QApplication 类

QApplication class manages the GUI application's control flow and main settings. 更多...

Header: #include <QApplication>
qmake: QT += widgets
继承: QGuiApplication



QApplication (int & argc , char ** argv )
virtual ~QApplication ()
QString styleSheet () const


virtual bool notify (QObject * receiver , QEvent * e ) override


void aboutQt ()
bool autoSipEnabled () const
void closeAllWindows ()
void setAutoSipEnabled (const bool enabled )
void setStyleSheet (const QString & sheet )


void focusChanged (QWidget * old , QWidget * now )


QWidget * activeModalWidget ()
QWidget * activePopupWidget ()
QWidget * activeWindow ()
void alert (QWidget * widget , int msec = 0)
QWidgetList allWidgets ()
void beep ()
int cursorFlashTime ()
QDesktopWidget * desktop ()
int doubleClickInterval ()
int exec ()
QWidget * focusWidget ()
QFont font ()
QFont font (const QWidget * widget )
QFont font (const char * className )
QFontMetrics fontMetrics ()
QSize globalStrut ()
bool isEffectEnabled (Qt::UIEffect effect )
int keyboardInputInterval ()
Qt::NavigationMode navigationMode ()
QPalette palette (const QWidget * widget )
QPalette palette (const char * className )
void setActiveWindow (QWidget * active )
void setCursorFlashTime ( int )
void setDoubleClickInterval ( int )
void setEffectEnabled (Qt::UIEffect effect , bool enable = true)
void setFont (const QFont & font , const char * className = nullptr)
void setGlobalStrut ( const QSize & )
void setKeyboardInputInterval ( int )
void setNavigationMode (Qt::NavigationMode mode )
void setPalette (const QPalette & palette , const char * className = nullptr)
void setStartDragDistance (int l )
void setStartDragTime (int ms )
void setStyle (QStyle * style )
QStyle * setStyle (const QString & style )
void setWheelScrollLines ( int )
void setWindowIcon (const QIcon & icon )
int startDragDistance ()
int startDragTime ()
const QMetaObject staticMetaObject
QStyle * style ()
QWidget * topLevelAt (const QPoint & point )
QWidget * topLevelAt (int x , int y )
QWidgetList topLevelWidgets ()
int wheelScrollLines ()
QWidget * widgetAt (const QPoint & point )
QWidget * widgetAt (int x , int y )
QIcon windowIcon ()


virtual bool event (QEvent * e ) override


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

QApplication specializes QGuiApplication with some functionality needed for QWidget -based applications. It handles widget specific initialization, finalization.

For any GUI application using Qt, there is precisely one QApplication object, no matter whether the application has 0, 1, 2 or more windows at any given time. For non- QWidget based Qt applications, use QGuiApplication instead, as it does not depend on the QtWidgets 库。

Some GUI applications provide a special batch mode ie. provide command line arguments for executing tasks without manual intervention. In such non-GUI mode, it is often sufficient to instantiate a plain QCoreApplication to avoid unnecessarily initializing resources needed for a graphical user interface. The following example shows how to dynamically create an appropriate type of application instance:

QCoreApplication* createApplication(int &argc, char *argv[])
    for (int i = 1; i < argc; ++i)
        if (!qstrcmp(argv[i], "-no-gui"))
            return new QCoreApplication(argc, argv);
    return new QApplication(argc, argv);
int main(int argc, char* argv[])
    QScopedPointer<QCoreApplication> app(createApplication(argc, argv));
    if (qobject_cast<QApplication *>( {
       // start GUI version...
    } else {
       // start non-GUI version...
    return app->exec();

QApplication object is accessible through the instance () function that returns a pointer equivalent to the global qApp pointer.

QApplication 's main areas of responsibility are:

  • It initializes the application with the user's desktop settings such as palette (), font () 和 doubleClickInterval (). 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. By using sendEvent () 和 postEvent () you can send your own events to widgets.
  • It parses common command line arguments and sets its internal state accordingly. See the constructor documentation below for more details.
  • It defines the application's look and feel, which is encapsulated in a QStyle object. This can be changed at runtime with setStyle ().
  • It specifies how the application is to allocate colors. See setColorSpec() for details.
  • It provides localization of strings that are visible to the user via translate().
  • It provides some magical objects like the desktop () 和 clipboard ().
  • It knows about the application's windows. You can ask which widget is at a certain position using widgetAt (), get a list of topLevelWidgets () 和 closeAllWindows (), etc.
  • It manages the application's mouse cursor handling, see setOverrideCursor ()

Since the QApplication object does so much initialization, it must be created before any other objects related to the user interface are created. QApplication also deals with common command line arguments. Hence, it is usually a good idea to create it before any interpretation or modification of argv is done in the application itself.

另请参阅 QCoreApplication , QAbstractEventDispatcher , QEventLoop ,和 QSettings .


autoSipEnabled : bool

toggles automatic SIP (software input panel) visibility

把此特性设为 true to automatically display the SIP when entering widgets that accept keyboard input. This property only affects widgets with the WA_InputMethodEnabled attribute set, and is typically used to launch a virtual keyboard on devices which have very few or no keys.

The property only has an effect on platforms that use software input panels.

The default is platform dependent.

This property was introduced in Qt 4.5.


bool autoSipEnabled () const
void setAutoSipEnabled (const bool enabled )

cursorFlashTime : int

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 and setting this property sets the cursor flash time for all applications.

We recommend that widgets do not cache this value as it may change at any time if the user changes the global desktop settings.

注意: This property may hold a negative value, for instance if cursor blinking is disabled.


int cursorFlashTime ()
void setCursorFlashTime ( int )

doubleClickInterval : int

This property holds the time limit in milliseconds that distinguishes a double click from two consecutive mouse clicks

在 X11 默认值为 400 毫秒。在Windows 和 Mac OS 使用操作系统的值。


int doubleClickInterval ()
void setDoubleClickInterval ( int )

globalStrut : QSize

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.


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

默认情况下,此特性包含 QSize object with zero width and height.


QSize globalStrut ()
void setGlobalStrut ( const QSize & )

keyboardInputInterval : int

此特性以毫秒为单位保持 (区分键按下和两次连续键按下的) 时间限制

在 X11 默认值为 400 毫秒。在Windows 和 Mac OS 使用操作系统的值。

This property was introduced in Qt 4.2.


int keyboardInputInterval ()
void setKeyboardInputInterval ( int )

startDragDistance : int

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() >=

Qt 使用此值在内部,如在 QFileDialog .

The default value (if the platform doesn't provide a different default) is 10 pixels.


int startDragDistance ()
void setStartDragDistance (int l )

另请参阅 startDragTime (), QPoint::manhattanLength (),和 拖放 .

startDragTime : int

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.

默认值为 500 毫秒。


int startDragTime ()
void setStartDragTime (int ms )

另请参阅 startDragDistance () 和 拖放 .

styleSheet : QString

This property holds the application style sheet

By default, this property returns an empty string unless the user specifies the -stylesheet option on the command line when running the application.

This property was introduced in Qt 4.2.


QString styleSheet () const
void setStyleSheet (const QString & sheet )

另请参阅 QWidget::setStyle () 和 Qt 样式表 .

wheelScrollLines : int

This property holds the number of lines to scroll a widget, when the mouse wheel is rotated.

If the value exceeds the widget's number of visible lines, the widget should interpret the scroll operation as a single page up or page down . If the widget is an item view class , then the result of scrolling one line depends on the setting of the widget's scroll mode . Scroll one line can mean scroll one item or scroll one pixel .

By default, this property has a value of 3.


int wheelScrollLines ()
void setWheelScrollLines ( int )

另请参阅 QStyleHints::wheelScrollLines ().

windowIcon : QIcon



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

另请参阅 QWidget::setWindowIcon () 和 设置应用程序图标 .


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

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

警告: 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.).

注意: 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 depend on your system configuration. If you compiled Qt with additional styles or have additional styles as plugins these will be available to the -style command line option. You can also set the style for all Qt applications by setting the QT_STYLE_OVERRIDE 环境变量。
  • -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.

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

  • -stylesheet stylesheet , 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.

另请参阅 QCoreApplication::arguments ().

[virtual] QApplication:: ~QApplication ()

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

[static slot] void QApplication:: aboutQt ()

Displays a simple message box about Qt. The message includes the version number of Qt being used by the application.

This is useful for inclusion in the Help menu of an application, as shown in the 菜单 范例。

This function is a convenience slot for QMessageBox::aboutQt ().

[static] QWidget *QApplication:: activeModalWidget ()

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.

另请参阅 activePopupWidget () 和 topLevelWidgets ().

[static] QWidget *QApplication:: activePopupWidget ()

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.

另请参阅 activeModalWidget () 和 topLevelWidgets ().

[static] QWidget *QApplication:: activeWindow ()

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 (),例如:若该窗口的 Widget 不接受键事件。

另请参阅 setActiveWindow (), QWidget::setFocus (), QWidget::hasFocus (),和 focusWidget ().

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

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 macOS , 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.

[static] QWidgetList QApplication:: allWidgets ()

返回应用程序的所有 Widget 的列表。

列表是空的 ( QList::isEmpty ()) 若没有 Widget。

注意: 某些 Widget 可能被隐藏。


void updateAllWidgets()
    foreach (QWidget *widget, QApplication::allWidgets())

另请参阅 topLevelWidgets () 和 QWidget::isVisible ().

[static] void QApplication:: beep ()

响铃声,使用默认音量和声音。函数 not 可用于 Qt for Embedded Linux。

[static slot] void QApplication:: closeAllWindows ()

Closes all top-level windows.

This function is particularly useful for applications with many top-level windows. It could, for example, be connected to a Exit entry in the File menu:

    const QIcon exitIcon = QIcon::fromTheme("application-exit");
    QAction *exitAct = fileMenu->addAction(exitIcon, tr("E&xit"), qApp, &QApplication::closeAllWindows);
    exitAct->setStatusTip(tr("Exit the application"));

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.

另请参阅 quitOnLastWindowClosed , lastWindowClosed (), QWidget::close (), QWidget::closeEvent (), lastWindowClosed (), QCoreApplication::quit (), topLevelWidgets (),和 QWidget::isWindow ().

[static] QDesktopWidget *QApplication:: desktop ()

返回桌面 Widget (也称根窗口)。

桌面可能由多个屏幕组成,因此它将是不正确的,例如:试图 center some widget in the desktop's geometry. QDesktopWidget has various functions for obtaining useful geometries upon the desktop, such as QDesktopWidget::screenGeometry () 和 QDesktopWidget::availableGeometry ().

On X11, it is also possible to draw on the desktop.

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

Reimplemented from QGuiApplication::event ().

[static] int QApplication:: exec ()

进入主事件循环并等待,直到 exit () is called, then returns the value that was set to exit () (其为 0 若 exit () 被调用凭借 quit ()).

有必要调用此函数以启动事件处理。主事件循环从窗口系统接收事件,并将其分派给应用程序 Widget。

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 采用 0 超时。更高级空闲处理方案可以被达成,使用 processEvents ().

推荐把清理代码连接到 aboutToQuit() 信号,而非将其放入应用程序的 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.

另请参阅 quitOnLastWindowClosed , QCoreApplication::quit (), QCoreApplication::exit (), QCoreApplication::processEvents (),和 QCoreApplication::exec ().

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

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.

另请参阅 QWidget::setFocus (), QWidget::clearFocus (),和 Qt::FocusReason .

[static] QWidget *QApplication:: focusWidget ()

Returns the application widget that has the keyboard input focus, or 0 if no widget in this application has the focus.

另请参阅 QWidget::setFocus (), QWidget::hasFocus (), activeWindow (),和 focusChanged ().

[static] QFont QApplication:: font ()


另请参阅 setFont (), fontMetrics (),和 QWidget::font ().

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


返回默认字体,为 widget .

另请参阅 fontMetrics () 和 QWidget::setFont ().

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


Returns the font for widgets of the given className .

另请参阅 setFont () 和 QWidget::font ().

[static] QFontMetrics QApplication:: fontMetrics ()

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

另请参阅 font (), setFont (), QWidget::fontMetrics (),和 QPainter::fontMetrics ().

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

返回 true if effect 被启用;否则返回 false .

By default, Qt will try to use the desktop settings. To prevent this, call setDesktopSettingsAware (false).

注意: All effects are disabled on screens running at less than 16-bit color depth.

另请参阅 setEffectEnabled () 和 Qt::UIEffect .

Returns what kind of focus navigation Qt is using.

This feature is available in Qt for Embedded Linux only.

This function was introduced in Qt 4.6.

另请参阅 setNavigationMode () 和 keypadNavigationEnabled ().

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

Reimplemented from QGuiApplication::notify ().

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

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.

另请参阅 setPalette () 和 QWidget::palette ().

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


Returns the palette for widgets of the given className .

另请参阅 setPalette () 和 QWidget::palette ().

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

Sets the active window to the active widget in response to a system event. The function is called from the platform specific event handlers.

警告: This function does not set the keyboard focus to the active widget. Call QWidget::activateWindow () 代替。

It sets the activeWindow () 和 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.

另请参阅 activeWindow () 和 QWidget::activateWindow ().

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

Enables the UI effect effect if enable is true, otherwise the effect will not be used.

注意: All effects are disabled on screens running at less than 16-bit color depth.

另请参阅 isEffectEnabled (), Qt::UIEffect ,和 setDesktopSettingsAware ().

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

把默认应用程序字体改为 font 。若 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.

警告: Do not use this function in conjunction with Qt 样式表 . 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 }"

另请参阅 font (), fontMetrics (),和 QWidget::setFont ().

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

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

This feature is available in Qt for Embedded Linux only.

This function was introduced in Qt 4.6.

另请参阅 navigationMode () 和 keypadNavigationEnabled ().

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

Changes the default application palette to palette .

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 ().

警告: Do not use this function in conjunction with Qt 样式表 . 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".

注意: 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 Vista and macOS styles.

另请参阅 QWidget::setPalette (), palette (),和 QStyle::polish ().

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

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.



When switching application styles, the color palette is set back to the initial colors or the system defaults. This is necessary since certain styles have to adapt the color palette to be fully style-guide compliant.

Setting the style before a palette has been set, i.e., before creating QApplication , will cause the application to use QStyle::standardPalette () for the palette.

警告: Qt style sheets are currently not supported for custom QStyle subclasses. We plan to address this in some future release.

另请参阅 style (), QStyle , setPalette (),和 desktopSettingsAware ().

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


Requests a QStyle object for style QStyleFactory .

The string must be one of the QStyleFactory::keys (), typically one of "windows", "windowsvista", "fusion", or "macintosh". Style names are case insensitive.

返回 nullptr if an unknown style is passed, otherwise the QStyle object returned is set as the application's GUI style.

警告: To ensure that the application's style is set correctly, it is best to call this function before the QApplication constructor, if possible.

[static] QStyle *QApplication:: style ()

Returns the application's style object.

另请参阅 setStyle () 和 QStyle .

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

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

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


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

[static] QWidgetList QApplication:: topLevelWidgets ()

返回应用程序顶层 Widget (窗口) 的列表。

注意: 某些顶层 Widget 可能被隐藏,例如:工具提示,若目前未展示工具提示。


void showAllHiddenTopLevelWidgets()
    foreach (QWidget *widget, QApplication::topLevelWidgets()) {
        if (widget->isHidden())

另请参阅 allWidgets (), QWidget::isWindow (),和 QWidget::isHidden ().

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

返回 Widget,在全局屏幕位置 point , or 0 if there is no Qt widget there.


另请参阅 QCursor::pos (), QWidget::grabMouse (),和 QWidget::grabKeyboard ().

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


返回 Widget,在全局屏幕位置 ( x , y ), or 0 if there is no Qt widget there.



引用唯一应用程序对象的全局指针。它相当于 QCoreApplication::instance (), but cast as a QApplication pointer, so only valid when the unique application object is a QApplication .

另请参阅 QCoreApplication::instance () 和 qGuiApp .