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.
The event filter function set here is called for all messages received by all threads meant for all Qt objects. It is not called for messages that are not meant for Qt objects. The event filter function should return true if the message should be filtered, (i.e. It should return false to allow processing the message to continue. Hello: I intend to have the following use case scenario using QT: A Qialog is to be composed of a QPushButton and a widget derived from QTAbleWIdget. Each time the button is pushed a new row gets. QT slot function does not get called. Slot concept Qt in Education. A slot can return values, but not through connections. (vmvalue) return. This is the responsibility of all code that can emit signals – do not forget it in your own classes. Custom signals and slots class AngleObject: public QObject.
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]
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.
Qt Signal Slot Return Value
References[edit]
- ^'Signals & Slots - QtCore 5.1'. Qt Project. 2013-07-04. Retrieved 2013-07-04.