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    on_initial => sub {
       my ( $self ) = @_;
       $self->seek_to_last( "\n" );
    },

    on_read => sub {
       my ( $self, $buffref ) = @_;

       while( $$buffref =~ s/^(.*\n)// ) {
          print "Received a line $1";
       }

       return 0;
    },
 );

 $loop->add( $filestream );

 $loop->run;

=head1 DESCRIPTION

This subclass of L<IO::Async::Stream> allows reading the end of a regular file
which is being appended to by some other process. It invokes the C<on_read>
event when more data has been added to the file. 

This class provides an API identical to L<IO::Async::Stream> when given a
C<read_handle>; it should be treated similarly. In particular, it can be given
an C<on_read> handler, or subclassed to provide an C<on_read> method, or even
used as the C<transport> for an L<IO::Async::Protocol::Stream> object.

It will not support writing.

To watch a file, directory, or other filesystem entity for updates of other
properties, such as C<mtime>, see also L<IO::Async::File>.

=cut

=head1 EVENTS

The following events are invoked, either using subclass methods or CODE
references in parameters.

Because this is a subclass of L<IO::Async::Stream> in read-only mode, all the
events supported by C<Stream> relating to the read handle are supported here.
This is not a full list; see also the documentation relating to
L<IO::Async::Stream>.

=head2 $ret = on_read \$buffer, $eof

Invoked when more data is available in the internal receiving buffer.

Note that C<$eof> only indicates that all the data currently available in the
file has now been read; in contrast to a regular L<IO::Async::Stream>, this
object will not stop watching after this condition. Instead, it will continue
watching the file for updates.

=head2 on_truncated

Invoked when the file size shrinks. If this happens, it is presumed that the
file content has been replaced. Reading will then commence from the start of
the file.

=head2 on_initial $size

Invoked the first time the file is looked at. It is passed the initial size of
the file. The code implementing this method can use the C<seek> or
C<seek_to_last> methods to set the initial read position in the file to skip
over some initial content.

This method may be useful to skip initial content in the file, if the object
should only respond to new content added after it was created.

=cut

sub _init
{
   my $self = shift;
   my ( $params ) = @_;

   $self->SUPER::_init( $params );

   $params->{close_on_read_eof} = 0;

   $self->{last_size} = undef;

   $self->add_child( $self->{file} = IO::Async::File->new(
      on_devino_changed => $self->_replace_weakself( 'on_devino_changed' ),
      on_size_changed   => $self->_replace_weakself( 'on_size_changed' ),
   ) );
}

=head1 PARAMETERS

The following named parameters may be passed to C<new> or C<configure>, in
addition to the parameters relating to reading supported by
L<IO::Async::Stream>.

=head2 filename => STRING

Optional. If supplied, watches the named file rather than the filehandle given
in C<read_handle>. The file will be opened by the constructor, and then
watched for renames. If the file is renamed, the new filename is opened and
tracked similarly after closing the previous file.

=head2 interval => NUM

Optional. The interval in seconds to poll the filehandle using C<stat(2)>
looking for size changes. A default of 2 seconds will be applied if not
defined.

=cut

sub configure
{
   my $self = shift;
   my %params = @_;

   foreach (qw( on_truncated on_initial )) {
      $self->{$_} = delete $params{$_} if exists $params{$_};
   }

   foreach (qw( interval )) {
      $self->{file}->configure( $_ => delete $params{$_} ) if exists $params{$_};
   }
   if( exists $params{filename} ) {
      $self->{file}->configure( filename => delete $params{filename} );
      $params{read_handle} = $self->{file}->handle;
   }
   elsif( exists $params{handle} or exists $params{read_handle} ) {
      my $handle = delete $params{handle};
      defined $handle or $handle = delete $params{read_handle};

      $self->{file}->configure( handle => $handle );
      $params{read_handle} = $self->{file}->handle;
   }

   croak "Cannot have a write_handle in a ".ref($self) if defined $params{write_handle};

   $self->SUPER::configure( %params );

   if( $self->read_handle and !defined $self->{last_size} ) {
      my $size = (stat $self->read_handle)[7];

      $self->{last_size} = $size;

      local $self->{running_initial} = 1;
      $self->maybe_invoke_event( on_initial => $size );
   }
}

=head1 METHODS

=cut

# Replace IO::Async::Handle's implementation
sub _watch_read
{
   my $self = shift;
   my ( $want ) = @_;

   if( $want ) {
      $self->{file}->start if !$self->{file}->is_running;
   }
   else {
      $self->{file}->stop;
   }
}

sub _watch_write
{
   my $self = shift;
   my ( $want ) = @_;

   croak "Cannot _watch_write in " . ref($self) if $want;
}

sub on_devino_changed
{
   my $self = shift or return;

   $self->{renamed} = 1;
   $self->debug_printf( "read tail of old file" );
   $self->read_more;
}

sub on_size_changed
{
   my $self = shift or return;
   my ( $size ) = @_;

   if( $size < $self->{last_size} ) {
      $self->maybe_invoke_event( on_truncated => );
      $self->{last_pos} = 0;
   }

   $self->{last_size} = $size;

   $self->debug_printf( "read_more" );
   $self->read_more;
}

sub read_more
{
   my $self = shift;

   sysseek( $self->read_handle, $self->{last_pos}, SEEK_SET ) if defined $self->{last_pos};

   $self->on_read_ready;

   $self->{last_pos} = sysseek( $self->read_handle, 0, SEEK_CUR ); # == systell

   if( $self->{last_pos} < $self->{last_size} ) {
      $self->loop->later( sub { $self->read_more } );
   }
   elsif( $self->{renamed} ) {
      $self->debug_printf( "reopening for rename" );

      $self->{last_size} = 0;

      if( $self->{last_pos} ) {
         $self->maybe_invoke_event( on_truncated => );
         $self->{last_pos} = 0;