Slotscallable

Slotscallable is a term used in the context of signal-slot based event systems to describe a callable object that is registered as a slot. In such systems, signals are emitted and can be connected to one or more slots; a slotscallable is any function or method that can be invoked in response to a signal.

In Qt-inspired architectures, a slotscallable is implemented as a function that the framework can call through its internal meta-object or event loop. In C++ Qt, a member function is designated as a slot by declaring it under a slots section (for example, public slots: void onValueChanged(int); with the Q_SLOTS macro). In Python bindings such as PyQt, a slot is typically declared with a decorator like @pyqtSlot, which marks the method as a slotscallable and enables type information to be used by the framework.

Connecting a slotscallable to a signal creates a linkage whereby emitting the signal will invoke the connected slotscallables. The system may support direct (synchronous) connections, or queued (asynchronous) connections that post the call to the event loop, which is important for thread safety.

The design benefits include improved type safety, runtime introspection, and flexible wiring of components. Slotscallables can be overloaded or have specific type signatures that must match the emitting signal. They are central to decoupled architectures, allowing components to react to events without tight coupling.

Alternatives to slotscallables include plain function callbacks or other observer patterns. While slots are a core feature of Qt and its bindings, the general concept of a callable that handles events appears across many GUI frameworks and event-driven libraries.