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]}

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.

Alternative implementations[edit]

Thread Safety and Reentrancy Overview 16:14 Cross Thread Signals and Slots - Example1 21:58 Cross Thread Signals and Slots - Example2 13:43 Cross Thread Signals and Slots - Example3 06:31 Slots in QThread Subclass 09:43 Thread Safety and Reentrancy - Chapter Review. It is expected that slot objects will be created then connected to a signal in a single thread. Once they have been copied into a signal's slot list, they are protected by the mutex associated with each signal-slot connection. The signals2::trackable class does NOT provide thread-safe automatic connection management. In particular, it leaves. It is safe to connect signals and slots across different threads, thanks to a mechanism called queued connections. Another way to make code run in a separate thread, is to subclass QThread and reimplement run. Dec 29, 2012 Let's say I have a signal change connected to a slot notify. If the change signal is emitted, the notify slot will start executing. Now what happens if a second change signal is emitted and the first notify slot didn't finish its execution? Is the second slot launched concurrently with the first? And if so, is Qt handling the thread-safety. Signals are connected to some set of slots, which are callback receivers (also called event targets or subscribers), which are called when the signal is 'emitted.' Signals and slots are managed, in that signals and slots (or, more properly, objects that occur as part of the slots) can track connections and are capable of automatically.

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.

C++ Thread Signal

Libraries[edit]

Boost Signals And Slots

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

C++ Signal Slot Thread Safe

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

References[edit]

Are Qt Signals And Slots Thread Safe

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

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