qt3/doc/network.doc

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/*
    The text here is due to be replaced by networking.doc, once that's
    ready.
*/

/*!
\page network.html

\title Network Module

\if defined(commercial)
This module is part of the \link commercialeditions.html Qt Enterprise Edition \endlink.
\endif

\tableofcontents


\section1 Introduction

The network module offers classes to make network programming easier
and portable. Essentially, there are three sets of classes, first low
level classes like \l QSocket, \l QServerSocket, \l QDns, etc. which
allow you to work in a portable way with TCP/IP sockets. In addition,
there are classes like \l QNetworkProtocol, \l QNetworkOperation in
the Qt base library, which provide an abstract layer for implementing
network protocols and \c QUrlOperator which operates on such network
protocols. Finally the third set of network classes are the passive
ones, specifically \c QUrl and \c QUrlInfo which do URL parsing and
similar.

The first set of classes (\l QSocket, \l QServerSocket, \l QDns, \l
QFtp, etc.) are included in Qt's "network" module.

The QSocket classes are not directly related to the QNetwork classes,
but QSocket should and will be used for implementing network
protocols, which are directly related to the QNetwork classes. For
example, the QFtp class (which implements the FTP protocol) uses
QSockets. But QSockets don't need to be used for protocol
implementations, e.g. QLocalFs (which is an implementation of the
local filesystem as network protocol) uses QDir and doesn't use
QSocket. Using QNetworkProtocols you can implement everything which
fits into a hierarchical structure and can be accessed using URLs.
This could be, for example, a protocol which can read pictures from a
digital camera using a serial connection.


\section1 Working Network Protocol independently with QUrlOperator and QNetworkOperation

It is quite easy to just use existing network protocol implementations
and operate on URLs. For example, downloading a file from an FTP
server to the local filesystem can be done with following code:

\code
    QUrlOperator op;
    op.copy( "ftp://ftp.trolltech.com/qt/source/qt-2.1.0.tar.gz", "file:/tmp", FALSE );
\endcode

And that's all! Of course an implementation of the FTP protocol has to
be available and registered for doing that. More information on that
later.

You can also do things like creating directories, removing files,
renaming, etc. For example, to create a folder on a private FTP
account do

\code
    QUrlOperator op( "ftp://username:password@host.domain.no/home/username" );
    op.mkdir( "New Directory" );
\endcode

To see all available operations, look at the \c QUrlOperator class
documentation.

Since networking works asynchronously, the function call for an
operation will normally return before the operation has been
completed. This means that the function cannot return a value
indicating failure or success. Instead, the return value always is a
pointer to a \c QNetworkOperation, and this object stores
all the information about the operation.

For example, \c QNetworkOperation has a method which returns the state
of this operation. Using this you can find out the state of the
operation at any time. The object also makes available the arguments
you passed to the \c QUrlOperator method, the type of the operation
and some more information. For more details see the class
documentation of \c QNetworkOperation.

The \c QUrlOperator emits signals to inform you about the progress of
the operations. As you can call many methods which operate on a \c
QUrlOperator's URL, it queues up all the operations. So you can't know
which operation the \c QUrlOperator just processed. Clearly you will
want to know which operation just took place, so each signal's last
argument is a pointer to the \c QNetworkOperation object which was
just processed and which caused the signal to be emitted.

Some of these operations send a \c start() signal at the beginning (if
this makes sense), and some of them send some signals during
processing. All operations send a \c finished() signal after they are
done. To find that out if an operation finished successfully you can
use the \c QNetworkOperation pointer you got with the \c finished()
signal. If \c QNetworkOperation::state() equals \c
QNetworkProtocol::StDone the operation finished successfully, if it is
\c QNetworkProtocol::StFailed the operation failed.

Example: A slot which you might connect to the 
\c{QUrlOperator::finished( QNetworkOperation * )}
\code
void MyClass::slotOperationFinished( QNetworkOperation *op )
{
    switch ( op->operation() ) {
    case QNetworkProtocol::OpMkDir: 
        if ( op->state() == QNetworkProtocol::StFailed )
            qDebug( "Couldn't create directory %s", op->arg( 0 ).latin1() );
        else
            qDebug( "Successfully created directory %s", op->arg( 0 ).latin1() );
	break;
    // ... and so on
    }
}
\endcode

As mentioned earlier, some operations send other signals too. Let's
take the list children operation as an example (e.g. read a directory
on a FTP server):

\code
QUrlOperator op;

MyClass::MyClass() : QObject(), op( "ftp://ftp.trolltech.com" )
{
    connect( &op, SIGNAL( newChildren( const QValueList<QUrlInfo> &, QNetworkOperation * ) ),
	     this, SLOT( slotInsertEntries( const QValueList<QUrlInfo> &, QNetworkOperation * ) ) );
    connect( &op, SIGNAL( start( QNetworkOperation * ) ),
	     this, SLOT( slotStart( QNetworkOperation *) ) );
    connect( &op, SIGNAL( finished( QNetworkOperation * ) ),
	     this, SLOT( slotFinished( QNetworkOperation *) ) );
}

void MyClass::slotInsertEntries( const QValueList<QUrlInfo> &info, QNetworkOperation * )
{
    QValueList<QUrlInfo>::ConstIterator it = info.begin();
    for ( ; it != info.end(); ++it ) {
        const QUrlInfo &inf = *it;
        qDebug( "Name: %s, Size: %d, Last Modified: %s",
	    inf.name().latin1(), inf.size(), inf.lastModified().toString().latin1() );
    }
}

void MyClass::slotStart( QNetworkOperation * )
{
    qDebug( "Start reading '%s'", op.toString().latin1() );
}

void MyClass::slotFinished( QNetworkOperation *operation )
{
    if ( operation->operation() == QNetworkProtocol::OpListChildren ) {
        if ( operation->state() == QNetworkProtocol::StFailed )
            qDebug( "Couldn't read '%s'! Following error occurred: %s",
                op.toString().latin1(), operation->protocolDetail().latin1() );
        else
            qDebug( "Finished reading '%s'!", op.toString().latin1() );
    }
}

\endcode

These examples demonstrate now how to use the \c QUrlOperator and \c
QNetworkOperations. The network extension also contains useful example
code.


\section2 Implementing your own Network Protocol

\c QNetworkProtocol provides a base class for implementations
of network protocols and an architecture for the a dynamic
registration and de-registration of network protocols. If you use this
architecture you don't need to care about asynchronous programming, as
the architecture hides this and does all the work for you.

\e{Note} It is difficult to design a base class for network protocols
which is useful for all network protocols. The architecture described
here is designed to work with all kinds of hierarchical structures,
like filesystems. So everything which can be interpreted as
hierarchical structure and accessed via URLs, can be implemented as
network protocol and easily used in Qt. This is not limited to
filesystems only!

To implement a network protocol create a class derived from
\c QNetworkProtocol.

Other classes will use this network protocol implementation
to operate on it. So you should reimplement following protected members

\code
    void QNetworkProtocol::operationListChildren( QNetworkOperation *op );
    void QNetworkProtocol::operationMkDir( QNetworkOperation *op );
    void QNetworkProtocol::operationRemove( QNetworkOperation *op );
    void QNetworkProtocol::operationRename( QNetworkOperation *op );
    void QNetworkProtocol::operationGet( QNetworkOperation *op );
    void QNetworkProtocol::operationPut( QNetworkOperation *op );
\endcode

Some notes on reimplementing these methods: You always get a pointer
to a \c QNetworkOperation as argument. This pointer holds all the
information about the operation in the current state. If you start
processing such an operation, set the state to \c
QNetworkProtocol::StInProgress. If you finished processing the
operation, set the state to \c QNetworkProtocol::StDone if it was
successful or \c QNetworkProtocol::StFailed if an error occurred. If
an error occurred you must set an error code (see
\c{QNetworkOperation::setErrorCode()}) and if you know some details
(e.g. an error message) you can also set this message to the operation
pointer (see \c{QNetworkOperation::setProtocolDetail()}). Also you get
all the relevant information (type, arguments, etc.) about the
operation from the \c QNetworkOperation pointer. For details about
which arguments you can get and set look at \c{QNetworkOperation}'s
class documentation.

If you reimplement an operation function, it's very important to emit
the correct signals at the correct time: In general always emit \c
finished() at the end of an operation (when you either successfully
finished processing the operation or an error occurred) with the
network operation as argument. The whole network architecture relies
on correctly emitted \c finished() signals! Then there are some more
specialized signals which are specific to operations:
\list
    \i Emit in \c operationListChildren:
    \list
	\i  \c start() just before starting to list the children
	\i  \c newChildren() when new children are read
    \endlist
    \i Emit in \c operationMkDir:
    \list
	\i  \c createdDirectory() after the directory has been created
	\i  \c newChild() (or newChildren()) after the directory has been
	    created (since a new directory is a new child)
    \endlist
    \i Emit in \c operationRemove:
    \list
	\i  \c removed() after a child has been removed
    \endlist
    \i Emit in \c operationRename:
    \list
	\i  \c itemChanged() after a child has been renamed
    \endlist
    \i Emit in \c operationGet:
    \list
	\i  \c data() each time new data has been read
	\i  \c dataTransferProgress() each time new data has been read to
	    indicate how much of the data has been read now.
    \endlist
    \i Emit in \c operationPut:
    \list
	\i  \c dataTransferProgress() each time data has been written to
	    indicate how much of the data has been written. Although you
	    know the whole data when this operation is called, it's
	    suggested not to write the whole data at once, but to do it
	    step by step to avoid blocking the GUI. Doing things
	    incrementally also means that progress can be made visible
	    to the user.
    \endlist
\endlist

And remember, always emit the \c finished() signal at the end!

For more details about these signals' arguments look at the \c
QNetworkProtocol class documentation.

Here is a list of which \c QNetworkOperation arguments you can get and
which you must set in which function:

(To get the URL on which you should work, use the \c
QNetworkProtocol::url() method which returns a pointer to the URL
operator. Using that you can get the path, host, name filter, etc.)

\list
    \i In \c operationListChildren:
    \list
       \i   Nothing.
    \endlist
    \i In \c operationMkDir:
    \list
	\i  \c QNetworkOperation::arg( 0 ) contains the name of the directory which should be created
    \endlist
    \i In \c operationRemove:
    \list
	\i  \c QNetworkOperation::arg( 0 ) contains the name of the file
	    which should be removed. Normally this is a relative name. But
	    it could be absolute. Use QUrl( op->arg( 0 ) ).fileName()
	    to get the filename.
    \endlist
    \i In \c operationRename:
    \list
	\i  \c QNetworkOperation::arg( 0 ) contains the name of the file
	    which should be renamed
	\i  \c QNetworkOperation::arg( 1 ) contains the name to which it
	    should be renamed.
    \endlist
    \i In \c operationGet:
    \list
	\i  \c QNetworkOperation::arg( 0 ) contains the full URL of the
	    file which should be retrieved.
    \endlist
    \i In \c operationPut:
    \list
	\i  \c QNetworkOperation::arg( 0 ) contains the full URL of the
	    file in which the data should be stored.
	\i  \c QNetworkOperation::rawArg( 1 ) contains the data which
	    should be stored in \c QNetworkOperation::arg( 0 )
    \endlist
\endlist

In summary: If you reimplement an operation function, you must emit
some special signals and at the end you must \e always emit a \c
finished() signal, regardless of success or failure. Also you must
change the state of the \c QNetworkOperation during processing. You 
can also get and set \c QNetworkOperation arguments as the operation
progresses.

It may occur that the network protocol you implement only requires a
subset of these operations. In such cases, simply reimplement the
operations which are supported by the protocol. Additionally you must
specify which operations you support. This is achieved by
reimplementing

\code
    int QNetworkProtocol::supportedOperations() const;
\endcode

In your implementation of this method return an \c int value
which is constructed by OR-ing together the correct values
(supported operations) of the following enum (of \c QNetworkProtocol):

\list
\i \c OpListChildren
\i \c OpMkDir
\i \c OpRemove
\i \c OpRename
\i \c OpGet
\i \c OpPut
\endlist

For example, if your protocol supports listing children and renaming
them, your implementation of \c supportedOperations() should do this:

\code
    return OpListChildren | OpRename;
\endcode

The last method you must reimplement is

\code
    bool QNetworkProtocol::checkConnection( QNetworkOperation *op );
\endcode

Here you must return TRUE, if the connection is up and okay (this means
operations on the protocol can be done). If the connection is not okay,
return FALSE and start to try opening it. If you cannot open the
connection at all (e.g. because the host is not found), emit a \c finished()
signal and set an error code and the \c QNetworkProtocol::StFailed state to
the \c QNetworkOperation pointer you get here.

Now, you never need to check before doing an operation yourself, if
the connection is okay. The network architecture does this, which
means it uses \c checkConnection() to see if an operation can be done
and if not, it tries it again and again for some time, only calling an
operation function if the connection is okay.

To be able to use a network protocol with a QUrlOperator (and so, for
example, in the QFileDialog), you must register the network
protocol implementation. This can be done like this:

\code
    QNetworkProtocol::registerNetworkProtocol( "myprot", new QNetworkProtocolFactory<MyProtocol> );
\endcode

In this case \c MyProtocol would be a class you implemented as
described here (derived from \c QNetworkProtocol) and the name of the
protocol would be "myprot". So to use it, you would do something like

\code
    QUrlOperator op( "myprot://host/path" );
    op.listChildren();
\endcode

Finally, as example of a network protocol implementation you could
look at the implementation of QLocalFs. The network extension also
contains an example implementation of a network protocol.


\section2 Error Handling

Error handling is important for both implementing new network
protocols for and using them (through \c QUrlOperator).

After processing an operation has been finished the network operation
the QUrlOperator emits the \c finished() signal. This has as argument
a pointer to the processed \c QNetworkOperation. If the state of this
operation is \c QNetworkProtocol::StFailed, the operation contains
some more information about this error. The following error codes are
defined in \c QNetworkProtocol:

\table
\header \i Error \i Meaning
\row \i \c QNetworkProtocol::NoError 
     \i No error occurred
\row \i \c QNetworkProtocol::ErrValid 
     \i The URL you are operating on is not valid
\row \i \c QNetworkProtocol::ErrUnknownProtocol 
     \i There is no protocol implementation available for the protocol
	of the URL you are operating on (e.g. if the protocol is http
	and no http implementation has been registered)
\row \i \c QNetworkProtocol::ErrUnsupported 
     \i The operation is not supported by the protocol
\row \i \c QNetworkProtocol::ErrParse 
     \i Parse error of the URL
\row \i \c QNetworkProtocol::ErrLoginIncorrect 
     \i You needed to login but the username or password are wrong
\row \i \c QNetworkProtocol::ErrHostNotFound 
     \i The specified host (in the URL) couldn't be found
\row \i \c QNetworkProtocol::ErrListChildren 
     \i An error occurred while listing the children
\row \i \c QNetworkProtocol::ErrMkDir 
     \i An error occurred when creating a directory
\row \i \c QNetworkProtocol::ErrRemove 
     \i An error occurred while removing a child
\row \i \c QNetworkProtocol::ErrRename 
     \i An error occurred while renaming a child
\row \i \c QNetworkProtocol::ErrGet 
     \i An error occurred while getting (retrieving) data
\row \i \c QNetworkProtocol::ErrPut 
     \i An error occurred while putting (uploading) data
\row \i \c QNetworkProtocol::ErrFileNotExisting 
     \i A file which is needed by the operation doesn't exist
\row \i \c QNetworkProtocol::ErrPermissionDenied 
     \i The permission for doing the operation has been denied
\endtable

\c QNetworkOperation::errorCode() returns one of these codes or
perhaps a different one if you use your an own network protocol
implementation which defines additional error codes.

\c QNetworkOperation::protocolDetails() may also return a string which
contains an error message then which might be suitable for display to
the user.

If you implement your own network protocol, you must report any
errors which occurred. First you always need to be able to
access the \c QNetworkOperation which is being processed at the
moment. This is done using \c
QNetworkOperation::operationInProgress(), which returns a pointer to
the current network operation or 0 if no operation is processed at the
moment.

Now if an error occurred and you need to handle it, do this:
\code
    if ( operationInProgress() ) {
        operationInProgress()->setErrorCode( error_code_of_your_error );
        operationInProgress()->setProtocolDetails( detail ); // optional
        emit finished( operationInProgress() );
        return;
    }
\endcode

That's all. The connection to the \c QUrlOperator and so on is done
automatically. Additionally, if the error was really bad so that no
more operations can be done in the current state (e.g. if the host
couldn't be found), call \c QNetworkProtocol::clearOperationStack() \e
before emitting \c finished().

Ideally you should use one of the predefined error codes of \c
QNetworkProtocol. If this is not possible, you can add own error codes
- they are just normal \c{int}s. Just be careful that the value of the
error code doesn't conflict with an existing one.

An example to look at is in qt/examples/network/ftpclient.
This is the implementation of a fairly complete FTP client, which
supports uploading and downloading files, making directories, etc.,
all done using \c QUrlOperators.

You might also like to look at QFtp (in qt/src/network/qftp.cpp) or at
the example in qt/examples/network/networkprotocol/nntp.cpp.

*/