Understanding Signals and Slot in Qt
This article shows how to use Qt's Signals and Slots mechanism.
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Introduction
The Signals and slots mechanism is fundamentally used to bind the object together without the object knowing any thing about each other.
Here slots are almost identical to C++ member functions which can be public, protected, or private. They can be directly invoked like any other C++ member functions. The difference is that a slot can also be connected to a signal, in which case it is automatically called each time the signal is emitted. We can use a connect statement to connect a signal and a slot. The connect() statement looks like this:
connect(sender, SIGNAL(signal), receiver, SLOT(slot));
Here is the example of it.
QObject::connect(spinBox, SIGNAL(valueChanged(int)),slider, SLOT(setValue(int)));
In the above example a signal is emitted each time the value of the spinbox is changed and this signal is received by a slider and it called the corresponding function in the slot.Where sender and receiver are pointers to QObject and where signal and slot are function signatures.
There are many other possibilities for connecting a signal and a slot in a different way.
• One signal can be connected to many slots:
connect(slider, SIGNAL(valueChanged(int)),spinBox, SLOT(setValue(int)));
connect(slider, SIGNAL(valueChanged(int)),this, SLOT(updateStatusBarIndicator(int)));
When the signal is emitted, the slots are called one after the other, in an unspecified order.
For Example of signal asn slots visit: Archived:How to use QSpinBox and QSlider
For More details in Slgnals and Slote visit: Signals and Slots in Qt
• Many signals can be connected to the same slot:
connect(sender0, SIGNAL(overflow()),receiver1, SLOT(handleMathError()));
connect(sender1, SIGNAL(divisionByZero()),receiver1, SLOT(handleMathError()));
• A signal can be connected to another signal:
connect(sender1, SIGNAL(function1()),receiver, SIGNAL(function2()));
When the first signal is emitted, the second signal is emitted as well. Apart from that, signal–signal connections are indistinguishable from signal–slot connections. • Disconnect can be used to remove the connection.
disconnect(sender0, SIGNAL(overflow()),receiver1, SLOT(handleMathError()));
In practice disconnect statement are rarely used because Qt removes all the connection automatically when it delete the objects.
Declaring your own slots
If your are going to declare your own slots then you have to define your slots in the header file:
pulbic slots:
void slots();
To connect this slots we have to write
connect(but1,SIGNAL(clicked()),this,SLOT(slots()));
For more information about signals and slots see Signals and Slots
28 Sep
2009
Signals and slots are one of the powerful feature of Qt. It enables communication between objects, irrespective of type of object. Other frameworks achieve this type of communication by callbacks, but they are not type-safe and the callback is strongly coupled to the processing function since the processing function must know which callback to call.
A signal is emitted when a particular event occurs. A slot is a function that is called in response to a particular signal. The signals and slots mechanism is type safe: The signature of a signal must match the signature of the receiving slot.The signals and slots mechanism is type safe: The signature of a signal must match the signature of the receiving slot. Signals and slots are loosely coupled: A class which emits a signal neither knows nor cares which slots receive the signal.
This article explains in the concept of signal and slots in brief. A code shows how sender will send signals and receiver receive that signals using slots, for example sender object,spinBox , will emit signal valueChanged() which receiver object ,slider , will receive in setValue().
Although this article does not have detailed explanation of signals and slots mechanism, the example given explains how this mechanism will works. Before starting programming in Qt one must learn this mechanism.