Hi, ich arbeite mich grad in QT ein und möchte ein ganz einfaches Beispiel erstellen. Ich möchte ein Fenster mit einem Button und einem Edit-Feld haben und. Nov. So jetzt das ganze noch umgekehrt also von einer C++ Funktion informationen per Signal zu unserem Qml Ui senden. Dazu erstellen wir in. Nov. Wir wollen über Signal und Slot mit unserer QML Datei kommunizieren wie wir es von unseren normalen Qt UI Anwendungen gewohnt sind.
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:: If you want to get some information about what these macros do, please read the last section of this chapter.
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.
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.
The hypothetical class used to check the progress might have a signal that have this signature:. 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.
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, it restores "Hello World".
QPushButton does not implement such a specific slot, so we have to implement it on our own. We also add our custom slot.
Since we are trying to react from the button being checked, and since the corresponding signal is. Most of the time, by convention, we implement private and protected slots by prefixing them with "slot".
Here, we are not interested in exposing this slot as a public function, we can make it private. The new header is then. We need to make the button checkable, and establish the connection, we have to add this code in the constructor:.
Based on the previous example, we want to close the application if the button is clicked checked or unchecked 10 times.
We first need to implement a counter that will count the number of clicks. These modifications implement it:. Even if the signal is declared as a method, there is no need to implement it.
The meta-object compiler is used to do this. This is because of the meta-object compiler not being run on a class that should have meta-object.
Radio button is a standard GUI component. It is often used to make a unique choice from a list. In Qt, the QRadioButton is used to create radio buttons.
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 QButtonGroup.
This class can be used like this: We allocate a new button group and attach it to the parent object. Note that the parent object might be the mainwindow, or "this":.
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.
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.
You can also add icons on the buttons, or resize them. Obviously, the "Quit" button should be more important, so why not make it bigger? 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. Qt documentation on 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. 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 All classes. Another interesting page is the page that lists All modules. This page provides information about the different components in Qt.
The search function is also quite important. 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 online , or in the filter in the index offline.
You can also search for methods and enumerations in these fields. Let's take the QTextEdit class as an example. T foo const ; void setFoo const T ;.
QFont font "Courier" ; button. Retrieved from " https: Navigation menu Personal tools Sign in. Views Read View source View history. Navigation Main page Recent changes Random page Help.
This page was last modified on 27 June , at Moreover, does existing better approach? That's definitly not what you want. What you want is:. Moreover, your function void chkState Ui:: What I advise you to do, if it's only for tests so you know that parent is the type of MainWindow , you can do:.
Can someone make an simple example? QMainWindow parent , ui new Ui:: Have you ever read the docs? Yes, but the doc dont help me: Hey, please use your brain before asking.
Maybe he is just lost. Qt is not that simple to learn. There is a good documentation and example here too.
I understand how to use it, but the syntax of it bothers me. What is "private slots: Vadim Kotov 4, 5 32 Justin 1, 1 11 Lookup QT signals and slots.
Christophe Weis 1, 4 19 Russell Davis 5, 3 30 Thanks, Qt's preprocessor is what I was missing in my mental model of what was going on.
Not correct, the code compiles all the time as "signals" and "slots" are empty defines so the compiler never sees them.
These macros are hints for moc, which generates additional code. What would fail is linking, as the moc-generated definitions signal definitions, metaobject, etc.
Is the slots keyword necessary? My experiments show that: Qt updated the connect syntax to allow for connecting a signal to an arbitrary function, including lambdas.
Because of this, slots is not necessary. However, the slots keyword still affects the way that an object's QMetaObject is built.
So, although the connection will still work, the method will not show up in introspection tools. But slots are also "normal" methods.
When you call them using the traditional way, the access specifiers are considered. In Qt5 the connect method can use function pointers which has advantages.
The access specifiers do not restrict your ability to connect signals to slots; that is, a private slot can be connected to any signal. The access specifier does, however, protect the member function from its class in the typical way while it's being invoked.
If you declare signal and slot as private you are saying that only this class can manage then but function member pointers do not have access restrictions: Euri Pinhollow 2 The Qt-meta-object-compiler moc wraps the function, are marked as slot -s, in relatively large code-wrapper.
Roman Rdgz 5, 29 85 I am not sure if it is a duplicated, but this might help: To make this happen your class should meet the following requirements: You are right, but IMHO it should be used only in rare cases.
Using slots is more elegant than using tr A lot like short cirquit if statements, they are useful in a very narrow set of circumstances, and in those cases increase readability and ease of maintenance.
But, they should only be used then, not as a general rule. As I could not find example code, I added the following: Robert Knight 2, 17 Sign up or log in Sign up using Google.
Sign up using Facebook. Sign up using Email and Password. Post as a guest Name.