Quite expect the new signal and slot mechanism of Qt5 Looks like it is not text-base again and able to co-work with bind, function and lambda. SaveFocusWidget is a slot which will be called whenever focus changes, and allows us to store the newly focused widget in lastFocusedWidget, so that its focus can be restored if it loses it to the input panel. ButtonClicked is a slot which will be called by the signalMapper whenever it receives a clicked signal from any of the buttons.

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

Signals & Slots Signals and slots are used for communication between objects. The signals and slots mechanism is a central feature of Qt and probably the part that differs most from the features provided by other frameworks. Signals and slots are made possible by Qt's meta-object system.

This class is obsolete. It is provided to keep old source code working. We strongly advise against using it in new code.

Public Functions

• 32 public functions inherited from QObject

Public Slots

• 1 public slot inherited from QObject

Signals

• 2 signals inherited from QObject

Additional Inherited Members

• 1 property inherited from QObject
• 11 static public members 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.

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)

This function is deprecated.

Constructs a QSignalMapper with parent parent.

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().

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[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)

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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.

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Qt allows us to connect multiple signals to the samesignal or slot. This can be useful when weprovide the user with many ways of performing the same operation.Sometimes, however, we would like the slot to behave slightlydifferently depending on which widget invoked it. In this article weexplore various solutions, including the use of QSignalMapper.

To illustrate the problem, we will implement a Keypad widget thatprovides ten QPushButtons, numbered 0 to 9, and adigitClicked(int) signal that is emitted when the user clicks abutton. We will review four solutions and discuss their respectivemerits.

The most straightforward solution to our problem (but also thesilliest) is to connect the ten QPushButton objects'clicked() signals to ten distinct slots calledbutton0Clicked() to button9Clicked(), each of which emits thedigitClicked(int) signal with a different parameter value (0 to9). Here's the definition of the Keypad class:

This is the Keypad constructor:

In the constructor, we create the QPushButtons, and we tediouslyconnect each button's clicked() signal to the correspondingprivate slot.

Each slot simply emits the digitClicked(int) signal with adifferent hard-coded argument.

Needless to say, this approach is inflexible and error-prone. It ispracticable for a small number of connections, but even for a10-key Keypad, the copy/paste is almost unbearable. Let's moveon to a better solution.

The next step is to merge thebuttonNClicked() slots into one private slotthat emits the digitClicked(int) signal with the correctparameter, depending on which button was clicked. This is possibleusing the QObject::sender() function, aswe will see shortly. The Keypad constructor becomes:

And this is the code for the buttonClicked() slot:

We start by calling sender() to retrieve a pointer to the QObjectthat emitted the signal that invoked this slot. In this particularexample, we know that the sender is a QPushButton, so we can castsender()'s return value to a QPushButton*. Then we emitthe digitClicked(int) signal with the digit value shown on thebutton.

The drawback of this approach is that we need a private slot to dothe demultiplexing. The code in buttonClicked() isn't veryelegant; if you suddenly replace the QPushButtons with anothertype of widget and forget to change the cast, you will get a crash.Similarly, if you change the text on the buttons (for example, 'NIL'instead of '0'), the digitClicked(int) signal will be emittedwith an incorrect value.

Finally, the use of sender() leads to tightly coupled components,which many people consider to be bad programming style. It isn't quiteso bad in this example, because Keypad already knows about thebutton objects, but if buttonClicked() was a slot in anotherclass, the use of sender() would have the unfortunate effect oftying that class to an implementation detail of the Keypad class.

Our third approach requires no private slot in Keypad; instead,we make sure that the buttons themselves emit a clicked(int)signal that can be directly connected to Keypad'sdigitClicked(int) signal. (When connecting a signal to anothersignal, the target signal is emitted whenever the first signal isemitted.) This requires subclassing QPushButton as follows:

Whenever QPushButton emits the clicked() signal, we interceptit in our KeypadButton subclass and emit the clicked(int)signal with the correct digit as the argument.

The Keypad constructor then looks like this:

This approach is both flexible and clean, but it is quite cumbersometo write, because it forces us to subclass QPushButton.

The fourth and last approach does not require any private slots,nor does it need a QPushButton subclass. Instead, the entiresignal-related logic is implemented in the Keypad class'sconstructor:

First, we create a QSignalMapper object. QSignalMapperinherits from QObject and provides a means of establishing arelationship between a set of zero-parameter signals and aone-parameter signal or slot. The call to setMapping() inside thefor loop establishes a mapping between a button and an integervalue; for example, buttons[3] is associated with the integervalue 3.

When the clicked() signal of a button is emitted,QSignalMapper's map() slot is invoked (thanks to theconnect() call in the for loop). If a mapping exists for theobject that emitted the signal, the mapped(int) signal is emittedwith the integer value set using setMapping(). That signal is inturn connected to the Keypad's digitClicked(int) signal.The setMapping() function exists in two versions: one that takesan int and one that takes a QString as the second argument.This makes it possible to associate an arbitrary string with a senderobject, instead of an integer. When such a mapping exists,QSignalMapper emits a mapped(const QString &) signal.

QSignalMapper does not directly support any other data types. Thismeans that, for example, if you were to implement a palette toolallowing the user to choose a color from a set of standard colors andneeded to emit a colorSelected(const QColor &) signal, your bestbet would be to use the sender() approach or the subclass approachdescribed above. If you already use a subclass of, say,QToolButton to represent a color, it doesn't cost you much to adda clicked(const QColor &) slot to it.