that inherits QApplication and reimplement this function, you get direct access to all QWS (Q Window System) events that the are received from the QWS master process. 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.
Set Qtopia Core custom color table.
Qtopia Core on 8-bpp displays allocates a standard 216 color cube. The remaining 40 colors may be used by setting a custom color table in the QWS master process before any clients connect.
colorTable is an array of up to 40 custom colors. start is the starting index (0-39) and numColors is the number of colors to be set (1-40).
This method is non-portable. It is available only in Qtopia Core.
Set the QWSDecoration derived class to use for decorating the Qtopia Core windows to decoration.
This method is non-portable. It is only available in Qtopia Core.
See also QDecoration.
This is an overloaded member function, provided for convenience.
Requests a QDecoration object for decoration from the QDecorationFactory.
The string must be one of the QDecorationFactory::keys(). Keys are case insensitive.
A later call to the QApplication constructor will override the requested style when a "-style" option is passed in as a commandline parameter.
Returns 0 if an unknown decoration is passed, otherwise the QStyle object returned is set as the application's GUI style.
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.
Note that 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 session manager 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().
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.
See also font(), fontMetrics(), and QWidget::setFont().
This function replaces the QInputContext instance used by the application with inputContext.
See also inputContext().
Sets whether Qt should use focus navigation suitable for use with a minimal keypad.
If enabled, Qt::Key_Up and Qt::Key_Down are used to change focus.
This feature is available in Qtopia Core only.
See also keypadNavigationEnabled().
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().
See also QWidget::setPalette(), palette(), and QStyle::polish().
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.
Warning: To ensure that the application's style is set correctly, it is best to call this function before the QApplication constructor, if possible.
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.
Note that 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.
See also style(), QStyle, setPalette(), and desktopSettingsAware().
This is an overloaded member function, provided for convenience.
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.
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 member function, provided for convenience.
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 that 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().
This is an overloaded member function, provided for convenience.
Returns the widget at global screen position (x, y), or 0 if there is no Qt widget there.
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.
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().
A global pointer referring to the unique application object. It is equivalent to the pointer returned by the QCoreApplication::instance() function.
Only one application object can be created.
See also QCoreApplication::instance().
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