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Introduction

Signal slot is the core mechanism of Qt and also the mechanism of object communication in PyQt programming.In Qt, the QObject object and all controls in PyQt that inherit from QWidget support the slot mechanism.When the signal is transmitted, the connected slot function is automatically executed.In PyQt5, the signal and slot functions are connected by the object.signal.connect method. It also knows the type of the signal arguments so it can do the proper type conversion. We use ListLeft to only pass the same number as argument as the slot, which allows connecting a signal with many arguments to a slot with less arguments. QObject::connectImpl is the private internal function that will perform the connection.

Remember old X-Window call-back system? Generally it isn't type safe and flexible. There are many problems with them. Qt offer new event-handling system - signal-slot connections. Imagine alarm clock. When alarm is ringing, signal is sending (emitting). And you're handling it as a slot.

1. Every QObject class may have as many signals of slots as you want.
2. You can emit signal only from that class, where signal is.
3. You can connect signal with another signal (make chains of signals);
4. Every signal and slot can have unlimited count of connections with other.
5. ATTENTION! You can't set default value in slot attributes. e.g.void mySlot(int i = 0);

Connection

You can connect signal with this template:QObject::connect ( const QObject * sender, const char * signal, const QObject * receiver, const char * method);You have to wrap const char * signal and const char * method into SIGNAL () and SLOT() macros.

And you also can disconnect signal-slot:QObject::disconnect ( const QObject * sender, const char * signal, const QObject * receiver, const char * method);

Deeper

Widgets emit signals when events occur. For example, a button will emit a 'clicked' signal when it is clicked. A developer can choose to connect to a signal by creating a function (a 'slot') and calling the connect() function to relate the signal to the slot. Qt's signals and slots mechanism does not require classes to have knowledge of each other, which makes it much easier to develop highly reusable classes. Since signals and slots are type-safe, type errors are reported as warnings and do not cause crashes to occur.

For example, if a Quit button's clicked() signal is connected to the application's quit() slot, a user's click on Quit makes the application terminate. In code, this is written as

connect(button, SIGNAL (clicked()), qApp, SLOT (quit()));

Connections can be added or removed at any time during the execution of a Qt application, they can be set up so that they are executed when a signal is emitted or queued for later execution, and they can be made between objects in different threads.

The signals and slots mechanism is implemented in standard C++. The implementation uses the C++ preprocessor and moc, the Meta-Object Compiler, included with Qt. Code generation is performed automatically by Qt's build system. Developers never have to edit or even look at the generated code.

published at 20.08.2015 15:28 by Jens Weller

This is the 7th blog post in my series about writing applications with C++ using Qt and boost. This time it is about how to notify one part of our application that something has happened somewhere else. I will start with Qt, as it brings with signals and slots a mechanism to do exactly that. But, as I have the goal not to use Qt mainly in the UI Layer, I will also look on how to notify other parts of the application, when things are changing. The last episode was about QWidgets and data.

The video for this episode:

Signals and Events in Qt

But lets start with Qt. Qt offers two different systems for our needs, Qt signal/slot and QEvents. While Qt signal/slot is the moc driven signaling system of Qt (which you can connect to via QObject::connect), there is a second Event interface informing you about certain system-like events, such as QMouseEvent, QKeyEvent or QFocusEvent. Usually you have to overwrite a method to receive such events, or use an event filter, like I showed in my last post for QFocusEvents. Some classes translate QEvents to signals, such as the TreeView, which has a signal for displaying context menus. But as this blog post is more on signaling then system events...

Qt has had its own signaling mechanism for a long time now, so when you use Qt, you also will use QSignals. Qt also uses its own keywords for this: signals, slots and emit. There is an option to turn this of, and use the macros Q_SIGNAL/S,Q_SLOT/S and Q_EMIT instead: CONFIG += no_keywords. This allows to use 3rd party libraries which use these terms, e.g. boost::signal. Qt signal/slot implementation is thread safe, so that you can use it to send messages between different QThreads, this is especially important, as anything UI related should run in the main thread of Qt, anything that could block your UI should not run in this thread, so running jobs in a QThreadPool and emitting the finished result as a signal is a common pattern. Maybe I will touch this in a later post...

For now, lets see the basics of using signals and slots in Qt. This is the code from my MainWindow class constructor, connecting several signals to slots:

So, the traditional, moc driven connect method is QObject* derived sender, the SIGNAL macro defining the signal to connect to, followed by the QObject* derived receiver, then SLOT(...) is the last argument, naming the slot to connect to. There is a fifth defaultet parameter: the ConnectionType. The last line contains the new, lambda based connection option, where you again have the sender and its slot, this time as a method-pointer, and then followed by a lambda acting as the receiving slot.

This syntax can lead to a rare error, when ever a signal is overloaded, like QComboBox::currentIndexChanged, which is available with an int or QString parameter. Then you'll need an ugly static_cast to tell the compiler which version you'd like:

In this case I didn't even needed the argument from the slot. It is fairly easy to use your own signals and slots, all you need is a QObject derived class, which is processed by the moc. Mostly of course you already have classes derived from QObject indirectly, which then use signals and slots, like the page panel class:

So, slots and signals are normal member functions, declared after the qt-specific keyword signals/slots. When you want to emit a signal, its enough to just write 'emit my_signal();', and all observers on this signal will get notified. Slots are often used to react to certain events in the UI, like the currentIndexChanged signal in this case. In the widget editor of QtCreator you get an overview of available signals when right clicking and selecting 'go to slot...', this will create a slot for this signal in your QWidget derived class.

There is also the option to map certain widgets to certain values when a signal fires, this is done via QSignalMapper. I use this in a different program to have one widget for editing flag like settings, where each flag is a bit in a settings value:

The constructor only takes a QStringList for the option names, and an int for how many columns of check boxes the current use case should have. The QSignalMapper is a member variable, and each QCheckBox connects its clicked signal to the map() slot of QSignalMapper. With setMapping the connection between the sender and the value is set up. QSignalMapper offers int, QObject*, QWidget* and QString as mapping values. QVariant or a generic interface is not provided by Qt. In the clicked slot I simply toggle the bit for the corresponding flag.

When working in Qt, most of it types provide support for signals and slots through deriving from QObject, which offers connect/disconnect methods to manage your slot connections. This brings again the disadvantages of QObject and the moc, as templates can't be used in this context, all classes using signal/slot must be concrete classes. Deriving your classes from templates (CRTP e.g.) can help here to mix in a generic layer.

While Qt is fairly well prepared to manage its own messaging needs, what alternatives exist, that could be used in the non Qt related code? The C++ standard offers currently only std::function, which can be used to implement a callback mechanism. But this has its limitations, of a 1:1 or 1:many connection this is a viable option. I use it to notify my MainWindow class that a node in the tree has changed its name. Also its useful to implement classes which execute a callback in a certain context, like EventFilter in the last blog post in this series. But std::function is not an implementation of the observer pattern, and implementing your own with it would be reinventing the wheel. Boost has had for a long time a signal library, which now is available as version 2: boost::signals2.

Using boost::signals2

Honestly, if I could avoid using signals2, I would, as it has one certain disadvantage: build times increase. So far my project is kind of small, has only a few classes, which most of are less then 100 loc. Adding boost::signals2 to a class makes it hard to build a project quickly for debugging or just seeing if the work of the past hour still compiles.

The need for signals2 came in my application, when I began to understand, that there are some events, which go from the Qt layer into the boost/standard C++ layer, and then need to travel back into the Qt layer. Each Page has a shared_ptr to a layout object, which is part of a LayoutItem holding the list of layouts for a document. There is one LayoutPanel to edit, create and delete layouts in LayoutItem, and each PagePanel has a QComboBox, so that the user can select the layout for the page. Now, when a user creates/renames a layout, each PagePanel needs to be notified, but when it gets deleted, also page needs to change. This could be implemented in the Qt layer, each Qt class involved has access to the boost/C++ layer, and can make the necessary changes. But then, this important business logic of removing a layout will only work through the UI. When I use boost::signals2, it can be done in the boost/standard C++ layer.

boost::signals2 has a signal template, which has the signature as the argument, this signal type also then has the typedef for the slot type, signal::connect returns a connection object:

When ever an object subscribes to the layout signals, it must to so for all three, the vector should invoke RVO. Currently, PagePanel is the only subscriber, it simply connects to the signals using boost::bind:

One detail here is, that I do use scoped_connection, which will call disconnect() on its destruction, while the default boost::signals2::connection class does not. scoped_connection can be moved, but not copied. But once it is in the vector, it will stay there. Also, you should forward declare the connection classes, so that you don't have to include the boost/signals2.hpp headers, this prevents leaking into other sources.

But boost::signals2 can do far more. I have no use for code that depends on the order of slots called, but you can specify this with signal::contect(int group, slot):

In some context it is interesting to handle the return value of a signal, for this boost::signal2 offers a combiner, which is the second template parameter to signal: signal<float(float,float), aggregate_combiner<std::vector<float> > >. This combiner then also overwrites the return value of the signal, which is now std::vector instead of float. Another feature is that you can block a connection with shared_connection_block.

Qt Signal Slot Mechanism Lock

boost::signal2 is currently header only, thread safe and offers a few more customization points, for example you can change the mutex, but also the signature type, which currently is boost::function.

Qt Signal Slot Mechanism Car Bodies

Alternatives to boost::signals2

If you know very well what you are doing, you could use boost::signal instead of its new version, signals2. This might improve your compile times, but boost::signals is not any more maintained. Also, while signals2 is header-only, signals is not. The thread safety is a key feature of signals2, which at some time sooner or later will come into play in your code base. I don't want to introduce a 3rd party library into my project just to have signaling/observer pattern, but you should know, that there are a few alternatives (I googled that too):

• libsigslot
  • has open bugs from 2003 - 2011, memory leaks and other issues. But seems to do the job.
• libsigc++
  • a standard C++ implementation, inspired by Qt, you (might) have to derive your objects from a base class. Virtual function calls are the base of this library it seems, at least for method slots, which the call has to be derived from sigc::trackable.
  • gtkmm and glibmm seem to use this for their signaling needs.
  • the 5 open bugs seem to be feature requests mostly (and nil is a keyword in Object-C, well...)
  • the library has been rewritten using modern C++ idioms (claims the site)
• This codeproject article from 2005 gives some insights, but C++11 changes some of them I think.
• slimsig
  • seems to be a header only alternative to boost::signals2
  • 2 open bugs, no change in one year
• boost::synapse
  • this library is proposed for boost, but has not yet been reviewed.
  • I think it could be a more lightweight alternative to signals2
  • Currently its not threadsafe.

Qt Signal Slot Not Working

The only disadvantage of boost::signal2 is really its impact on compile and link time, which can be reduced through pimple and other isolation techniques, so that a recompilation is only triggered when really needed. One idea which came in my mind during this blog post is a std_signal2 header, which replaces the boost types (function, mutex etc.) with the corresponding std types. I'm not sure how this would work out, but boost::signals2 seems to be pretty well build to do this, a lot of template parameters have default values which then configure the library, and are hidden from the day to day usage.

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Qt Signal Slot Mechanism