• Qt Signal Slot Const Reference Generator
• Qt Signal Slot Const Reference Guide
• Qt Signal Slot Const Reference Example
• Qt Signal Slot Const Reference Sheet

• This function was introduced in Qt 4.8. See also multicastInterface, joinMulticastGroup, and leaveMulticastGroup. Qint64 QUdpSocket:: writeDatagram ( const char. data, qint64 size, const QHostAddress & address, quint16 port) Sends the datagram at data of size size to the host address address at port port. Returns the number of bytes sent.
• The QTimer class provides timer signals and single-shot timers. It uses timer events internally to provide a more versatile timer. QTimer is very easy to use: create a QTimer, call start to start it and connect its timeout to the appropriate slots. When the time is up it will emit the timeout signal.
• signal void QGroupBox:: clicked (bool checked = false) This signal is emitted when the check box is activated (i.e., pressed down then released while the mouse cursor is inside the button), or when the shortcut key is typed. Notably, this signal is not emitted if you call setChecked.

This is the sequel of my previous article explaining the implementation details of the signals and slots. In the Part 1, we have seen the general principle and how it works with the old syntax.In this blog post, we will see the implementation details behind the new function pointer based syntax in Qt5. 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.

The Bluetooth Chat example shows how to use the Qt Bluetooth API to communicate with another application on a remote device using Bluetooth.

The Bluetooth Chat example implements a simple chat program between multiple parties. The application always acts as both a server and a client eliminating the need to determine who should connect to whom.

Running the Example

To run the example from Qt Creator, open the Welcome mode and select the example from Examples. For more information, visit Building and Running an Example.

Chat Server

The chat server is implemented by the ChatServer class. The ChatServer class is declared as:

The first thing the chat server needs to do is create an instance of QRfcommServer to listen for incoming Bluetooth connections. Our clientConnected() slot will be called whenever a new connection is created.

The chat server is only useful if others know that it is there. To enable other devices to discover it, a record describing the service needs to be published in the systems SDP (Service Discovery Protocol) database. The QBluetoothServiceInfo class encapsulates a service record.

We will publish a service record that contains some textural descriptions of the services, a UUID that uniquely identifies the service, the discoverability attribute, and connection parameters.

The textural description of the service is stored in the ServiceName, ServiceDescription, and ServiceProvider attributes.

Bluetooth uses UUIDs as unique identifiers. The chat service uses a randomly generated UUID.

A Bluetooth service is only discoverable if it is in the PublicBrowseGroup.

The ProtocolDescriptorList attribute is used to publish the connection parameters that the remote device requires to connect to our service. Here we specify that the Rfcomm protocol is used and set the port number to the port that our rfcommServer instance is listening to.

Finally, we register the service record with the system.

As mentioned earlier, incoming connections are handled in the clientConnected() slot where pending connections are connected to the readyRead() and disconnected() signals. The signals notify others that a new client has connected.

The readSocket() slot is called whenever data is ready to be read from a client socket. The slot reads individual lines from the socket, converts them from UTF-8, and emits the messageReceived() signal.

The clientDisconnected() slot is called whenever a client disconnects from the service. The slot emits a signal to notify others that a client has disconnected, and deletes the socket.

The sendMessage() slot is used to send a message to all connected clients. The message is converted into UTF-8 and appended with a newline before being sent to all clients.

When the chat server is stopped the service record is removed from the system SDP database, all connected client sockets are deleted, and the QRfcommServer instance is deleted.

Chat Client

The chat client is implemented by the ChatClient class. The ChatClient class is declared as:

The client creates a new QBluetoothSocket and connects to the remote service described by the remoteService parameter. Slots are connected to the sockets readyRead(), connected() and disconnected() signals.

On successful socket connection we emit a signal to notify others.

Similarly to the chat server, the readSocket() slot is called when data is available from the socket. Lines are read individually and converted from UTF-8. The messageReceived() signal is emitted.

The sendMessage() slot is used to send a message to the remote device. The message is converted to UTF-8 and a newline is appended.

Qt Signal Slot Const Reference Generator

To disconnect from the remote chat service, the QBluetoothSocket instance is deleted.

Chat Dialog

The main window of this example is the chat dialog, implemented in the Chat class. This class displays a chat session between a single ChatServer and zero or more ChatClients. The Chat class is declared as:

First we construct the user interface

We create an instance of the ChatServer and respond to its clientConnected(), clientDiconnected(), and messageReceived() signals.

In response to the clientConnected() and clientDisconnected() signals of the ChatServer, we display the typical 'X has joined chat.' and 'Y has left.' messages in the chat session.

Incoming messages from clients connected to the ChatServer are handled in the showMessage() slot. The message text tagged with the remote device name is displayed in the chat session.

In response to the connect button being clicked, the application starts service discovery and presents a list of discovered chat services on remote devices. A ChatClient for the service is selected by the user.

In reponse to the connected() signals from ChatClient, we display the a 'Joined chat with X.' message in the chat session.

Messages are sent to all remote devices via the ChatServer and ChatClient instances by emitting the sendMessage() signal.

We need to clean up ChatClient instances when the remote device forces a disconnect.

Files:

Introduction

Qt Signal Slot Const Reference Guide

Remember old X-Windows call-back system? Generally it isn't type safe and flexible. There are many problems with them. Qt offers a new event handling system: signal-slot connections. Imagine an alarm clock. When alarm is ringing, a signal is being sent (emit). And you're handling it in a slot.

• Every QObject class may have as many signals and slots as you want
• You can emit signals only from within that class, where the signal is located
• You can connect signal with another signal (make chains of signals);
• Every signal and slot can have unlimited count of connections with other.
• 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 (

);

You have to wrap const char * signal and const char * method into SIGNAL() and SLOT() macros.

And you also can disconnect signal-slot:

QObject::disconnect (

);

Qt Signal Slot Const Reference Example

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.

Qt Signal Slot Const Reference Sheet

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.

In addition to handling signals and slots, the Meta Object Compiler supports Qt's translation mechanism, its property system, and its extended runtime type information. It also makes runtime introspection of C++ programs possible in a way that works on all supported platforms.

To make moc compile the meta object classes don't forget to add the Q_OBJECT macro to your class.