WIP-How to create a simple chat application

Introduction

This article will illustrate a simple chat client and server communicating over TCP. The aim is to clarify aspects of QTcpSocket/QTcpServer that are not developed in the official Qt Fortune example. This has no intention to be a fully featured chat application.

You can find the source code relating to this example on GitHub

Pre-Requisites

• Qt 5.7 or later
• An intermediate level of knowledge of Qt5
• Familiarity with C++11 concepts, especially lambdas and std::bind

The Logic

This application will use a central server that will manage the communication among clients via JSON messages. We'll implement 2 versions of the server, one that runs in a single server and one that distributes the sockets among multiple threads.

The Client

The client in this example is a simple QtWidgets application, the ui is minimal, just a button to connect to the server, a list view to display the messages received, a line edit to type your messages and a button to send them.

The core of the functionality is in the ChatClient class.

class ChatClient : public QObject
{
    Q_OBJECT
    Q_DISABLE_COPY(ChatClient)
public:
    explicit ChatClient(QObject *parent = nullptr);
public slots:
    void connectToServer(const QHostAddress &address, quint16 port);
    void login(const QString &userName);
    void sendMessage(const QString &text);
    void disconnectFromHost();
private slots:
    void onReadyRead();
signals:
    void connected();
    void loggedIn();
    void loginError(const QString &reason);
    void disconnected();
    void messageReceived(const QString &sender, const QString &text);
    void error(QAbstractSocket::SocketError socketError);
    void userJoined(const QString &username);
    void userLeft(const QString &username);
private:
    QTcpSocket *m_clientSocket;
    bool m_loggedIn;
    void jsonReceived(const QJsonObject &doc);
};

Let's break down what each of these methods and member does:

Private Members
m_clientSocket	The socket that will interact with the server
m_loggedIn	Used to record whether the client already agreed a username with the server or not
Private Methods and Slots
onReadyRead()	Slot that reacts to data being available on the socket. It will download the data and pass it to jsonReceived()
jsonReceived()	Method to process the messages received from the server
Signals
connected()	The client connected to the server successfully
disconnected()	The client is not connected any more
loggedIn()	The client successfully agreed a username with the server
loginError()	The server rejected the username provided
messageReceived()	The client received a chat message
userJoined()	A new user joined the chat
userLeft()	A user left the chat
error()	Just a forward of the QTcpSocket::error signal
Public Slots
connectToServer()	Attempts the connection to a server
login()	Attempts to agree the given username with the server
disconnectFromHost()	Closes the connection with the server
sendMessage()	Sends a chat message to the server

Let's now look at the implementation:

ChatClient::ChatClient(QObject *parent)
    : QObject(parent)
    , m_clientSocket(new QTcpSocket(this))
    , m_loggedIn(false)
{

In the constructor we initialize the parent class and the members, then we take care of connecting to the signals coming from the socket.

// Forward the connected and disconnected signals
connect(m_clientSocket, &QTcpSocket::connected, this, &ChatClient::connected);
connect(m_clientSocket, &QTcpSocket::disconnected, this, &ChatClient::disconnected);
// connect readyRead() to the slot that will take care of reading the data in
connect(m_clientSocket, &QTcpSocket::readyRead, this, &ChatClient::onReadyRead);
// Forward the error signal, QOverload is necessary as error() is overloaded, see the Qt docs
connect(m_clientSocket, QOverload<QAbstractSocket::SocketError>::of(&QAbstractSocket::error), this, &ChatClient::error);
// Reset the m_loggedIn variable when we disconnec. Since the operation is trivial we use a lambda instead of creating another slot
connect(m_clientSocket, &QTcpSocket::disconnected, this, [this]()->void{m_loggedIn = false;});