Introduction
Signals and Slots
A signal is a void message defined within a class. It has a list of parameters but no function body.
It resembles a function but is not invoked like a regular function. Instead, it is emitted by an object of the class.
A slot is a void function that can be called normally but can also be invoked indirectly by the QMetaObject system. A signal from one class can be connected to one or more slots of another class. This requires that the objects exist at the time of the connection (and during its use). Additionally, the parameter lists must match between the signal and the slot; if they do not match, the signal and slot cannot be connected.
There is a specific function that allows establishing the connection between a signal and a particular slot, as shown in the following example:
bool QObject::connect(senderQObjectPtr, SIGNAL(signalName(argList)), receiverQObjectPtr, SLOT(slotName(argList)), optConnectionType);
Any QObject with a signal can emit it. Each time the signal is emitted, the connected slots are automatically invoked. The function parameters serve as a way to pass information from the signal to the slots. The optionalConnectionType parameter enables you to specify whether the slot calls should be synchronous (blocking) or asynchronous (queued).
If necessary, a slot can know which signal was emitted based on the sender() function.
QObject Lifecycle
Any QObject must be created on the heap and not defined as static, as these objects may depend on a QApplication whose lifetime can extend beyond the scope of the main() function, where stack objects might be created.
QWidget
A QWidget es un QObject que puede ser representado visualmente. Para ello un QWidget hereda pulbicamente del QObject y tambien de una clase llamada QPaintDevice. QPaintDevice es cualquier objeto que se puede pintar en el screen.
Si un QWidget tiene un parent QWidget, las coordenadas seran dentro del QWidget parent indicando que en casos normales siempre estara dentro del QWidget parent. Como un QWidget es un QObject tambien tendra eventos y signals and slots.
A QWidget is a QObject that can be visually represented. To achieve this, a QWidget publicly inherits from QObject and also from a class called QPaintDevice. QPaintDevice refers to any object that can be painted on the screen. If a QWidget has a parent QWidget, its coordinates will be defined within the coordinate system of the parent QWidget, meaning that under normal circumstances, it will always be contained within the parent. Additionally, since a QWidget is also a QObject, it will have access to events and the signals and slots mechanism.