NAME

MooX::AttributeFilter - Implements 'filter' option for Moo-class attributes

VERSION

version 0.002903

SYNOPSIS

package My::Class;
use Moo;
use MooX::AttributeFilter;

has field => (
    is     => 'rw',
    filter => 'filterField',
);

has lazyField => (
    is      => 'rw',
    lazy    => 1,
    builder => sub { [1, 2, 3 ] },
    filter  => 1,
);

has incremental => (
    is => 'rw',
    filter => sub {
        my $this = shift;
        my ($val, $oldVal) = @_;
        if ( @_ > 1 && defined $oldVal ) {
            die "incremental attribute value may only increase"
                unless $val > $oldVal;
        }
        return $_[0];
    }
);

sub filterField {
    my $this = shift;
    return "filtered($_[0])";
}

sub _filter_lazyField {
    my $this = shift;
    my @a = @{$_[0]};
    push @a, -1;
    return \@a;
}

package main;
my $obj = My::Class->new( field => "initial" );
($obj->field eq "filtered(initial)")  # True!
$obj->lazyField;                      # [ 1, 2, 3, -1 ]
$obj->field( "value" );               # "filtered(value)"
$obj->incremental( -1 );              # -1
$obj->incremental( 10 );              # 10
$obj->incremental( 9 );               # dies...

$obj = My::Class->new( incremental => 1 ); # incremental is set to 1
$obj->incremental( 0 );                    # dies too.

DESCRIPTION

The idea behind this extension is to overcome the biggest deficiency of coercion: its ignorance about the object it is acting for. While triggers are executed as methods, they don't receive the previous attribute value; and they're called after the attribute is set.

Filter is a method which is called right before attribute value is about to be set. It receives one or two arguments of which the first is the new attribute value; the second is the old value. Number of arguments passed depends on what stage the filter get called at: one is for the construction, two is when set by writer.

Note: When an attribute was never set before and a writer is used then the old value filter argument will be undefined.

It is also worth mentioning that a filter is called always upon writing a value into attribute, including initialization from constructor arguments or lazy builders. See the "SYNOPSIS". In both cases the filter gets called with a single argument.

I.e.:

package LazyOne {
    use Moo;
    use MooX::AttributeFilter;
    
    has lazyField => (
        is => 'rw',
        lazy => 1,
        default => "value",
        filter => sub {
            my $this = shift;
            say "Arguments: ", scalar(@_);
            return $_[0];
        },
    );
}

my $obj = LazyOne->new;
$obj->lazyField;        # Arguments: 1
$obj->lazyField("foo"); # Arguments: 2

$obj = LazyOne->new( lazyField => "bar" );  # Arguments: 1
$obj->lazyField( "foobar" );                # Arguments: 2

Filter method must always return a (possibly modified) value.

Filter is called before any other attribute handlers. Its return value is then subject for passing through isa and coerce.

Use cases

Filters are of the most use when attribute value (or allowed values) depends on other attributes of its object (or even other linked objects). The dependency could be hard (isa-like) – i.e. an exception must be thrown if value doesn't pass validation. Or it could be soft: by storing a value calling code suggest what it would like to see in the attribute but the result might be changed depending on the current environment. For example:

package ChDir;
use File::Spec;
use Moo;
extends qw<Project::BaseClass>;
use MooX::AttributeFilter;

has curDir => (
    is => 'rw',
    filter => 'fullPath',
);

sub fullPath {
    my $this = shift;
    my ( $subdir ) = @_;
    
    return File::Spec->catdir(
        $this->testMode ? $this->baseTestDir : $this->baseDir,
        $subdir
    );
}
}

Inflation to Moose

This module inflates into MooseX::AttributeFilter.

CAVEATS

* The code relies on low-level functionality of Method::Generate family of modules. For this reason it may become incompatible with their future versions if they get drastically changed.

ACKNOWLEDGEMENTS

This work is a result of refusal to include filtering functionality into the Moo core. Since the refusal was backed by strong reasoning while the functionality itself is badly wanted there was no other choice but to create the module... So, my great thanks to Graham Knopp <haarg@haarg.org> for his advises, sample code, and Moo itself, of course!

My special thanks to Princess Kitten <littleprincess@kittymail.com> who implemented similar module for Moose framework and made the inflation code working.

AUTHOR

Vadim Belman <vrurg@cpan.org>

COPYRIGHT AND LICENSE

This software is copyright (c) 2018 by Vadim Belman.

This is free software; you can redistribute it and/or modify it under the same terms as the Perl 5 programming language system itself.