Signals and slots is a language construct introduced in Qt for communication between objects^[1] which makes it easy to implement the observer pattern while avoiding boilerplate code. The concept is that GUI widgets can send signals containing event information which can be received by other widgets / controls using special functions known as slots. This is similar to C/C++ function pointers, but signal/slot system ensures the type-correctness of callback arguments.^{[citation needed]}

1. Pyside2 Signal Slot
2. Signals And Slots Thread Safety

The signal/slot system fits well with the way graphical user interfaces are designed. Similarly, the signal/slot system can be used for other non-GUI usages, for example asynchronous I/O (including sockets, pipes, serial devices, etc.) event notification or to associate timeout events with appropriate object instances and methods or functions. It is easy to use and no registration/deregistration/invocation code need to be written, because Qt's metaobject compiler (MOC) automatically generates the needed infrastructure.

A commonly used metaphor is a spreadsheet. A spreadsheet has cells that observe the source cell(s). When the source cell is changed, the dependent cells are updated from the event.

Qt is well known for its signals and slots mechanism. But how does it work? In this blog post, we will explore the internals of QObject and QMetaObject and discover how signals and slot work under the hood. In this blog article, I show portions of Qt5 code, sometimes edited for formatting and brevity. Provide a very simple header-only C interface for creating and managing signal-slot-connections in an easy and thread-safe manner.

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Sep 08, 2015  lsignal (or lightweight signal) is a very little and fast C thread-safe implementation of signal and slot system which is based on modern C11 code. What I don't like about most of the signal libraries is, that the signals are members of each object, taking place in them, even when they are not really used. I was looking for something similar GObject does, but in C, typesafe, thread safe and with asynchronous dispatch. On a side note one of the side effects of using signals and slots is you shouldn't need to use mutexes directly. The signal slot connection (with a few rare exceptions) is safe across thread boundaries. So Just making it is usually enough to ensure race conditions don't occur. SignalsandSlots in C SarahThompson∗ March2002 1 Introduction This paper introduces the sigslot library, which implements a type-safe, thread-safe signal/slot mech-anism in C. The library is implemented entirely in C, and does not require source code.

Alternative implementations[edit]

Pyside2 Signal Slot

There are some implementations of signal/slot systems based on C++ templates, which don't require the extra metaobject compiler, as used by Qt, such as libsigc++, sigslot, vdk-signals, nano-signal-slot, neosigslot, Signals, boost.signals2, Synapse, Cpp::Events, Platinum and JBroadcaster. Common Language Infrastructure (CLI) languages such as C# also supports a similar construct although with a different terminology and syntax: events play the role of signals, and delegates are the slots. Another implementation of signals exists for ActionScript 3.0, inspired by C# events and signals/slots in Qt. Additionally, a delegate can be a local variable, much like a function pointer, while a slot in Qt must be a class member declared as such. The C based GObject system also provides similar functionality via GSignal.In D it is implemented by std.signals.

See also[edit]

Libraries[edit]

Java: sig4j - multi-threaded, type-safe, based on the FunctionalInterface annotation introduced in Java 8.

C++: vdk-signals - thread-safe, type-safe, written in C++11 with atomic variables.

References[edit]

Signals And Slots Thread Safety

1. ^'Signals & Slots - QtCore 5.1'. Qt Project. 2013-07-04. Retrieved 2013-07-04.