8.5. Signals and Slots

8.5. Signals and Slots
	Chapter 8. QObject, QApplication, Signals, and Slots

[ fromfile: signals.xml id: signals ]

A signal is a message presented in a class definition like a void function declaration.
It has a parameter list but no function body.
A signal is part of the interface of a class.
It looks like a function, but it is not invoked the same way – it is emitted by an object of that class.

A slot is usually a void member function.
It can be called as a normal member function
it can be invoked indirectly by the QMetaObject system.

A signal of one object can be connected to the slots of one or more objects, provided the objects exist and the parameter lists are compatible from the signal to the slot.^[41]

The syntax of the connect statement is

  bool QObject::connect(senderQObjectPtr,
                       SIGNAL(signalName(argumentList)),
                       receiverQObjectPointer,
                       SLOT^[42](slotName(argumentList))
                       optionalConnectionType);

Any QObject that has a signal can emit that signal.
This produces indirect calls to all connected slots.

Arguments passed in the emit statement are accessible as parameters in the slot function, similar to a direct function call.
The optionalConnectionType enables you to specify if you want synchronous (blocking) or asynchronous (queued) calls to the destination slots from the emit point. ^[43]

Figure 8.5. QInputDialog

	Tip
	If you have multiple signals connected to the same slot, and need to know which `QObject` emitted the signal, you can call `sender()` from inside the slot, and it returns a pointer to that object.

^[41] When the lists are assignment compatible, this means that corresponding parameters must be compatible. In Qt, the slot must have at least as many paramaters as the signal. The slot can ignore extra arguments.

^[42]or signal

^[43]Connection is not restricted to the current thread Section 17.2


8.4. `Q_OBJECT` and `moc`: A checklist		8.5.1. Points of Departure