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

VERSION
    version 0.001006

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
            );
        }
    }

CAVEATS
    * This module doesn't inflate into Moose.

    * 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!

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.

