QAbstractButton Class Reference
[QtGui module]

Detailed Description

The QAbstractButton class is the abstract base class of button widgets, providing functionality common to buttons.

This class implements an abstract button. Subclasses of this class handle user actions, and specify how the button is drawn.

QAbstractButton provides support for both push buttons and checkable (toggle) buttons. Checkable buttons are implemented in the QRadioButton and QCheckBox classes. Push buttons are implemented in the QPushButton and QToolButton classes; these also provide toggle behavior if required.

Any button can display a label containing text and an icon. setText() sets the text; setIcon() sets the icon. If a button is disabled, its label is changed to give the button a "disabled" appearance.

If the button is a text button with a string containing an ampersand ('&'), QAbstractButton automatically creates a shortcut key. For example:

 QPushButton *button = new QPushButton(tr("Ro&ck && Roll"), this);

The Alt+C shortcut is assigned to the button, i.e., when the user presses Alt+C the button will call animateClick(). See the QShortcut documentation for details (to display an actual ampersand, use '&&').

You can also set a custom shortcut key using the setShortcut() function. This is useful mostly for buttons that do not have any text, because they have no automatic shortcut.

 button->setIcon(QIcon(":/images/print.png"));
 button->setShortcut(tr("Alt+F7"));

All of the buttons provided by Qt (QPushButton, QToolButton, QCheckBox, and QRadioButton) can display both text and icons.

A button can be made the default button in a dialog are provided by QPushButton::setDefault() and QPushButton::setAutoDefault().

QAbstractButton provides most of the states used for buttons:

• isDown() indicates whether the button is pressed down.
• isChecked() indicates whether the button is checked. Only checkable buttons can be checked and unchecked (see below).
• isEnabled() indicates whether the button can be pressed by the user.
• setAutoRepeat() sets whether the button will auto-repeat if the user holds it down. autoRepeatDelay and autoRepeatInterval define how auto-repetition is done.
• setCheckable() sets whether the button is a toggle button or not.

The difference between isDown() and isChecked() is as follows. When the user clicks a toggle button to check it, the button is first pressed then released into the checked state. When the user clicks it again (to uncheck it), the button moves first to the pressed state, then to the unchecked state (isChecked() and isDown() are both false).

QAbstractButton provides four signals:

1. pressed() is emitted when the left mouse button is pressed while the mouse cursor is inside the button.
2. released() is emitted when the left mouse button is released.
3. clicked() is emitted when the button is first pressed and then released, when the shortcut key is typed, or when click() or animateClick() is called.
4. toggled() is emitted when the state of a toggle button changes.

To subclass QAbstractButton, you must reimplement at least paintEvent() to draw the button's outline and its text or pixmap. It is generally advisable to reimplement sizeHint() as well, and sometimes hitButton() (to determine whether a button press is within the button). For buttons with more than two states (like tri-state buttons), you will also have to reimplement checkStateSet() and nextCheckState().

See also QButtonGroup.

Property Documentation

autoExclusive : bool

This property holds whether auto-exclusivity is enabled.

If auto-exclusivity is enabled, checkable buttons that belong to the same parent widget behave as if they were part of the same exclusive button group. In an exclusive button group, only one button can be checked at any time; checking another button automatically unchecks the previously checked one.

The property has no effect on buttons that belong to a button group.

autoExclusive is off by default, except for radio buttons.

Access functions:

• bool autoExclusive () const
• void setAutoExclusive ( bool )

See also QRadioButton.

autoRepeat : bool

This property holds whether autoRepeat is enabled.

If autoRepeat is enabled, then the pressed(), released(), and clicked() signals are emitted at regular intervals when the button is down. autoRepeat is off by default. The initial delay and the repetition interval are defined in milliseconds by autoRepeatDelay and autoRepeatInterval.

Note: If a button is pressed down by a shortcut key, then auto-repeat is enabled and timed by the system and not by this class. The pressed(), released(), and clicked() signals will be emitted like in the normal case.

Access functions:

• bool autoRepeat () const
• void setAutoRepeat ( bool )

autoRepeatDelay : int

This property holds the initial delay of auto-repetition.

If autoRepeat is enabled, then autoRepeatDelay defines the initial delay in milliseconds before auto-repetition kicks in.

This property was introduced in Qt 4.2.

Access functions:

• int autoRepeatDelay () const
• void setAutoRepeatDelay ( int )

See also autoRepeat and autoRepeatInterval.

autoRepeatInterval : int

This property holds the interval of auto-repetition.

If autoRepeat is enabled, then autoRepeatInterval defines the length of the auto-repetition interval in millisecons.

This property was introduced in Qt 4.2.

Access functions:

• int autoRepeatInterval () const
• void setAutoRepeatInterval ( int )

See also autoRepeat and autoRepeatDelay.

checkable : bool

This property holds whether the button is checkable.

By default, the button is not checkable.

Access functions:

• bool isCheckable () const
• void setCheckable ( bool )

See also checked.

checked : bool

This property holds whether the button is checked.

Only checkable buttons can be checked. By default, the button is unchecked.

Access functions:

• bool isChecked () const
• void setChecked ( bool )

See also checkable.

down : bool

This property holds whether the button is pressed down.

If this property is true, the button is pressed down. The signals pressed() and clicked() are not emitted if you set this property to true. The default is false.

Access functions:

• bool isDown () const
• void setDown ( bool )

icon : QIcon

This property holds the icon shown on the button.

The icon's default size is defined by the GUI style, but can be adjusted by setting the iconSize property.

Access functions:

• QIcon icon () const
• void setIcon ( const QIcon & icon )

iconSize : QSize

This property holds the icon size used for this button.

The default size is defined by the GUI style. This is a maximum size for the icons. Smaller icons will not be scaled up.

Access functions:

• QSize iconSize () const
• void setIconSize ( const QSize & size )

shortcut : QKeySequence

This property holds the mnemonic associated with the button.

Access functions:

• QKeySequence shortcut () const
• void setShortcut ( const QKeySequence & key )

text : QString

This property holds the text shown on the button.

If the button has no text, the text() function will return a an empty string.

If the text contains an ampersand character ('&'), a shortcut is automatically created for it. The character that follows the '&' will be used as the shortcut key. Any previous shortcut will be overwritten, or cleared if no shortcut is defined by the text. See the QShortcut documentation for details (to display an actual ampersand, use '&&').

There is no default text.

Access functions:

• QString text () const
• void setText ( const QString & text )

Member Function Documentation

QAbstractButton::QAbstractButton ( QWidget * parent = 0 )

Constructs an abstract button with a parent.

QAbstractButton::~QAbstractButton ()

Destroys the button.

void QAbstractButton::animateClick ( int msec = 100 )   [slot]

Performs an animated click: the button is pressed immediately, and released msec milliseconds later (the default is 100 ms).

Calling this function again before the button was released will reset the release timer.

All signals associated with a click are emitted as appropriate.

This function does nothing if the button is disabled.

See also click().

void QAbstractButton::checkStateSet ()   [virtual protected]

This virtual handler is called when setChecked() was called, unless it was called from within nextCheckState(). It allows subclasses to reset their intermediate button states.

See also nextCheckState().

void QAbstractButton::click ()   [slot]

Performs a click.

All the usual signals associated with a click are emitted as appropriate. If the button is checkable, the state of the button is toggled.

This function does nothing if the button is disabled.

See also animateClick().

void QAbstractButton::clicked ( bool checked = false )   [signal]

This signal is emitted when the button is activated (i.e. pressed down then released while the mouse cursor is inside the button), when the shortcut key is typed, or when click() or animateClick() is called. Notably, this signal is not emitted if you call setDown(), setChecked() or toggle().

If the button is checkable, checked is true if the button is checked, or false if the button is unchecked.

See also pressed(), released(), and toggled().

QButtonGroup * QAbstractButton::group () const

Returns the group that this button belongs to.

If the button is not a member of any QButtonGroup, this function returns 0.

See also QButtonGroup.

bool QAbstractButton::hitButton ( const QPoint & pos ) const   [virtual protected]

Returns true if pos is inside the clickable button rectangle; otherwise returns false.

By default, the clickable area is the entire widget. Subclasses may reimplement this function to provide support for clickable areas of different shapes and sizes.

void QAbstractButton::nextCheckState ()   [virtual protected]

This virtual handler is called when a button is clicked. The default implementation calls setChecked(!isChecked()) if the button isCheckable(). It allows subclasses to implement intermediate button states.

See also checkStateSet().

void QAbstractButton::pressed ()   [signal]

This signal is emitted when the button is pressed down.

See also released() and clicked().

void QAbstractButton::released ()   [signal]

This signal is emitted when the button is released.

See also pressed(), clicked(), and toggled().

void QAbstractButton::toggle ()   [slot]

Toggles the state of a checkable button.

See also checked.

void QAbstractButton::toggled ( bool checked )   [signal]

This signal is emitted whenever a checkable button changes its state. checked is true if the button is checked, or false if the button is unchecked.

This may be the result of a user action, click() slot activation, or because setChecked() was called.

The states of buttons in exclusive button groups are updated before this signal is emitted. This means that slots can act on either the "off" signal or the "on" signal emitted by the buttons in the group whose states have changed.

For example, a slot that reacts to signals emitted by newly checked buttons but which ignores signals from buttons that have been unchecked can be implemented using the following pattern:

 void MyWidget::reactToToggle(bool checked)
 {
    if (checked) {
       // Examine the new button states.
       ...
    }
 }

Button groups can be created using the QButtonGroup class, and updates to the button states monitored with the QButtonGroup::buttonClicked() signal.

See also checked and clicked().