Qt Signal Slot Custom Class

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The Problem

Sometimes you might want to use a custom class as data of a model that you'll then display in QML.While the process is pretty straightforward, it might be difficult to put together all the infos you need so we'll summarise the process using a small example.

Qt signal/slot mechanism needs metainformation about your custom types, to be able to send them in emitted signals. To achieve that, register your type with qRegisterMetaType ('MyDataType'); Consult official QMetaType documentation for more information about this. The receivers of signals are called Slots in Qt terminology. A number of standard slots are provided on Qt classes to allow you to wire together different parts of your application. However, you can also use any Python function as a slot, and therefore receive the message yourself.

Letting QML use your custom class

Say we have a pretty simple custom class, 3 members with getter and setter functions plus a const method that does a simple calculation

1. include <QString>
2. include <QDate>

class Person{public:

private:

};

The first thing we need to do is allow QML to interact with this class. To achieve this we'll make it a Q_GADGET.A Q_GADGET is a lighter version of Q_OBJECT: it doesn't allow you to use signals, slots and the parent/child ownership system but allows you to use Q_PROPERTY and Q_INVOKABLE.

1. include <QString>
2. include <QDate>
3. include <QObject>

class Person{

public:

private:

};

Here we declared 4 properties of the object, 3 for the members and one for the age calculation.

Using your class in a C++ model

This part is straightforward, we'll build a QObject that fills a list model with dummy data and exposes it in a way QML can manage

1. include <QStandardItemModel>
2. include 'person.h'

class People : public QObject{

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public:

private:

};We expose the model through a property and we also added a slot to add a new person to the list. We used QStandardItemModel here just for convenience.

Passing to QML

The main function is very simple, the only change we made to QtCreator's default is creating the People object and passing it to the root context of QtQuick

1. include <QGuiApplication>
2. include <QQmlContext>
3. include <QQmlApplicationEngine>
4. include 'people.h'

int main(int argc, char *argv[]){

1. if defined(Q_OS_WIN)

1. endif

}

Using the data in QML

We'll display the data in a ListView import QtQml 2.2import QtQuick 2.7import QtQuick.Window 2.2import QtQuick.Layouts 1.3import QtQuick.Controls 2.3

Window {

}The first thing to notice is that we used the model property from People as the ListView's model.Now let's focus on the delegate, as you can see accessing our data is very easy we just use the roleName.propertyName syntax. Since we saved our data in Qt::EditRole we used edit as role name (a list of the default role names is In QAbstractItemModel's documentation) and then accesses directly all the properties of Person directly

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:

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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.

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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)

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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.

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There may be at most one object for each sender.

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