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Qt Slots

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In Qt, we have an alternative to the callback technique: We use signals and slots. A signal is emitted when a particular event occurs. Qt's widgets have many predefined signals, but we can always subclass widgets to add our own signals to them.

A slot is a function that is called in response to a particular signal. Qt's widgets have many pre-defined slots, but it is common practice to subclass widgets and add your own slots so that you can handle the signals that you are interested in.

The signals and slots mechanism is type safe: 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. Since the signatures are compatible, the compiler can help us detect type mismatches.

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. They are completely type safe. All classes that inherit from QObject or one of its subclasses e.

Signals are emitted by objects when they change their state in a way that may be interesting to other objects. This is all the object does to communicate.

It does not know or care whether anything is receiving the signals it emits. This is true information encapsulation, and ensures that the object can be used as a software component.

Slots can be used for receiving signals, but they are also normal member functions. Just as an object does not know if anything receives its signals, a slot does not know if it has any signals connected to it.

This ensures that truly independent components can be created with Qt. You can connect as many signals as you want to a single slot, and a signal can be connected to as many slots as you need.

It is even possible to connect a signal directly to another signal. This will emit the second signal immediately whenever the first is emitted.

The QObject -based version has the same internal state, and provides public methods to access the state, but in addition it has support for component programming using signals and slots.

This class can tell the outside world that its state has changed by emitting a signal, valueChanged , and it has a slot which other objects can send signals to.

They must also derive directly or indirectly from QObject. Slots are implemented by the application programmer. Here is a possible implementation of the Counter::setValue slot:.

The emit line emits the signal valueChanged from the object, with the new value as argument. In the following code snippet, we create two Counter objects and connect the first object's valueChanged signal to the second object's setValue slot using QObject::connect :.

Calling a. Then b emits the same valueChanged signal, but since no slot has been connected to b 's valueChanged signal, the signal is ignored.

Note that the setValue function sets the value and emits the signal only if value! This prevents infinite looping in the case of cyclic connections e.

By default, for every connection you make, a signal is emitted; two signals are emitted for duplicate connections. You can break all of these connections with a single disconnect call.

Any object that inherits from QObject can have a parent and children. This hierarchy tree makes many things convenient:. You can also note that when the application is closed, button1 , which is allocated on the stack, is deallocated.

Since button2 has button1 as a parent, it is deleted also. You can even test this in Qt Creator in the analyze section, by searching for a memory leak — there won't be any.

There is clearly no benefit in putting a button inside a button, but based on this idea, we might want to put buttons inside a container, that does not display anything.

This container is simply the QWidget. Note that we create a fixed size widget that acts as a window using setFixedSize.

This method has the following signature:. Until now, we have put all of our code in the main function. This was not a problem for our simple examples, but for more and more complex applications we might want to split our code into different classes.

What is often done is to create a class that is used to display a window, and implement all the widgets that are contained in this window as attributes of this class.

You can see that Qt Creator automatically generates a class template. All these elements will be explained in the next chapter, and none of them are needed now.

Implementing the window is done in the constructor. We can declare the size of the window, as well as the widgets that this window contains and their positions.

Nearly all UI toolkits have a mechanism to detect a user action, and respond to this action. Some of them use callbacks , others use listeners , but basically, all of them are inspired by the observer pattern.

Observer pattern is used when an observable object wants to notify other observers objects about a state change.

Here are some concrete examples:. Observer pattern is used everywhere in GUI applications, and often leads to some boilerplate code.

Qt was created with the idea of removing this boilerplate code and providing a nice and clean syntax, and the signal and slots mechanism is the answer.

Instead of having observable objects and observers, and registering them, Qt provides two high level concepts: signals and slots.

Here are some examples of signals and slots from our well known QPushButton class. As you can see, their names are quite explicit.

These signals are sent when the user clicked pressed then released , pressed or released the button. In order to respond to a signal, a slot must be connected to a signal.

Qt provides the method QObject:: connect. If you want to get some information about what these macros do, please read the last section of this chapter.

While the notion of a signal as a method is unusual, a slot is actually a real method, and can be called as usual in other methods, or whilst responding to a signal.

The signals and slots mechanism is useful to respond to buttons clicks, but it can do much more than that.

For example, It can also be used to communicate information. Let's say while playing a song, a progress bar is needed to show how much time remains before the song is over.

A media player might have a class that is used to check the progress of the media. An instance of this class might periodically send a tick signal, with the progress value.

This signal can be connected to a QProgressBar , that can be used to display the progress. You can see that the signal and the slot have the same kind of parameters, especially the type.

If you connect a signal to a slot that does not share the same kind of parameters, when the connection is done at run-time you will get a warning like:.

This is because the signal transmits the information to the slot using the parameters. The first parameter of the signal is passed to the first one of the slot, and the same for second, third, and so forth.

You may also provide the name of the variable if you want. It is actually even better. Remember our button app? Let's try to actually make something with this app, like being able to close it while clicking on the button.

We already know that QPushButton provides the clicked signal. We also have to know that QApplication provides the quit slot, that closes the application.

In order to make a click on a button close the app, we have to connect the signal clicked of the button to the quit slot of QApplication instance.

We can modify the code from the previous chapter to do this, but before that, you might wonder how to access to the QApplication instance while you are in another class.

Actually, it is pretty simple, since there exists a static function in QApplication , with the following signature, that is used to get it:. Here is a simpler example for information transmission.

It only displays a progress bar and a slider created by QSlider inside a window, and while the slider is moved, the value of the progress bar is synced with a very simple connection.

QSlider automatically emits the signal valueChanged with the new value passed as a parameter when the value is changed, and the method setValue of QProgressBar, is used, as we have seen, to set the value of the progress bar.

This section can be skipped for now if you only want to program with Qt. If you want to know how Qt works, it is better to read this.

Qt provides a meta-object system. To use such meta-object capabilities in an application, one can subclass QObject and mark it so that the meta-object compiler moc can interpret and translate it.

Code produced by moc includes signals and slots signatures, methods that are used to retrieve meta-information from those marked classes, properties handling All this information can be accessed using the following method:.

QMetaObject class contains all the methods that deal with meta-objects. When a signal is emitted, the meta-object system is used to compare the signature of the signal, to check the connection, and to find the slot using it's signature.

These macros are actually used to convert the provided method signature into a string that matches the one stored in the meta-object. Creating custom slots and signals is really simple.

Slots are like normal methods, but with small decorations around, while signals need little to no implementation at all. In order to implement a slot, we first need to make the class be able to send signals and have slots see the previous chapter.

After that, a slot should be declared in the corresponding section, and implemented as a normal method. Signals should also be declared in the signals section, and there is no need for them to be implemented.

Note that in order to send signals that have parameters, you have to pass them in the signal emission:. We might want to remove our previous connection that makes the application quit while clicking the button.

So from Qt 4. While developing with Qt, you only need to know about the absolute method index. But while browsing the Qt's QObject source code, you must be aware of the difference between those three.

The first thing Qt does when doing a connection is to find out the index of the signal and the slot. Qt will look up in the string tables of the meta object to find the corresponding indexes.

Then a QObjectPrivate::Connection object is created and added in the internal linked lists. What information needs to be stored for each connection?

We need a way to quickly access the connections for a given signal index. Since there can be several slots connected to the same signal, we need for each signal to have a list of the connected slots.

Each connection must contain the receiver object, and the index of the slot. We also want the connections to be automatically destroyed when the receiver is destroyed, so each receiver object needs to know who is connected to him so he can clear the connection.

Each object has then a connection vector: It is a vector which associates for each of the signals a linked lists of QObjectPrivate::Connection.

Each object also has a reversed lists of connections the object is connected to for automatic deletion. It is a doubly linked list. Linked lists are used because they allow to quickly add and remove objects.

That is because we don't really point to the previous node, but rather to the pointer to the next in the previous node. This pointer is only used when the connection is destroyed, and not to iterate backwards.

It allows not to have a special case for the first item. Here is an annotated version of its implementation from qobject.

We saw how connections are made and how signals slots are emitted. What we have not seen is the implementation of the new Qt5 syntax , but that will be for another post.

Hire us! Services Products Videos Blog. In this blog article, I show portions of Qt5 code, sometimes edited for formatting and brevity. Hover over the code to see fancy tool tips powered by the Woboq Code Browser!

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Qt Slots Video

Tutorial Qt Creator - Sintaxis alternativa de signals \u0026 slots

Qt Slots - Einführung

Bei den Signal- und Steckplatzmechanismen hängt es vom Verbindungstyp ab. Qt Creator benötigt zum Erstellen einen Compiler. Als nächstes wollen wir einen Button implementieren, der anzeigt wie oft er bereits gedrückt wurde.