This article is the most comprehensive description of signals and slots in QML compared to all previous articles on this site.

In this article, I will try to explain the following when working with Qt/QML + Qt/C++:

• ways to declare signals and slots, also called methods in the C ++ class, which will be registered in the QML layer
• ways to connect to signals of classes declared in C ++ as context
• work with Q_PROPERTY, which also requires signals and slots
• ways to connect signals and slots in QML
• etc.

In this case, the signal slot connections are set automatically. In QML, you can connect and disconnect signal / slot connections using the following syntax: object1.signal.connect (object2.slot) object1.signal.disconnect (object2.slot) Signals in QML can also be connected to other signals, as is done in Qt / C.

1. From the signals slots documentation: The signature of a signal must match the signature of the receiving slot. (In fact a slot may have a shorter signature than the signal it receives because it can ignore extra arguments.) This means that a signal of the form. Signal(int, int, QString) can only be connected with slots with the following.
2. Slots and signals must have same parameters. Otherwise, the connection will not occur. Not only for connection, slot function must have same parameters with signal. For example, this sample doesn’t work: QObject::connect(ui.comboBox, SIGNAL (activated(int)), this, SLOT (onComboboxActivated)); But it works.
3. Displaying Signals and Slots. If an instance of a class is derived from QObject, and you would like to find all other objects connected to one of your object's slots using Qt's signals and slots mechanism, select Tools Options Debugger Locals and Expressions Use Debugging Helper.

Signals and slots from the C++ class

Let's create our first class that will work with signals and slots in QML. This is one of the very first examples that I have already shown, but I will repeat this example so that the article is as complete as possible.

In this example, I want to create an application that has one button and by pressing this button increases the counter that is inside the C++ class. This C++ class will be registered as a context property in the QML engine of our application.

App appearance will be next

AppCore.h

Signals And Slots Qt Creator Game

Declaring signals and slots in C ++ code will not differ much from the classical Qt/C++.

AppCore.cpp

As well as the implementation of the methods themselves.

main.cpp

main.qml

And now the most interesting. How to use an object loaded in a QML context and how to connect to its signals.

As you remember, we loaded the object into the context QML under the name appCore , we will use this object to access it. But to connect to the signal, we will need to use the QML type Connections .

Thus, you can access the object that was loaded into the context of the QML engine, call its slot, and process the signal from this object.

It is also not necessary to declare receiveFromQml() as a slot in this case. This method can also be declared as Q_INVOKABLE method.

Using Q_PROPERTY

The next option is to use the Q_PROPERTY macro. A classic property in Qt might look like this for our task

This property has the following components:

• type of property, as well as its name: int counter , which are bound to the variable int m_counter inside the class, this is the logic of code generation in Qt
• name of the method to read, matches the name of the property: counter
• method name for setting the value: setCounter
• signal that reports property changes: counterChanged

You can also pass additional parameters to this macro, but this is beyond the scope of this article. And also the property can be read only, that is, without a setter.

Signals And Slots Qt Creator Software

Now look at the full code using Q_PROPERTY

AppCore.h

AppCore.cpp

main.qml

Here you will see that connecting the property and accessing it has become easier thanks to the declarative style of QML code. Of course, you cannot always use properties, sometimes you just need to use signals, slots, and Q_INVOKABLE methods. But for variables like counter, properties are likely to be much more convenient.

Connecting signals inside QML files

Now consider the option of connecting signals and slots (functions) inside QML files. There will no longer be any C ++ code.

Among other things, you can use and disable signals from slots

Connect a signal to a signal

Also in QML there is still the ability to connect a signal to a signal, as in Qt/C++. Look at the following artificial example.

In this case, the counter will continue to increase when the button is pressed. But the button press signal is not connected directly to the counter increase function, but is forwarded through the signal.

Using Variables in Signals

QML also has the ability to use variables in signals.

Conclusion

For the most part, this entire article fits into several points:

• In C ++, to interact with the QML layer, you can use signals, slots, Q_INVOKABLE methods, as well as create properties using the Q_PROPERTY macro
• In order to respond to signals from objects, you can use the QML type Connections
• Q_PROPERTY obeys the declarative style of QML and, when a property is changed, it can automatically set new values, if the property has been added to any object in QML. In this case, the signal slot connections are set automatically.
• In QML, you can connect and disconnect signal / slot connections using the following syntax:
  • object1.signal.connect (object2.slot)
  • object1.signal.disconnect (object2.slot)
• Signals in QML can also be connected to other signals, as is done in Qt / C ++
• Signals in QML may also have arguments

Signals and slots is a language construct introduced also in Qt^[1] for communication between objects which makes it easy to implement the observer pattern while avoiding boilerplate code. The concept is that GUI widgets can send signals containing event information which can be received by other widgets / controls using special functions known as slots. This is similar to C/C++ function pointers, but signal/slot system ensures the type-correctness of callback arguments.^{[citation needed]}

The signal/slot system fits well with the way graphical user interfaces are designed.^{[citation needed]} Similarly, the signal/slot system can be used for other non-GUI usages, for example asynchronous I/O (including sockets, pipes, serial devices, etc.) event notification or to associate timeout events with appropriate object instances and methods or functions. It is easy to use and no registration/deregistration/invocation code need to be written, because Qt's metaobject compiler (MOC) automatically generates the needed infrastructure.

A commonly used metaphor^{[according to whom?]} is a spreadsheet. A spreadsheet has cells that observe the source cell(s). When the source cell is changed, the dependent cells are updated from the event.

Alternative implementations[edit]

There are some implementations of signal/slot systems based on C++ templates, which don't require the extra metaobject compiler, as used by Qt, such as libsigc++, sigslot, vdk-signals, nano-signal-slot, neosigslot, Signals, boost.signals2, Synapse, Cpp::Events, Platinum and JBroadcaster. Common Language Infrastructure (CLI) languages such as C# also supports a similar construct although with a different terminology and syntax: events play the role of signals, and delegates are the slots. Another implementation of signals exists for ActionScript 3.0, inspired by C# events and signals/slots in Qt. Additionally, a delegate can be a local variable, much like a function pointer, while a slot in Qt must be a class member declared as such. The C based GObject system also provides similar functionality via GSignal.In D it is implemented by std.signals.

See also[edit]

Libraries[edit]

Java: sig4j - multi-threaded, type-safe, based on the FunctionalInterface annotation introduced in Java 8.

C++: vdk-signals - thread-safe, type-safe, written in C++11 with atomic variables.

References[edit]

1. ^'Signals & Slots - QtCore 5.1'. Qt Project. 2013-07-04. Retrieved 2013-07-04.