Qt Signals & Slots: How they work 7. Dezember 2016 5. The one thing that confuses the most people in the beginning is the Signal & Slot mechanism of Qt. But it’s actually not that difficult to understand. In general Signals & Slots are used to loosely connect classes. Signals and Slots. In Qt, we have an alternative to the callback technique: We use signals and slots. A signal is emitted when a particular event occurs. Qt's widgets have many predefined signals, but we can always subclass widgets to add our own signals to them. A slot is a function that is called in response to a particular signal. Traditional syntax: SIGNAL and SLOT QtCore.SIGNAL and QtCore.SLOT macros allow Python to interface with Qt signal and slot delivery mechanisms. This is the old way of using signals and slots. The example below uses the well known clicked signal from a QPushButton.The connect method has a non python-friendly syntax.

• Qt Debug Signal And Slot
• Signal And Slot In Qt
• Qt Connect Signal Slot
• Qt Signal Slot 5.6
• Qt Signals And Slots Tutorial

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

Properties

• 58 properties inherited from QWidget
• 1 property inherited from QObject

Public Functions

• 211 public functions inherited from QWidget
• 31 public functions inherited from QObject
• 14 public functions inherited from QPaintDevice

Public Slots

• 19 public slots inherited from QWidget
• 1 public slot inherited from QObject

Signals

• 3 signals inherited from QWidget
• 2 signals inherited from QObject

Protected Functions

Reimplemented Protected Functions

• 35 protected functions inherited from QWidget
• 9 protected functions inherited from QObject
• 1 protected function inherited from QPaintDevice

Additional Inherited Members

• 5 static public members inherited from QWidget
• 11 static public members inherited from QObject
• 1 protected slot inherited from QWidget

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:

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, and therefore can't have any automatic shortcut.

All 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 by means of 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.

  Note: As opposed to other widgets, buttons derived from QAbstractButton accept mouse and context menu events when disabled.

• 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.

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Access functions:

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:

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:

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:

See also autoRepeat and autoRepeatDelay.

checkable : bool

This property holds whether the button is checkable

By default, the button is not checkable.

Access functions:

Mohegan sun pa slot machines. 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.

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Access functions:

Notifier signal:

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:

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:

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:

Qt Debug Signal And Slot

shortcut : QKeySequence

This property holds the mnemonic associated with the button

Access functions:

text : QString

This property holds the text shown on the button

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

Signal And Slot In Qt

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:

Member Function Documentation

QAbstractButton::QAbstractButton(QWidget *parent = Q_NULLPTR)

Constructs an abstract button with a parent.

QAbstractButton::~QAbstractButton()

Destroys the button.

[slot] void QAbstractButton::animateClick(intmsec = 100)

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 is released resets 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().

[virtual protected] void QAbstractButton::changeEvent(QEvent *e)

Reimplemented from QWidget::changeEvent().

[virtual protected] void QAbstractButton::checkStateSet()

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

See also nextCheckState().

[slot] void QAbstractButton::click()

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().

[signal] void QAbstractButton::clicked(boolchecked = false)

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().

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

Reimplemented from QObject::event().

[virtual protected] void QAbstractButton::focusInEvent(QFocusEvent *e)

Reimplemented from QWidget::focusInEvent().

[virtual protected] void QAbstractButton::focusOutEvent(QFocusEvent *e)

Reimplemented from QWidget::focusOutEvent().

QButtonGroup *QAbstractButton::group() const

Returns the group that this button belongs to.

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

See also QButtonGroup.

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

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.

[virtual protected] void QAbstractButton::keyPressEvent(QKeyEvent *e)

Reimplemented from QWidget::keyPressEvent().

Qt Connect Signal Slot

[virtual protected] void QAbstractButton::keyReleaseEvent(QKeyEvent *e)

Reimplemented from QWidget::keyReleaseEvent().

[virtual protected] void QAbstractButton::mouseMoveEvent(QMouseEvent *e)

Reimplemented from QWidget::mouseMoveEvent().

[virtual protected] void QAbstractButton::mousePressEvent(QMouseEvent *e)

Reimplemented from QWidget::mousePressEvent().

[virtual protected] void QAbstractButton::mouseReleaseEvent(QMouseEvent *e)

Reimplemented from QWidget::mouseReleaseEvent().

[virtual protected] void QAbstractButton::nextCheckState()

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().

[pure virtual protected] void QAbstractButton::paintEvent(QPaintEvent *e)

Reimplemented from QWidget::paintEvent().

[signal] void QAbstractButton::pressed()

This signal is emitted when the button is pressed down.

See also released() and clicked().

[signal] void QAbstractButton::released()

This signal is emitted when the button is released.

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

[virtual protected] void QAbstractButton::timerEvent(QTimerEvent *e)

Reimplemented from QObject::timerEvent().

[slot] void QAbstractButton::toggle()

Toggles the state of a checkable button.

See also checked.

[signal] void QAbstractButton::toggled(boolchecked)

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() is 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:

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

Note: Notifier signal for property checked.

Qt Signal Slot 5.6

See also checked and clicked().

Qt Signals And Slots Tutorial

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