=for rcs $Id: File.pod,v 1.4 2002/10/16 21:24:23 dk Exp $

=head1 NAME

Prima::File - asynchronous stream I/O.

=head1 DESCRIPTION

Prima::File provides access to the I/O stream events,
that are called when a file handle becomes readable, writable
or if an exception occurred. Registering file handles to Prima::File
objects makes possible the stream operations coexist with the event loop. 

=head1 USAGE

Prima::File is a descendant of Prima::Component.
Objects of Prima::File class must be binded to a valid file handle object,
before the associated events can occur:

  my $f = Prima::File-> create();
  $f-> file( *STDIN);

When a file handle, binded via the C<::file> property becomes readable,
writable or when an exception signaled, one of three correspondent
events called - C<Read>, C<Write> or C<Exception>. When a handle is
always readable, or always writable, or, some of these events are
desired to be blocked, the file event mask can be set via the C<::mask>
property:

  $f-> mask( fe::Read | fe::Exception);

NB. Due to different system implementations, the only handles,
currently supported on all systems, are socket handle and disk file 
handles. Pipes only work on unix platforms. The example file I<socket.pl>
elucidates the use of sockets together with Prima::File.

When a file handle is not needed anymore, it is expected to
be detached from an object explicitly:

  $f-> file( undef);

However, if the system detects that a file handle is no longer valid,
it is automatically detached. It is possible to check, if a file handle
is still valid by calling the C<is_active()> method.

Prima::File events are basically the same I/O callbacks, provided by
a system C<select()> call. See documentation of your system's select()
for the implementation details.

=head1 API

=head2 Properties

=over

=item file HANDLE

Selects a file handle, that is to be monitored for stream I/O events.
If HANDLE is C<undef>, object is returned to a passive state, and
the previously binded file handle is de-selected.

=item mask EVENT_MASK

Selects a event mask, that is a combination of C<fe::XXX> integer constants,
each representing an event:

   fe::Read
   fe::Write
   fe::Exception

The omitted events are effectively excluded from the system file event
multiplexing mechanism.

=back

=head2 Methods

=over

=item get_handle

Returns C<sprintf("0x%08x", fileno( file ))> string.
If C<::file> is C<undef>, -1 is used instead fileno() result.

=item is_active AUTODETACH = 0

Returns a boolean flag, indicating if a file handle is valid.
If AUTODETACH is 1, and the file handle is not valid, 
C<file(undef)> is called.

=back

=head2 Events

=over

=item Read

Called when a file handle becomes readable. The callback procedure
is expected to call a non-blocking read() on the file handle.

=item Write

Called when a file handle becomes writable. The callback procedure
is expected to call a non-blocking write() on the file handle.

=item Exception

Called when an exception is signaled on a file handle.
The exceptions are specific to handle type and the operating system.
For example, a unix socket signals C<Exception> when a control status 
data for a pseudo terminal or an out-of-band data arrives.

=back

=head1 AUTHOR

Dmitry Karasik, E<lt>dmitry@karasik.eu.orgE<gt>.

=head1 SEE ALSO

L<Prima>, L<Prima::Object>

