A developer can choose to connect to a signal by creating a function (a slot) and calling the connect function to relate the signal to the slot. Qt's signals and slots mechanism does not require classes to have knowledge of each other, which makes it much easier to develop highly reusable classes. Signals and slots are loosely coupled: A class which emits a signal neither knows nor cares which slots receive the signal. Qt's signals and slots mechanism ensures that if you connect a signal to a slot, the slot will be called with the signal's parameters at the right time. Signals and slots can take any number of arguments of any type. The signals and slots mechanism is fundamental to Qt programming. It enables the application programmer to bind objects together without the objects knowing anything about each other. We have already connected some signals and slots together, declared our own signals and slots, implemented our own slots, and emitted our own signals. Signal/slot mechanism is a central feature of Qt and probably the part that differs most from other toolkits. In most GUI toolkits widgets have a callback for each action they can This callback is a.

1. Qt Signal Slot Mechanism
2. Qt Signal Slot Not Working

Introduction

Remember old X-Window call-back system? Generally it isn't type safe and flexible. There are many problems with them. Qt offer new event-handling system - signal-slot connections. Imagine alarm clock. When alarm is ringing, signal is sending (emitting). And you're handling it as a slot.

1. Every QObject class may have as many signals of slots as you want.
2. You can emit signal only from that class, where signal is.
3. You can connect signal with another signal (make chains of signals);
4. Every signal and slot can have unlimited count of connections with other.
5. ATTENTION! You can't set default value in slot attributes. e.g.void mySlot(int i = 0);

Connection

You can connect signal with this template:QObject::connect ( const QObject * sender, const char * signal, const QObject * receiver, const char * method);You have to wrap const char * signal and const char * method into SIGNAL () and SLOT() macros.

Qt Signal Slot Mechanism

And you also can disconnect signal-slot:QObject::disconnect ( const QObject * sender, const char * signal, const QObject * receiver, const char * method);

Deeper

Widgets emit signals when events occur. For example, a button will emit a 'clicked' signal when it is clicked. A developer can choose to connect to a signal by creating a function (a 'slot') and calling the connect() function to relate the signal to the slot. Qt's signals and slots mechanism does not require classes to have knowledge of each other, which makes it much easier to develop highly reusable classes. Since signals and slots are type-safe, type errors are reported as warnings and do not cause crashes to occur.

For example, if a Quit button's clicked() signal is connected to the application's quit() slot, a user's click on Quit makes the application terminate. In code, this is written as

connect(button, SIGNAL (clicked()), qApp, SLOT (quit()));

Connections can be added or removed at any time during the execution of a Qt application, they can be set up so that they are executed when a signal is emitted or queued for later execution, and they can be made between objects in different threads.

The signals and slots mechanism is implemented in standard C++. The implementation uses the C++ preprocessor and moc, the Meta-Object Compiler, included with Qt. Code generation is performed automatically by Qt's build system. Developers never have to edit or even look at the generated code.

Qt Signal Slot Not Working