POE::Filter(3) User Contributed Perl Documentation POE::Filter(3)NAMEPOE::Filter - protocol abstractions for POE::Wheel and standalone use
SYNOPSIS
To use with POE::Wheel classes, pass a POE::Filter object to one of the
"...Filter" constructor parameters:
#!perl
use POE qw(Filter::Line Wheel::FollowTail);
POE::Session->create(
inline_states => {
_start => sub {
$_[HEAP]{tailor} = POE::Wheel::FollowTail->new(
Filename => "/var/log/system.log",
InputEvent => "got_log_line",
Filter => POE::Filter::Line->new(),
);
},
got_log_line => sub {
print "Log: $_[ARG0]\n";
}
}
);
POE::Kernel->run();
exit;
Standalone use without POE:
#!perl
use warnings;
use strict;
use POE::Filter::Line;
my $filter = POE::Filter::Line->new( Literal => "\n" );
# Prints three lines: one, two three.
$filter->get_one_start(["one\ntwo\nthr", "ee\nfour"]);
while (1) {
my $line = $filter->get_one();
last unless @$line;
print $line->[0], "\n";
}
# Prints two lines: four, five.
$filter->get_one_start(["\nfive\n"]);
while (1) {
my $line = $filter->get_one();
last unless @$line;
print $line->[0], "\n";
}
DESCRIPTIONPOE::Filter objects plug into the wheels and define how the data will
be serialized for writing and parsed after reading. POE::Wheel objects
are responsible for moving data, and POE::Filter objects define how the
data should look.
POE::Filter objects are simple by design. They do not use POE
internally, so they are limited to serialization and parsing. This may
complicate implementation of certain protocols (like HTTP 1.x), but it
allows filters to be used in stand-alone programs.
Stand-alone use is very important. It allows application developers to
create lightweight blocking libraries that may be used as simple
clients for POE servers. POE::Component::IKC::ClientLite is a notable
example. This lightweight, blocking event-passing client supports thin
clients for gridded POE applications. The canonical use case is to
inject events into an IKC application or grid from CGI interfaces,
which require lightweight resource use.
POE filters and drivers pass data in array references. This is
slightly awkward, but it minimizes the amount of data that must be
copied on Perl's stack.
PUBLIC INTERFACE
All POE::Filter classes must support the minimal interface, defined
here. Specific filters may implement and document additional methods.
new PARAMETERS
new() creates and initializes a new filter. Constructor parameters
vary from one POE::Filter subclass to the next, so please consult the
documentation for your desired filter.
clone
clone() creates and initializes a new filter based on the constructor
parameters of the existing one. The new filter is a near-identical
copy, except that its buffers are empty.
Certain components, such as POE::Component::Server::TCP, use clone().
These components accept a master or template filter at creation time,
then clone() that filter for each new connection.
my $new_filter = $old_filter->clone();
get_one_start ARRAYREF
get_one_start() accepts an array reference containing unprocessed
stream chunks. The chunks are added to the filter's internal buffer
for parsing by get_one().
The "SYNOPSIS" shows get_one_start() in use.
get_one
get_one() parses zero or one complete item from the filter's internal
buffer. The data is returned as an ARRAYREF suitable for passing to
another filter or a POE::Wheel object. Filters will return empty
ARRAYREFs if they don't have enough raw data to build a complete item.
get_one() is the lazy form of get(). It only parses only one item at a
time from the filter's buffer. This is vital for applications that may
switch filters in mid-stream, as it ensures that the right filter is in
use at any given time.
The "SYNOPSIS" shows get_one() in use. Note how it assumes the return
is always an ARRAYREF, and it implicitly handles empty ones.
get ARRAYREF
get() is the greedy form of get_one(). It accepts an array reference
containing unprocessed stream chunks, and it adds that data to the
filter's internal buffer. It then parses as many full items as
possible from the buffer and returns them in another array reference.
Any unprocessed data remains in the filter's buffer for the next call.
As with get_one(), get() will return an empty array reference if the
filter doesn't contain enough raw data to build a complete item.
In fact, get() is implemented in POE::Filter in terms of
get_one_start() and get_one().
Here's the get() form of the SYNOPSIS stand-alone example:
#!perl
use warnings;
use strict;
use POE::Filter::Line;
my $filter = POE::Filter::Line->new( Literal => "\n" );
# Prints three lines: one, two three.
my $lines = $filter->get(["one\ntwo\nthr", "ee\nfour"]);
foreach my $line (@$lines) {
print "$line\n";
}
# Prints two lines: four, five.
$lines = $filter->get(["\nfive\n"]);
foreach my $line (@$lines) {
print "$line\n";
}
get() should not be used with wheels that support filter switching.
Its greedy nature means that it often parses streams well in advance of
a wheel's events. By the time an application changes the wheel's
filter, it's too late: The old filter has already parsed the rest of
the received data.
Consider a stream of letters, numbers, and periods. The periods signal
when to switch filters from one that parses letters to one that parses
numbers.
In our hypothetical application, letters must be handled one at a time,
but numbers may be handled in chunks. We'll use POE::Filter::Block
with a BlockSize of 1 to parse letters, and POE::FIlter::Line with a
Literal terminator of "." to handle numbers.
Here's the sample stream:
abcdefg.1234567.hijklmnop.890.q
We'll start with a ReadWrite wheel configured to parse characters.
$_[HEAP]{wheel} = POE::Wheel::ReadWrite->new(
Filter => POE::Filter::Block->new( BlockSize => 1 ),
Handle => $socket,
InputEvent => "got_letter",
);
The "got_letter" handler will be called 8 times. One for each letter
from a through g, and once for the period following g. Upon receiving
the period, it will switch the wheel into number mode.
sub handle_letter {
my $letter = $_[ARG0];
if ($letter eq ".") {
$_[HEAP]{wheel}->set_filter(
POE::Filter::Line->new( Literal => "." )
);
$_[HEAP]{wheel}->event( InputEvent => "got_number" );
}
else {
print "Got letter: $letter\n";
}
}
If the greedy get() were used, the entire input stream would have been
parsed as characters in advance of the first handle_letter() call. The
set_filter() call would have been moot, since there would be no data
left to be parsed.
The "got_number" handler receives contiguous runs of digits as period-
terminated lines. The greedy get() would cause a similar problem as
above.
sub handle_numbers {
my $numbers = $_[ARG0];
print "Got number(s): $numbers\n";
$_[HEAP]->{wheel}->set_filter(
POE::Filter::Block->new( BlockSize => 1 )
);
$_[HEAP]->{wheel}->event( InputEvent => "got_letter" );
}
So don't do it!
put ARRAYREF
put() serializes items into a stream of octets that may be written to a
file or sent across a socket. It accepts a reference to a list of
items, and it returns a reference to a list of marshalled stream
chunks. The number of output chunks is not necessarily related to the
number of input items.
In stand-alone use, put()'s output may be sent directly:
my $line_filter = POE::Filter::Line->new();
my $lines = $line_filter->put(\@list_of_things);
foreach my $line (@$lines) {
print $line;
}
The list reference it returns may be passed directly to a driver or
filter. Drivers and filters deliberately share the same put()
interface so that things like this are possible:
$driver->put(
$transfer_encoding_filter->put(
$content_encoding_filter->put(
\@items
)
)
);
1 while $driver->flush(\*STDOUT);
get_pending
get_pending() returns any data remaining in a filter's input buffer.
The filter's input buffer is not cleared, however. get_pending()
returns a list reference if there's any data, or undef if the filter
was empty.
POE::Wheel objects use get_pending() during filter switching.
Unprocessed data is fetched from the old filter with get_pending() and
injected into the new filter with get_one_start().
use POE::Filter::Line;
use POE::Filter::Stream;
my $line_filter = POE::Filter::Line->new();
$line_filter->get_one_start([ "not a complete line" ]);
my $stream_filter = POE::Filter::Stream->new();
my $line_buffer = $line_filter->get_pending();
$stream_filter->get_one_start($line_buffer) if $line_buffer;
print "Stream: $_\n" foreach (@{ $stream_filter->get_one });
Full items are serialized whole, so there is no corresponding "put"
buffer or accessor.
SEE ALSO
The SEE ALSO section in POE contains a table of contents covering the
entire POE distribution.
POE is bundled with the following filters:
POE::Filter::Block POE::Filter::Grep POE::Filter::HTTPD
POE::Filter::Line POE::Filter::Map POE::Filter::RecordBlock
POE::Filter::Reference POE::Filter::Stackable POE::Filter::Stream
BUGS
In theory, filters should be interchangeable. In practice, stream and
block protocols tend to be incompatible.
AUTHORS & COPYRIGHTS
Please see POE for more information about authors and contributors.
perl v5.14.2 2011-12-15 POE::Filter(3)
