[Qt-C++ 03] ?Signals y Slots!
This is a translation of my original article, originally written in french.
Qt is a C++ framework used to build graphical interfaces able to run on many platforms namely Linux, Windows and OS X with a single source code.
The only step needed to make a Qt software available for a platform is to recompile it.
The Qt framework can be downloaded independently, but can also be used through its IDE called Qt Creator.
Qt Creator allows to design Qt graphical interfaces visually.
This framework doesn't only stops at graphical interfaces.
Qt is filled with a lot of options allowing, among other, to manage files, SQL request and databases connections, new characters strings with many options QStringQVariant a variable class that allows to change the underlying data type on the go, from a character to a double precision number, to an integer.
Qt ensures the compability with the underlying operating system at compile time, allowing to keep the exact same source code for an application running on any operating system with a C++ compiler.
Also, Qt uses it's own form of makefiles called ".
One particular thing about the Qt framework is that any "QObject" the base class whose any Qt class inherits is created with an interface containing "slots" and "signals".
Any signal of any object can be connected to any slot of any other or not object.
Hence, signals and slots allows for communication between objects to make event-based programming.
To make it short, Qt is a powerful programming tool allowing to do event-based programming in C++, usually in the context of making fully-fledged graphical interfaces easily.
The following articles supposes you have a very basic understanding of C++, such a class definition and creation.
Nothing very complicated will be used in this article.
Signals A "Signal" qt private public slots, in the Qt Framework, an event emitted by a class instance inheriting from the QObject parent class.
All classes from the Qt framework inherit from QObject, and any class that you might create that uses signals and slots should inherit from it.
A signal is defined by it's signature in a class header.
In example, here is a demonstration class called "DemoClass" inheriting from QObject, where we are defining a signal called "myFirstSignal ".
But this signal isn't magically emitted when we want to.
To use it, we have to emit it manually so the Qt event engine qt private public slots catch it and associate it to the slot it has been connected to.
Let's make a very simple function called "triggerMyFirstSignal " that will allow us to send that signal whenever we want.
Let's reuse our original DemoClass.
Just like any other function, you can write the implementation in a.
Now we just need to connect that signal to a slot.
But before, we need to know how to make a slot.
Slots A slot is a method or function that can be connected to a signal to be called every time that signal is emitted.
The only difference between a normal method or function is continue reading it is defined in a class inheriting from QObject under the category "public slot" or "private slot" depending on the visibility that you expect.
That way, if we reuse our previous class and add a very originally named slot "myFirstSlot", our DemoClass.
The only thing needed is that the function definition is under "public slot" or private slot to allow this function to be used by the Qt event engine.
It's important to note that a slot can be used like any other function or method.
You can call and use a slot just like any other function In example, this article source of code can be used without any problem.
Democlass demo ; demo.
The link between signal and slot We are now able to create a signal, to emit it, and to create a slot.
Now we only need to link a signal to a slot.
Right now, if a signal is emitted, no slot is going to catch it.
To realize this link, we are going to use the QObject::connect function.
The second one is the name of the signal of that instance that is going to qt private public slots connected The third one is another instance pointer to a QObject class that slim usb2 0 slot in dvdrw be the receiver of that signal.
The fourth parameter is obviously now the name of the signal that should be qt private public slots when the signal is received.
The last parameter is about the connection type.
Here, there is a default value see more to Qt::AutoConnection.
If you want to know the subtle nuances between the connection type, the Qt documentation will give you any information you might need.
But you're not limited to connecting a signal and a slot from the same instance.
For example, this is a valid way to connect signals and slots.
And from the same mechanism, any signal from any class can be connnected to any slot of any other class.
As a general rule Any signal can be linked to any slot, with no limitation from the originating instance and class of the signal or the slot.
Advanced case: signal with parameters Until now, we only did some very basic signals.
The signal may not be in itself enough of an information.
It's common in Qt classes to emit a signal when a value has been modified.
The signal that is emitted is then passed with a parameter, corresponding to the new value that has been set.
It is then possible to pass parameters and variables to a signal, that would be received by the corresponding slot.
Now for a better example Now let's illustrate how signals and slots work with a bigger and better example.
We will try to emulate a table tennis game between two players in a very simplistic way by using signals and slots.
Start by creating a new project inside Qt Creater the official Qt IDE.
Here, we are not going to use anything graphical, so choose "console application" Let's start by imagining our "Player" Class.
A player has a name and remembers the number of rounds he won.
He can service and start the game, try to send the ball back and if his opponent fails, he wins a round.
When he plays, he obviously might miss his shot.
And this information, him missing or succeeding, will be useful for his opponent to know if he should try to send the ball back or if he earned a round victory.
Let's do it this way then.
When he wins a round, he checks if he won the game or not.
And to make it easier, we consider the Player always manages to service correctly.
As you can see, every information needed here is a signal.
Our actions are slots.
Here, successfulHit is created with a value corresponding to the number of hits in the rally.
That way we can implement another class: the Ball!
It's a magic ball.
And when someone misses it, it does "Whoops!
This simple class is then easily defined: it doesn't emit any information, only answers to others.
Here is our "ball.
The line return a.
Everything that needs to be added should be added between these lines.
We obviously need two players now, and a single ball.
Let's make it easy for us here and only use pointer to class instances.
When one of our player lands article source hit, it is supposed to do "Ping!
When one of our player failes, it's supposed to do "Whoops!
Now we just need to take care of the communication between our players.
When one of them lands a hit, the other must try to hit too.
When one of them misses a hit, the other must count this winning round and start the slot shops kent by doing a service, or announcing his victory if he won the game.
QObject::connect jean,SIGNAL successfulHit int ,pierre,SLOT tryToHit int ; QObject::connect pierre,SIGNAL successfulHit int ,jean,SLOT continue reading int ; QObject::connect jean,SIGNAL failedHit ,pierre,SLOT winARound ; QObject::connect pierre,SIGNAL failedHit ,jean,SLOT winARound ; Start!
After an epic match, Pierre won this game.
Here, the Qt event engine qt private public slots all the informations between the players and the ball.
This illustrates how easy it is to do event programming with Qt, even if that particular example was very simple for the purpose of showing the possibilities of the engine.
But with such tools, it is possible to easily implement event-based logic, which is a particularly important concept in any graphic interface.
As a last note, here the entire "main.
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