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/*!
    \example network/qftp
    \title FTP Example

    \brief 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.

    \dots
    \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
    \dots
    \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
    directory.

    \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
    user.

    \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
    disabled.
*/