13 Signal/Slot (Let's make the text editor)
En Ar Bg De El Es Fa Fi Fr Hi Hu It Ja Kn Ko Ms Nl Pl Pt Ru Sq Th Tr Uk Zh Warning : Be sure to have some knowledge of C++ before starting!
Remark : This tutorial series target mainly Qt4.
Even if most of these tutorials are also valid for Qt5, the case of Qt5 is discussed in a separate part.
It completely describes a method or function.
Don't know how to start?
Then this wiki page is for you!
It is a step by step tutorial that presents all specificities and features signals and slots editor Qt.
Introduction to Qt Qt pronounced as "cute", not "cu-tee" is a cross-platform framework that is usually used as a graphical toolkit, although it is also very helpful in creating CLI applications.
It runs on the three major desktop OSes, as well as on mobile OSes, such as Symbian, Nokia Belle, Meego Harmattan, MeeGo or BB10, and on embedded devices.
Ports for Android Necessitas and iOS are also in development.
They can be build from source, or better, be downloaded as an SDK from.
This SDK includes a lot of features, like cross compilers for Symbian and the Nokia N9.
You might choose not to install them by selecting "custom install".
NB : On linux, it is better to use the packages that your distribution provides.
Qt Creator should be available in nearly all distributions, and installing it should install all dependancies, like libraries, compilers, and developement headers.
Note: See the official page for an alternative tutorial.
We are now ready to create our first window.
And it will be as usual, a hello signals and slots editor />Qt Creator features Before writing our first GUI app, let's discover Qt Creator.
Qt Creator is yet another IDE for C++, but it is very well suited for coding Qt applications.
It provides a doc browser and the "designer", which makes creation of windows easier, all wrapped in a well-designed user interface.
It's also one of the fastest IDE's available.
Our first window Let's start by creating our first project.
Qt uses a command line tool that parses these project files in order to generate "makefiles", files that are used by compilers to build an application.
This tool is called qmake.
But, we shouldn't bother too much about qmake, since Qt Creator will do the job for us.
It can be an application, a library, or simply subdirectories.
Since our first app is a small GUI, we will need QtCore and QtGui.
Let's now add the entry point of our application.
Follow the wizard once again, naming the file "main", and you are done.
The event loop is a loop that waits for user input in GUI applications.
Let's compile this application.
By clicking on the green arrow on the bottom left, Qt Creator will compile and execute it.
The application seems to be launched and not responding.
That is actually normal.
The event loop is running and waiting for events, like mouse clicks on a GUI, but we did not provide any event continue reading be processed, so it will run indefinitely.
Let's add something to be displayed.
How a Qt program is compiled Qt Creator does the job of invoking the build system for us, but it might be interesting to know how Qt programs are compiled.
For small programs, it is easy to compile everything by hand, creating objects files, then linking them.
But for bigger projects, the command line easily becomes hard to write.
If you are familiar with Linux, you may know that all the programs are compiled using a makefile that describes all these command lines to execute.
But for some projects, even writing a makefile can become tedious.
With a simple syntax, it produces the makefile that is used to compile a Qt program.
But that is not its only goal.
Qt uses meta-objects to extend C++ functionalities, and qmake is responsible for preparing a makefile that contains this meta-object extraction phase.
You will see this in another chapter.
It will cover widgets properties, the inheritance scheme that is used in widgets, and also the parenting system.
Now that we have our button, we may want to customize it a bit.
Qt objects have a lot of attributes that can be modified using getters and setters.
In Qt, if an attribute is called foo, the associated getter and setter will have these signatures T foo const ; void setFoo const T ; In fact, Qt extends this system of attributes and getters and setters to something called property.
A property is a value of any type that can be accessed, be modified or constant, and can notify a change.
The property system is useful, especially in the third part QML.
For now, we will use "attribute" or "property" to do the same thing.
So we can use these to customize the button.
In Qt, a font is represented with the class.
The documentation provides a lot of information.
We are especially concerned here with one of the constructors of QFont.
The following snippet will change the font to Courier.
QFont font "Courier" ; button.
Setting an icon is not very difficult either.
An icon is represented with the class.
And you can create an icon provided that it has an absolute or relative path in the filesystem.
I recommend providing the absolute path in this example.
But for deployment considerations, you might use the relative path, or better, the resource system.
The following graph shows some of these inheritances: is the most basic class in Qt.
Most of classes in Qt inherit from this class.
All widgets inherit from QObject.
The most basic widget is the.
QWidget contains most properties that are used to describe a window, or a widget, like position and size, mouse cursor, tooltips, etc.
Remark : in Qt, a widget can also be a window.
In the previous section, we displayed a button that is a widget, but it appears directly as a window.
There is no need for a "QWindow" class.
Nearly all graphical elements inherit from QWidget.
We can more info for example: QAbstractButton, a base class for all button types QPushButton QCheckBox QRadioButton QFrame, that displays a frame QLabel, that displays text or picture This inheritance is done in order to facilitate properties management.
Shared properties like size and cursors can be used on other graphical components, and provides basic properties that are shared by all buttons.
Parenting system Parenting system is a convenient way of dealing with objects in Qt, especially widgets.
Any object that inherits from can have a parent and children.
So, calling delete becomes optional in certain cases.
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.
This method has the following signature: void QWidget :: setFixedSize int widthint height ; We also positioned the button using setGeometry.
This method has the following signature: void QWidget :: setGeometry int xint yint widthint height ; Subclassing QWidget 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.
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.
For example, implementing the previous window that contains a button can be done in this way : main.
Further Reading A better overview of is given in this wiki page The observer pattern 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 is used when an observable object wants to notify other observers objects about a state change.
Observer pattern is used everywhere in GUI applications, and often leads to some.
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.
Signals and slots 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 class.
These signals are sent when the user clicked pressed then releasedpressed or released the button.
Qt provides the method QObject:: connect.
You have to write the signature of the signal and the slot inside the two macros SIGNAL and SLOT.
If you want to get some information about what these macros do, please read the last section of this chapter.
Remark : Basically, signals and slots are methods, that might or might not have arguments, but that never return anything.
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.
Transmitting information 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 athat can be used to display the progress.
The hypothetical class used to check the progress might have a signal that have this signature : void MediaProgressManager :: tick int miliseconds ; and we know from the documentation, that the QProgressBar has this slot: void QProgressBar :: setValue int value ; You can see that the signal and the slot have the same kind of parameters, especially the type.
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.
The second signal is sent if the first signal is sent.
Examples Responding to an event 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 provides the clicked signal.
We also have to know that 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.
Transmitting information with signals and slots Here is a simpler example for information transmission.
It only displays a progress bar and a slider created by inside a window, and while the slider is moved, the value of the progress bar is synced with a very simple connection.
The interesting signals and slots are: void QSlider :: valueChanged int value ; void QProgressBar :: setValue int value ; 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.
Just know that you need to put SIGNAL and SLOT around the signals and slots while calling connect.
If you want to know how Qt works, it is better to read this.
The Meta Object Qt provides a meta-object system.
Code produced by moc includes signals and slots signatures, methods that are used to retrieve meta-information from those marked classes, properties handling.
Signal-Slot connections and their syntax cannot be interpreted by a regular C++ compiler.
The moc is provided to translate the QT syntax like "connect", "signals", "slots", etc into regular C++ syntax.
SIGNAL and SLOT are also two very important and useful macros.
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 signals and slots This chapter covers the second part of signals and slots: implementing custom signals and slots.
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.
Creating custom signals and slots is very simple.
After that, a slot should be declared in the corresponding section, and implemented as a normal method.
Finally, slots are connected to signals.
Signals should also be declared in the signals section, and there is no need for them to be implemented.
They are emitted using the emit keyword: emit mySignal ; Note that in order to send signals that have parameters, you have to pass them more info the signal emission: emit mySignal firstParametersecondParameter … ; Example Creating custom slots Let's start with our window with the button: window.
Now, we want that, when clicking on the button, the text is changed.
More precisely, we want that the button can be checked, and that, when checked, it displays "checked", and when unchecked, signals and slots editor restores "Hello World".
QPushButton does not implement such a specific slot, so we have to implement it on our own.
Since we are trying to react from the button being checked, and since the corresponding signal is void Window :: slotButtonClicked bool checked ; Most of the time, by convention, we implement private and protected slots by prefixing them with "slot".
Here, we are not interested in source this slot as a public function, we can make it private.
The new header is then window.
We first need to implement a counter that will count the number check this out clicks.
The meta-object compiler is used to do this.
Now we need to emit the signal when the counter reaches 10.
Connecting the newly created signal to the quit slot is done as usual: connect thisSIGNAL counterReachedQApplication :: instanceSLOT quit ; The final code is: window.
Widgets Radio button is a standard GUI component.
It is often used to make a unique choice from a list.
In Qt, the is used to create radio buttons.
Thanks to a nice heritance, a QRadioButton behaves just like a QPushButton.
All properties of the QPushButton are also the same in the QRadioButton, and everything that was learned in the second chapter can be reused here.
By default, QRadioButtons are not grouped, so many of them can be checked at the same time.
In order to have the "exclusive" behaviour of many radio buttons, we need to use.
This class can be used like this: We allocate a new button group and attach it to the parent object.
What we want is to create a menu picker.
In a window, a list of yummy plates should be displayed with radio buttons, and a push button that is used to select the chosen plate should be displayed.
Obviously, nothing will happen now when the buttons are clicked.
Signals and slots Here is an example about signals and slots.
We are going to write an application with two buttons.
The first button should display information about Qt.
Obviously, the "Quit" button should be more important, so why not make it bigger?
But we really recommend you try and figure it out by yourself how to solve these exercises.
Qt for beginners — Finding information in the documentation Qt documentation is a very valuable piece of information.
It is the place to find everything related to Qt.
But, Qt documentation is not a tutorial on how to use Qt.
It is a collection of all information related to classes, as well as some examples.
The goal of this chapter is to introduce you to the documentation as a basis for programming with Qt.
Where to find the documentation The best source of documentation is on the internet, in this developer network : It provides the full doc, as well as some DocNotes, that users can add.
These DocNotes give more examples and highlight some tricky points.
The online documentation also has a quite powerful search engine and contains also all the documentation for all versions of Qt.
While the online version requires an internet connection, the DocNotes are still available.
If the QtSDK was installed correctly, the documentation that matches the current version of Qt should have been installed, and the Help section of QtCreator should not be empty.
You can also use Qt Assistant, that is a standalone doc browser.
Important sections of the documentation If you are running the offline documentation viewer, in either Qt Creator, or Qt Assistant, you will find in the summary that there are documentations for different components of the Qt SDK.
Qt documentation provides a nice introduction of many components, and also the documentation for all the classes in Qt.
This list is listed in the page.
Another interesting page is the page that lists.
This page provides information about the different components in Qt.
If you know the class to use, and want to find the documentation, you can either type the name of this class in the search field onlineor in the filter in the index offline.
You can also search for methods and enumerations in these fields.
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