Signals and slots are loosely coupled: A class which emits a signal neither knows nor cares which slots receive the signal. Qt's signals and slots mechanism ensures that if you connect a signal to a slot, the slot will be called with the signal's parameters at the right time. Signals and slots can take any number of arguments of any type. I have a class X with a slot, and a class Y with a signal. I'm setting up the connection from class X, and created a public method in class Y to emit the signal from class X (I'm not sure this step was necessary). Then, if I call that method from class X, signal is emitted, and slot is executed. Qt allows signal relaying by connecting a signal to another signal if their signatures match. It's not the case here; the clicked signal has no parameter and the removed signal needs a Task. A lambda avoids the declaration of a verbose slot in Task. Qt 5 accepts a lambda instead of a slot in a connect, and both syntaxes can be used.

The QSignalMapper class bundles signals from identifiable senders. More...

Public Functions

• 32 public functions inherited from QObject

Public Slots

• 1 public slot inherited from QObject

Signals

• 2 signals inherited from QObject

Static Public Members

• 10 static public members inherited from QObject

Additional Inherited Members

• 1 property inherited from QObject
• 9 protected functions inherited from QObject

Detailed Description

The QSignalMapper class bundles signals from identifiable senders.

This class collects a set of parameterless signals, and re-emits them with integer, string or widget parameters corresponding to the object that sent the signal. Note that in most cases you can use lambdas for passing custom parameters to slots. This is less costly and will simplify the code.

The class supports the mapping of particular strings or integers with particular objects using setMapping(). The objects' signals can then be connected to the map() slot which will emit the mapped() signal with the string or integer associated with the original signalling object. Mappings can be removed later using removeMappings().

Example: Suppose we want to create a custom widget that contains a group of buttons (like a tool palette). One approach is to connect each button's clicked() signal to its own custom slot; but in this example we want to connect all the buttons to a single slot and parameterize the slot by the button that was clicked.

Here's the definition of a simple custom widget that has a single signal, clicked(), which is emitted with the text of the button that was clicked:

The only function that we need to implement is the constructor:

A list of texts is passed to the constructor. A signal mapper is constructed and for each text in the list a QPushButton is created. We connect each button's clicked() signal to the signal mapper's map() slot, and create a mapping in the signal mapper from each button to the button's text. Finally we connect the signal mapper's mapped() signal to the custom widget's clicked() signal. When the user clicks a button, the custom widget will emit a single clicked() signal whose argument is the text of the button the user clicked.

This class was mostly useful before lambda functions could be used as slots. The example above can be rewritten simpler without QSignalMapper by connecting to a lambda function.

See also QObject, QButtonGroup, and QActionGroup.

Member Function Documentation

QSignalMapper::QSignalMapper(QObject *parent = nullptr)

Constructs a QSignalMapper with parent parent.

[virtual] QSignalMapper::~QSignalMapper()

Destroys the QSignalMapper.

[slot] void QSignalMapper::map()

This slot emits signals based on which object sends signals to it.

[slot] void QSignalMapper::map(QObject *sender)

This slot emits signals based on the sender object.

[signal] void QSignalMapper::mapped(inti)

This signal is emitted when map() is signalled from an object that has an integer mapping set. The object's mapped integer is passed in i.

Note: Signal mapped is overloaded in this class. To connect to this signal by using the function pointer syntax, Qt provides a convenient helper for obtaining the function pointer as shown in this example:

See also setMapping().

[signal] void QSignalMapper::mapped(const QString &text)

This signal is emitted when map() is signalled from an object that has a string mapping set. The object's mapped string is passed in text.

Note: Signal mapped is overloaded in this class. To connect to this signal by using the function pointer syntax, Qt provides a convenient helper for obtaining the function pointer as shown in this example:

See also setMapping().

[signal] void QSignalMapper::mapped(QWidget *widget)

This signal is emitted when map() is signalled from an object that has a widget mapping set. The object's mapped widget is passed in widget.

Note: Signal mapped is overloaded in this class. To connect to this signal by using the function pointer syntax, Qt provides a convenient helper for obtaining the function pointer as shown in this example:

See also setMapping().

[signal] void QSignalMapper::mapped(QObject *object)

This signal is emitted when map() is signalled from an object that has an object mapping set. The object provided by the map is passed in object.

Note: Signal mapped is overloaded in this class. To connect to this signal by using the function pointer syntax, Qt provides a convenient helper for obtaining the function pointer as shown in this example:

See also setMapping().

QObject *QSignalMapper::mapping(intid) const

Returns the sender QObject that is associated with the id.

See also setMapping().

QObject *QSignalMapper::mapping(const QString &id) const

This function overloads mapping().

QObject *QSignalMapper::mapping(QWidget *widget) const

This function overloads mapping().

Returns the sender QObject that is associated with the widget.

QObject *QSignalMapper::mapping(QObject *object) const

This function overloads mapping().

Returns the sender QObject that is associated with the object.

void QSignalMapper::removeMappings(QObject *sender)

Removes all mappings for sender.

This is done automatically when mapped objects are destroyed.

Note: This does not disconnect any signals. If sender is not destroyed then this will need to be done explicitly if required.

void QSignalMapper::setMapping(QObject *sender, intid)

Adds a mapping so that when map() is signalled from the given sender, the signal mapped(id) is emitted.

There may be at most one integer ID for each sender.

See also mapping().

void QSignalMapper::setMapping(QObject *sender, const QString &text)

Adds a mapping so that when map() is signalled from the sender, the signal mapped(text ) is emitted.

There may be at most one text for each sender.

void QSignalMapper::setMapping(QObject *sender, QWidget *widget)

Adds a mapping so that when map() is signalled from the sender, the signal mapped(widget ) is emitted.

There may be at most one widget for each sender.

void QSignalMapper::setMapping(QObject *sender, QObject *object)

Adds a mapping so that when map() is signalled from the sender, the signal mapped(object ) is emitted.

There may be at most one object for each sender.

Qt Signal Slot Parameter

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