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QSignalMapper Class

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

Header: #include <QSignalMapper>
CMake: find_package(Qt6 REQUIRED COMPONENTS Core)
target_link_libraries(mytarget PRIVATE Qt6::Core)
qmake: QT += core
Inherits: QObject

Public Functions

QSignalMapper(QObject *parent = nullptr)
virtual ~QSignalMapper()
QObject *mapping(int id) const
QObject *mapping(QObject *object) const
QObject *mapping(const QString &id) const
void removeMappings(QObject *sender)
void setMapping(QObject *sender, QObject *object)
void setMapping(QObject *sender, const QString &text)
void setMapping(QObject *sender, int id)

Public Slots

void map()
void map(QObject *sender)

Signals

void mappedInt(int i)
void mappedObject(QObject *object)
void mappedString(const QString &text)

Detailed Description

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, integers, objects and widgets with particular objects using setMapping(). The objects' signals can then be connected to the map() slot which will emit a signal (it could be mappedInt(), mappedString() and mappedObject()) with a value 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:

class ButtonWidget : public QWidget
{
    Q_OBJECT

public:
    ButtonWidget(const QStringList &texts, QWidget *parent = nullptr);

signals:
    void clicked(const QString &text);

private:
    QSignalMapper *signalMapper;
};

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

ButtonWidget::ButtonWidget(const QStringList &texts, QWidget *parent)
    : QWidget(parent)
{
    signalMapper = new QSignalMapper(this);

    QGridLayout *gridLayout = new QGridLayout(this);
    for (int i = 0; i < texts.size(); ++i) {
        QPushButton *button = new QPushButton(texts[i]);
        connect(button, &QPushButton::clicked, signalMapper, qOverload<>(&QSignalMapper::map));
        signalMapper->setMapping(button, texts[i]);
        gridLayout->addWidget(button, i / 3, i % 3);
    }

    connect(signalMapper, &QSignalMapper::mappedString,
            this, &ButtonWidget::clicked);
}

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 mappedString() 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.

ButtonWidget::ButtonWidget(const QStringList &texts, QWidget *parent)
    : QWidget(parent)
{
    QGridLayout *gridLayout = new QGridLayout(this);
    for (int i = 0; i < texts.size(); ++i) {
        QString text = texts[i];
        QPushButton *button = new QPushButton(text);
        connect(button, &QPushButton::clicked, [this, text] { clicked(text); });
        gridLayout->addWidget(button, i / 3, i % 3);
    }
}

See also QObject, QButtonGroup, and QActionGroup.

Member Function Documentation

[explicit] QSignalMapper::QSignalMapper(QObject *parent = nullptr)

Constructs a QSignalMapper with parent parent.

[virtual noexcept] 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::mappedInt(int i)

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.

See also setMapping().

[signal] void QSignalMapper::mappedObject(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.

See also setMapping().

[signal] void QSignalMapper::mappedString(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.

See also setMapping().

QObject *QSignalMapper::mapping(int id) const

Returns the sender QObject that is associated with the id.

See also setMapping().

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

This function overloads mapping().

Returns the sender QObject that is associated with the object.

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

This function overloads mapping().

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, QObject *object)

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

There may be at most one object 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 mappedString(text ) is emitted.

There may be at most one text for each sender.

void QSignalMapper::setMapping(QObject *sender, int id)

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

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

See also mapping().

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