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/****************************************************************************
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**
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** Copyright (C) 2009 Nokia Corporation and/or its subsidiary(-ies).
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** All rights reserved.
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** Contact: Nokia Corporation (qt-info@nokia.com)
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**
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** This file is part of the documentation of the Qt Toolkit.
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**
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** $QT_BEGIN_LICENSE:LGPL$
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** No Commercial Usage
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** This file contains pre-release code and may not be distributed.
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** You may use this file in accordance with the terms and conditions
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** contained in the Technology Preview License Agreement accompanying
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** this package.
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**
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** GNU Lesser General Public License Usage
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** Alternatively, this file may be used under the terms of the GNU Lesser
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** General Public License version 2.1 as published by the Free Software
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** Foundation and appearing in the file LICENSE.LGPL included in the
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** packaging of this file. Please review the following information to
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** ensure the GNU Lesser General Public License version 2.1 requirements
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** will be met: http://www.gnu.org/licenses/old-licenses/lgpl-2.1.html.
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**
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** In addition, as a special exception, Nokia gives you certain additional
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** rights. These rights are described in the Nokia Qt LGPL Exception
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** version 1.1, included in the file LGPL_EXCEPTION.txt in this package.
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**
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** If you have questions regarding the use of this file, please contact
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** Nokia at qt-info@nokia.com.
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**
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**
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**
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**
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**
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**
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**
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**
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** $QT_END_LICENSE$
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**
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****************************************************************************/
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/*!
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\example network/fortuneclient
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\title Fortune Client Example
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The Fortune Client example shows how to create a client for a simple
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network service using QTcpSocket. It is intended to be run alongside the
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\l{network/fortuneserver}{Fortune Server} example or
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the \l{network/threadedfortuneserver}{Threaded Fortune Server} example.
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\image fortuneclient-example.png Screenshot of the Fortune Client example
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This example uses a simple QDataStream-based data transfer protocol to
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request a line of text from a fortune server (from the
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\l{network/fortuneserver}{Fortune Server} example). The client requests a
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fortune by simply connecting to the server. The server then responds with
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a 16-bit (quint16) integer containing the length of the fortune text,
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followed by a QString.
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QTcpSocket supports two general approaches to network programming:
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\list
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\o \e{The asynchronous (non-blocking) approach.} Operations are scheduled
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and performed when control returns to Qt's event loop. When the operation
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is finished, QTcpSocket emits a signal. For example,
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QTcpSocket::connectToHost() returns immediately, and when the connection
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has been established, QTcpSocket emits
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\l{QTcpSocket::connected()}{connected()}.
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\o \e{The synchronous (blocking) approach.} In non-GUI and multithreaded
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applications, you can call the \c waitFor...() functions (e.g.,
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QTcpSocket::waitForConnected()) to suspend the calling thread until the
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operation has completed, instead of connecting to signals.
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\endlist
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In this example, we will demonstrate the asynchronous approach. The
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\l{network/blockingfortuneclient}{Blocking Fortune Client} example
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illustrates the synchronous approach.
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Our class contains some data and a few private slots:
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\snippet examples/network/fortuneclient/client.h 0
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Other than the widgets that make up the GUI, the data members include a
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QTcpSocket pointer, a copy of the fortune text currently displayed, and
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the size of the packet we are currently reading (more on this later).
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The socket is initialized in the Client constructor. We'll pass the main
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widget as parent, so that we won't have to worry about deleting the
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socket:
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\snippet examples/network/fortuneclient/client.cpp 0
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\dots
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\snippet examples/network/fortuneclient/client.cpp 1
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The only QTcpSocket signals we need in this example are
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QTcpSocket::readyRead(), signifying that data has been received, and
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QTcpSocket::error(), which we will use to catch any connection errors:
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\dots
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\snippet examples/network/fortuneclient/client.cpp 3
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\dots
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\snippet examples/network/fortuneclient/client.cpp 5
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Clicking the \gui{Get Fortune} button will invoke the \c
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requestNewFortune() slot:
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\snippet examples/network/fortuneclient/client.cpp 6
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In this slot, we initialize \c blockSize to 0, preparing to read a new block
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of data. Because we allow the user to click \gui{Get Fortune} before the
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previous connection finished closing, we start off by aborting the
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previous connection by calling QTcpSocket::abort(). (On an unconnected
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socket, this function does nothing.) We then proceed to connecting to the
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fortune server by calling QTcpSocket::connectToHost(), passing the
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hostname and port from the user interface as arguments.
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As a result of calling \l{QTcpSocket::connectToHost()}{connectToHost()},
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one of two things can happen:
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\list
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\o \e{The connection is established.} In this case, the server will send us a
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fortune. QTcpSocket will emit \l{QTcpSocket::readyRead()}{readyRead()}
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every time it receives a block of data.
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\o \e{An error occurs.} We need to inform the user if the connection
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failed or was broken. In this case, QTcpSocket will emit
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\l{QTcpSocket::error()}{error()}, and \c Client::displayError() will be
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called.
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\endlist
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Let's go through the \l{QTcpSocket::error()}{error()} case first:
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\snippet examples/network/fortuneclient/client.cpp 13
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We pop up all errors in a dialog using
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QMessageBox::information(). QTcpSocket::RemoteHostClosedError is silently
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ignored, because the fortune server protocol ends with the server closing
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the connection.
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Now for the \l{QTcpSocket::readyRead()}{readyRead()} alternative. This
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signal is connected to \c Client::readFortune():
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\snippet examples/network/fortuneclient/client.cpp 8
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\codeline
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\snippet examples/network/fortuneclient/client.cpp 10
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The protocol is based on QDataStream, so we start by creating a stream
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object, passing the socket to QDataStream's constructor. We then
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explicitly set the protocol version of the stream to QDataStream::Qt_4_0
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to ensure that we're using the same version as the fortune server, no
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matter which version of Qt the client and server use.
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Now, TCP is based on sending a stream of data, so we cannot expect to get
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the entire fortune in one go. Especially on a slow network, the data can
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be received in several small fragments. QTcpSocket buffers up all incoming
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data and emits \l{QTcpSocket::readyRead()}{readyRead()} for every new
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block that arrives, and it is our job to ensure that we have received all
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the data we need before we start parsing. The server's response starts
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with the size of the packet, so first we need to ensure that we can read
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the size, then we will wait until QTcpSocket has received the full packet.
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\snippet examples/network/fortuneclient/client.cpp 11
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\codeline
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\snippet examples/network/fortuneclient/client.cpp 12
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We proceed by using QDataStream's streaming operator to read the fortune
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from the socket into a QString. Once read, we can call QLabel::setText()
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to display the fortune.
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\sa {Fortune Server Example}, {Blocking Fortune Client Example}
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*/
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