Doc: Added documentation for the qftp example

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** Copyright (C) 2012 Digia Plc and/or its subsidiary(-ies).
** Contact:
** This file is part of the documentation of the Qt Toolkit.
** Commercial License Usage
** Licensees holding valid commercial Qt licenses may use this file in
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** GNU Free Documentation License Usage
** Alternatively, this file may be used under the terms of the GNU Free
** Documentation License version 1.3 as published by the Free Software
** Foundation and appearing in the file included in the packaging of
** this file. Please review the following information to ensure
** the GNU Free Documentation License version 1.3 requirements
** will be met:
\example network/qftp
\title FTP Example
The FTP example demonstrates a simple FTP client that can be used
to list the available files on an FTP server and download them.
\image ftp-example.png
The user of the example can enter the address or hostname of an
FTP server in the \gui {Ftp Server} line edit, and then push the
\gui Connect button to connect to it. A list of the server's
top-level directory is then presented in the \gui {File List} tree
view. If the selected item in the view is a file, the user can
download it by pushing the \gui Download button. An item
representing a directory can be double clicked with the mouse to
show the contents of that directory in the view.
The functionality required for the example is implemented in the
QFtp class, which provides an easy, high-level interface to the
file transfer protocol. FTP operations are requested through
\l{QFtp::Command}s. The operations are asynchronous. QFtp will
notify us through signals when commands are started and finished.
We have one class, \c FtpWindow, which sets up the GUI and handles
the FTP functionality. We will now go through its definition and
implementation - focusing on the code concerning FTP. The code for
managing the GUI is explained in other examples.
\section1 FtpWindow Class Definition
The \c FtpWindow class displays a window, in which the user can
connect to and browse the contents of an FTP server. The slots of
\c FtpWindow are connected to its widgets, and contain the
functionality for managing the FTP connection. We also connect to
signals in QFtp, which tells us when the
\l{QFtp::Command}{commands} we request are finished, the progress
of current commands, and information about files on the server.
\snippet examples/network/qftp/ftpwindow.h 0
We will look at each slot when we examine the \c FtpWindow
implementation in the next section. We also make use of a few
private variables:
\snippet examples/network/qftp/ftpwindow.h 1
The \c isDirectory hash keeps a history of all entries explored on
the FTP server, and registers whether an entry represents a
directory or a file. We use the QFile object to download files
from the FTP server.
\section1 FtpWindow Class Implementation
We skip the \c FtpWindow constructor as it only contains code for
setting up the GUI, which is explained in other examples.
We move on to the slots, starting with \c connectOrDisconnect().
\snippet examples/network/qftp/ftpwindow.cpp 0
If \c ftp is already pointing to a QFtp object, we QFtp::Close its
FTP connection and delete the object it points to. Note that we do
not delete the object using standard C++ \c delete as we need it
to finish its abort operation.
\snippet examples/network/qftp/ftpwindow.cpp 1
If we get here, \c connectOrDisconnect() was called to establish a
new FTP connection. We create a new QFtp for our new connection,
and connect its signals to slots in \c FtpWindow. The
\l{QFtp::}{listInfo()} signal is emitted whenever information
about a single file on the sever has been resolved. This signal is
sent when we ask QFtp to \l{QFtp::}{list()} the contents of a
directory. Finally, the \l{QFtp::}{dataTransferProgress()} signal
is emitted repeatedly during an FTP file transfer, giving us
progress reports.
\snippet examples/network/qftp/ftpwindow.cpp 2
The \gui {Ftp Server} line edit contains the IP address or
hostname of the server to which we want to connect. We first check
that the URL is a valid FTP sever address. If it isn't, we still
try to connect using the plain text in \c ftpServerLineEdit. In
either case, we assume that port \c 21 is used.
If the URL does not contain a user name and password, we use
QFtp::login(), which will attempt to log into the FTP sever as an
anonymous user. The QFtp object will now notify us when it has
connected to the FTP server; it will also send a signal if it
fails to connect or the username and password were rejected.
We move on to the \c downloadFile() slot:
\snippet examples/network/qftp/ftpwindow.cpp 3
\snippet examples/network/qftp/ftpwindow.cpp 4
We first fetch the name of the file, which we find in the selected
item of \c fileList. We then start the download by using
QFtp::get(). QFtp will send progress signals during the download
and a signal when the download is completed.
\snippet examples/network/qftp/ftpwindow.cpp 5
QFtp supports canceling the download of files. We make sure that
any file that is currently being written to is closed and removed,
and tidy up by deleting the file object.
\snippet examples/network/qftp/ftpwindow.cpp 6
The \c ftpCommandFinished() slot is called when QFtp has
finished a QFtp::Command. If an error occurred during the
command, QFtp will set \c error to one of the values in
the QFtp::Error enum; otherwise, \c error is zero.
\snippet examples/network/qftp/ftpwindow.cpp 7
After login, the QFtp::list() function will list the top-level
directory on the server. addToList() is connected to
QFtp::listInfo(), and will be invoked for each entry in that
\snippet examples/network/qftp/ftpwindow.cpp 8
When a \l{QFtp::}{Get} command is finished, a file has finished
downloading (or an error occurred during the download).
\snippet examples/network/qftp/ftpwindow.cpp 9
After a \l{QFtp::}{List} command is performed, we have to check if
no entries were found (in which case our \c addToList() function
would not have been called).
Let's continue with the \c addToList() slot:
\snippet examples/network/qftp/ftpwindow.cpp 10
When a new file has been resolved during a QFtp::List command,
this slot is invoked with a QUrlInfo describing the file. We
create a separate row for the file in \c fileList. If \c fileList
does not have a current item, we set the new item to be the
current item.
\snippet examples/network/qftp/ftpwindow.cpp 11
The \c processItem() slot is called when an item is double clicked
in the \gui {File List}. If the item represents a directory, we
want to load the contents of that directory with QFtp::list().
\snippet examples/network/qftp/ftpwindow.cpp 12
\c cdToParent() is invoked when the user requests to go to the
parent directory of the one displayed in the file list. After
changing the directory, we QFtp::List its contents.
\snippet examples/network/qftp/ftpwindow.cpp 13
The \c updateDataTransferProgress() slot is called regularly by
QFtp::dataTransferProgress() when a file download is in progress.
We use a QProgressDialog to show the download progression to the
\snippet examples/network/qftp/ftpwindow.cpp 14
The \c enableDownloadButton() is called whenever the current item
in \c fileList changes. If the item represents a file, the \gui
{Enable Download} Button should be enabled; otherwise, it is