• Why bother - let Qt do all the magic:) Just name your slot (in the mainWindow) like that. That guarantees, that your slot will be ok to connect it to the signal.
• So I used a namespace alias within my app. The project file uses the preprocessor to control which alias is declared and this allows my app to compile with the right library and to use the right namespace name. This works with everything except connect, signals, and slots. When I compile, I get errors that there is no namespace with the name alias.
• Signals are handled by slots, and so the QDialog will need to have slots for these signals. The slots (or handlers) are named reject and accept in this instance, and since they are default slots provided by Qt, they already exist in QDialog, so we won’t have to do anything (except if we want to override their default behavior).

• Categories: python , tutorials
• #pyqt , #python-3 , #qt , #rad
• 9 minutes read

QObject::connect(ui.btnStart, SIGNAL(clicked), this, SLOT(onButtonClicked)); If you want to get signals, you must connect these to slots. Slots are funct ions defined as slot like this example: private slots: void onButtonClicked; this code on header file.

Confession: I am the opposite of the lazy coder. I like doing things thehard way. Whether it’s developing Java in Vim without code completion,running JUnit tests on the command line (don’t forget to specify all 42dependencies in the colon-separated classpath!), creating a LaTeX graphthat looks “just right”, or writing sqlplus scripts instead of usingSQL Developer (GUIs are for amateurs), I always assumed that doing sowould make me a better programmer.

So when I was just a fledgling programmer, and I had to design andcreate some dialog windows for a side project of mine (an awesomeadd-on toAnkiSRS written in Python and Qt), I didwhat I always do: find a good resource to learnPyQt and then code everything by hand.Now, since Python is a dynamic language and I used to develop thisadd-on in Vim, I had no code completion whatsoever, and only the Qtdocumentation and that tutorial to go by. Making things even moreinteresting, is that the documentation on Qt is in C++, and is full ofstuff like this:

Obviously, I had no idea what all the asterisks and ampersands meant andhad to use good-old trial and error to see what would stick in Python.Now, on the plus side, I experimented quite a lot with the code, but nothaving code completion makes it really hard to learn the API. (And evennow, using PyCharm, code completion will not always work, because ofPython being a dynamically-typed language and all. I am still looking atthe Qt documentation quite a bit.)

One thing I noticed, though, is that the guy who develops Anki, DamienElmes, had all these .ui files lying around,and a bunch more files that read: “WARNING! All changes made to thisfile will be lost!”. Ugh, generated code! None of the dedicated,soul-cleansing and honest hard work that will shape you as a softwaredeveloper. It goes without saying I stayed far away from that kind oflaziness.

It took me some time to come around on this. One day, I actually got ajob as a professional software developer and I had much less time towork on my side-projects. So, when I wanted to implement another featurefor my Anki add-on, I found I had too little time to do it theold-fashioned way, and decided to give Qt Designer a go. Surprisingly, Ifound out it can actually help you tremendously to learn how Qt works(and also save you a bunch of time). Qt Designer allows you to visuallycreate windows using a drag-and-drop interface, then spews out an XMLrepresentation of that GUI, which can be converted to code. Thatgenerated code will show you possibilities that would have taken a longtime and a lot of StackOverflowing to figure out on your own.

So, to help other people discover the wonders of this program, here is alittle tutorial on how to do RAD and how to get a dialog window up andrunning with Qt Designer 4 and Python 3.

Installation

First, we need to install Qt Designer. On Arch Linux, it’s part of theqt4 package and you’ll also need python-pyqt4 for the Pythonbindings. (On Ubuntu, you have to install both qt4-designer and thepython-qt4 packages.)

Our goal

Our goal is to create a simple dialog window that has a text input fieldwhere we can type our name, and have a label that will display “Hellothere, $userName”. Yes, things will be that exciting. Along the way,we will learn how to assign emitted signals to slots and how to handleevents.

Creating a dialog

Fire up Qt Designer, and you will be presented with a “New form” dialog(if you do not see it, go to File > New…).

For this tutorial, we are going to choose a fairly small “Dialog withButtons Bottom”:

To the left are the widgets that we can add to our freshly createddialog, to the right, from top to bottom, we see the currently addedwidgets, the properties of those widgets and the signals and slotscurrently assigned to the dialog window.

I’m going to add a text label and a so-called line editor to our widget.To make sure they will align nicely, I will put them together in ahorizontal container, a QHBoxLayout:

In the object inspector, we can see the hierarchy of added widgets:

This is all simple drag-and-drop: we select a widget from the left pane,drag it to the desired location in the dialog and release the mouse.

Finally, we add two more label: one at the top, instructing the userwhat to do, and a second one near the bottom, where we soon will displayour message.

Qt Designer provides an easy way to connect signals to slots. If you goto Edit > Edit Signals/Slots (or press F4) you will be presented witha graphical overview of the currently assigned signals and slots. Whenwe start out, the button box at the bottom already emits two signals:rejected and accepted, from the Cancel and Ok button respectively:

The signals rejected() and accepted() are emitted when the canceland OK buttons are clicked respectively, and the ground symbols indicatethe object that is interested in these signals: in this case the dialogwindow itself. Signals are handled by slots, and so the QDialog willneed to have slots for these signals. The slots (or handlers) are namedreject() and accept() in this instance, and since they are defaultslots provided by Qt, they already exist in QDialog, so we won’t haveto do anything (except if we want to override their default behavior).

What we want to do now is “catch” the textEdited signal from the lineeditor widget and create a slot in the dialog window that will handleit. This new slot we’ll call say_hello. So we click the line editorwidget and drag a line to anywhere on the dialog window: a ground symbolshould be visible:

In the window that appears now, we can select the signal that we areinterested in (textEdited) and assign it to a predefined slot, or wecan click on Edit… and create our own slot. Let’s do that:

We click the green plus sign, and type in the name of our new slot(say_hello):

The result will now look like:

In Qt Designer, you can preview your creation by going to Form >Preview… (or pressing Ctrl + R). Notice, however, that typing text inthe line editor won’t do anything yet. This is because we haven’twritten any implementation code for it. In the next section we will seehow to do that.

You can also see the code that will be generated by going to Form >View code…, although this code is going to be in C++.

Okay, enough with designing our dialog window, let’s get to actualPython coding.

Generating code (or not)

When we save our project in Qt Designer, it will create a .ui file,which is just XML containing all the properties (widgets, sizes, signals& slots, etc.) that make up the GUI.

PyQt comes with a program, pyuic4, that can convert these .ui filesto Python code. Two interesting command-line options are --preview (or-p for short), that will allow you to preview the dynamically createdGUI, and --execute (or -x for short), that will generate Python codethat can be executed as a stand-alone. The --output switch (or -o)allows you to specify a filename where the code will be saved.

In our case, creating a stand-alone executable is not going to work,because we have added a custom slot (say_hello) that needs to beimplemented first. So we will use pyuic4 to generate the form class:

This is the result:

It is here that you can learn a lot about how PyQt works, even thoughsome statements seem a bit baroque, like the binding of thetextChanged signal to our custom slot:

If you would write this yourself, you would probably prefer:

(Incidentally, the text between the square brackets intextChanged[str] indicates that a single argument of this type ispassed to the slot.)

Bringing it all together

Having generated our form class, we can now create a base class thatwill use this class:

When we are passing self to self.ui.setupUi, we are passing thewidget (a QDialog) in which the user interface will be created.

We implement our custom slot by defining a method say_hello that takesa single argument (the user-provided text from the line editor). Wecraft a witty sentence with it, and set it as the label text.

As mentioned before, we could also dynamically create the GUI directlyfrom the .ui file:

Running the application

Running this will show the following (drum roll):

Signals and slots are used for communication between objects. The signals and slots mechanism is a central feature of Qt and probably the part that differs most from the features provided by other frameworks. Signals and slots are made possible by Qt's meta-object system.

Introduction

In GUI programming, when we change one widget, we often want another widget to be notified. More generally, we want objects of any kind to be able to communicate with one another. For example, if a user clicks a Close button, we probably want the window's close() function to be called.

Other toolkits achieve this kind of communication using callbacks. A callback is a pointer to a function, so if you want a processing function to notify you about some event you pass a pointer to another function (the callback) to the processing function. The processing function then calls the callback when appropriate. While successful frameworks using this method do exist, callbacks can be unintuitive and may suffer from problems in ensuring the type-correctness of callback arguments.

Signals and Slots

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 when using the function pointer-based syntax. The string-based SIGNAL and SLOT syntax will detect type mismatches at runtime. 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.g., QWidget) can contain signals and slots. 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.)

Together, signals and slots make up a powerful component programming mechanism.

Signals

Signals are emitted by an object when its internal state has changed in some way that might be interesting to the object's client or owner. Signals are public access functions and can be emitted from anywhere, but we recommend to only emit them from the class that defines the signal and its subclasses.

When a signal is emitted, the slots connected to it are usually executed immediately, just like a normal function call. When this happens, the signals and slots mechanism is totally independent of any GUI event loop. Execution of the code following the emit statement will occur once all slots have returned. The situation is slightly different when using queued connections; in such a case, the code following the emit keyword will continue immediately, and the slots will be executed later.

If several slots are connected to one signal, the slots will be executed one after the other, in the order they have been connected, when the signal is emitted.

Signals are automatically generated by the moc and must not be implemented in the .cpp file. They can never have return types (i.e. use void).

A note about arguments: Our experience shows that signals and slots are more reusable if they do not use special types. If QScrollBar::valueChanged() were to use a special type such as the hypothetical QScrollBar::Range, it could only be connected to slots designed specifically for QScrollBar. Connecting different input widgets together would be impossible.

Qt Connect Slot By Name

Slots

A slot is called when a signal connected to it is emitted. Slots are normal C++ functions and can be called normally; their only special feature is that signals can be connected to them.

Since slots are normal member functions, they follow the normal C++ rules when called directly. However, as slots, they can be invoked by any component, regardless of its access level, via a signal-slot connection. This means that a signal emitted from an instance of an arbitrary class can cause a private slot to be invoked in an instance of an unrelated class.

You can also define slots to be virtual, which we have found quite useful in practice.

Compared to callbacks, signals and slots are slightly slower because of the increased flexibility they provide, although the difference for real applications is insignificant. In general, emitting a signal that is connected to some slots, is approximately ten times slower than calling the receivers directly, with non-virtual function calls. This is the overhead required to locate the connection object, to safely iterate over all connections (i.e. checking that subsequent receivers have not been destroyed during the emission), and to marshall any parameters in a generic fashion. While ten non-virtual function calls may sound like a lot, it's much less overhead than any new or delete operation, for example. As soon as you perform a string, vector or list operation that behind the scene requires new or delete, the signals and slots overhead is only responsible for a very small proportion of the complete function call costs. The same is true whenever you do a system call in a slot; or indirectly call more than ten functions. The simplicity and flexibility of the signals and slots mechanism is well worth the overhead, which your users won't even notice.

Note that other libraries that define variables called signals or slots may cause compiler warnings and errors when compiled alongside a Qt-based application. To solve this problem, #undef the offending preprocessor symbol.

A Small Example

A minimal C++ class declaration might read:

A small QObject-based class might read:

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.

All classes that contain signals or slots must mention Q_OBJECT at the top of their declaration. 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.setValue(12) makes a emit a valueChanged(12) signal, which b will receive in its setValue() slot, i.e. b.setValue(12) is called. 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 != m_value. This prevents infinite looping in the case of cyclic connections (e.g., if b.valueChanged() were connected to a.setValue()).

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. If you pass the Qt::UniqueConnectiontype, the connection will only be made if it is not a duplicate. If there is already a duplicate (exact same signal to the exact same slot on the same objects), the connection will fail and connect will return false.

This example illustrates that objects can work together without needing to know any information about each other. To enable this, the objects only need to be connected together, and this can be achieved with some simple QObject::connect() function calls, or with uic's automatic connections feature.

A Real Example

The following is an example of the header of a simple widget class without member functions. The purpose is to show how you can utilize signals and slots in your own applications.

LcdNumber inherits QObject, which has most of the signal-slot knowledge, via QFrame and QWidget. It is somewhat similar to the built-in QLCDNumber widget.

The Q_OBJECT macro is expanded by the preprocessor to declare several member functions that are implemented by the moc; if you get compiler errors along the lines of 'undefined reference to vtable for LcdNumber', you have probably forgotten to run the moc or to include the moc output in the link command.

After the class constructor and public members, we declare the class signals. The LcdNumber class emits a signal, overflow(), when it is asked to show an impossible value.

If you don't care about overflow, or you know that overflow cannot occur, you can ignore the overflow() signal, i.e. don't connect it to any slot.

If on the other hand you want to call two different error functions when the number overflows, simply connect the signal to two different slots. Qt will call both (in the order they were connected).

A slot is a receiving function used to get information about state changes in other widgets. LcdNumber uses it, as the code above indicates, to set the displayed number. Since display() is part of the class's interface with the rest of the program, the slot is public.

Several of the example programs connect the valueChanged() signal of a QScrollBar to the display() slot, so the LCD number continuously shows the value of the scroll bar.

Note that display() is overloaded; Qt will select the appropriate version when you connect a signal to the slot. With callbacks, you'd have to find five different names and keep track of the types yourself.

Signals And Slots With Default Arguments

The signatures of signals and slots may contain arguments, and the arguments can have default values. Consider QObject::destroyed():

When a QObject is deleted, it emits this QObject::destroyed() signal. We want to catch this signal, wherever we might have a dangling reference to the deleted QObject, so we can clean it up. A suitable slot signature might be:

To connect the signal to the slot, we use QObject::connect(). There are several ways to connect signal and slots. The first is to use function pointers:

There are several advantages to using QObject::connect() with function pointers. First, it allows the compiler to check that the signal's arguments are compatible with the slot's arguments. Arguments can also be implicitly converted by the compiler, if needed.

You can also connect to functors or C++11 lambdas:

In both these cases, we provide this as context in the call to connect(). The context object provides information about in which thread the receiver should be executed. This is important, as providing the context ensures that the receiver is executed in the context thread.

The lambda will be disconnected when the sender or context is destroyed. You should take care that any objects used inside the functor are still alive when the signal is emitted.

The other way to connect a signal to a slot is to use QObject::connect() and the SIGNAL and SLOT macros. The rule about whether to include arguments or not in the SIGNAL() and SLOT() macros, if the arguments have default values, is that the signature passed to the SIGNAL() macro must not have fewer arguments than the signature passed to the SLOT() macro.

Qt Connect Slots By Name Search

All of these would work:

But this one won't work:

...because the slot will be expecting a QObject that the signal will not send. This connection will report a runtime error.

Note that signal and slot arguments are not checked by the compiler when using this QObject::connect() overload.

Advanced Signals and Slots Usage

For cases where you may require information on the sender of the signal, Qt provides the QObject::sender() function, which returns a pointer to the object that sent the signal.

Lambda expressions are a convenient way to pass custom arguments to a slot:

Using Qt with 3rd Party Signals and Slots

It is possible to use Qt with a 3rd party signal/slot mechanism. You can even use both mechanisms in the same project. Just add the following line to your qmake project (.pro) file.

It tells Qt not to define the moc keywords signals, slots, and emit, because these names will be used by a 3rd party library, e.g. Boost. Then to continue using Qt signals and slots with the no_keywords flag, simply replace all uses of the Qt moc keywords in your sources with the corresponding Qt macros Q_SIGNALS (or Q_SIGNAL), Q_SLOTS (or Q_SLOT), and Q_EMIT.

See also QLCDNumber, QObject::connect(), Digital Clock Example, Tetrix Example, Meta-Object System, and Qt's Property System.

Qt Connect Slots By Name Labels

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