package HTML::FormHandler::Manual::InflationDeflation;
# ABSTRACT: inflation and deflation of field values



=encoding UTF-8

=head1 NAME

HTML::FormHandler::Manual::InflationDeflation - inflation and deflation of field values

=head1 VERSION

version 0.40068


L<Manual Index|HTML::FormHandler::Manual>

How to inflate and deflate field values.


When working with the various ways that data can be transformed, in and out,
the meaning of the terms 'inflate' and 'deflate' starts to feel kind of slippery.
The one constant is that values presented in an HTML form must be in a string
format, or presented with select elements or checkboxes.

There are two general types of inflation/deflation provided by FormHandler.
The first, 'standard' type inflates values in order to validate them, and deflates them
in order to present them in string format via HTML. The other ('DB') type takes
values provided by defaults (usually a DB row, or item, but could also be
a field default or an init_object) and munges the values coming in and
changes them back going out.

=head2 Standard inflation/deflation

The standard type of inflation/deflation is implemented by using some of the following
options for inflation:

    transform (using 'apply')

..and the following options for deflation:

    deflation (field attribute)

When validation starts, the param input will be inflated by the inflate method,
allowing validation to be performed on the inflated object.

When the 'fif' fill-in-form value is returned for HTML generation, the deflation
is used to flatten the object, usually into a string format.

=head2 DB inflation/deflation

The 'DB' type of inflation/deflation uses 'inflate_default_method' for inflation,
and 'deflate_value_method' for deflation. Deflation could also be handled by changing
the value in one of the various validation methods.

This type of inflation/deflation is, logically, just a different way of providing
data munging around the defaults (item/init_object/default) and 'value' output.
The same effect could be achieved by performing a transformation outside of
FormHandler - if you were handling the database updates yourself. Since the DBIC
model enables automatic database updates, this kind of inflation/deflation
makes that easier.

One circumstance in which this type of inflation/deflation is useful is when
there's a single field in the database row object which you want to expand into
a compound field in the form.

=head2 Attributes used in deflation/inflation

=head3 Inflation methods

The field 'input' comes from the params that are passed in from the submission
of the form, so the input will always be in string format if it comes from an
HTTP request. It's also possible to pass in params in other formats, of course.
Or the params could be pre-processed before passing in to FormHandler.

You should not normally be changing the 'input' attribute of a field. If you
want the changed field value to be used when re-presenting the form, such as
when you're adopting a standard format for the field, you should set
C<< fif_from_value => 1 >>.

There are three options for standard inflation, or transforming the field's
'input' to the field's 'value':

=over 4

=item inflate_method

Provide a method on the field which inflates the field 'input' (from params):

   has_field 'futility' => ( inflate_method => \&inflate_field );
   sub inflate_field {
       my ( $self, $value ) = @_;
       return $value;

=item transform

In a sequence of 'apply' actions, changes the format of the 'value' that is
being validated. This might be useful if there are some validations that work
on one format of the value, and some that work on another.

=item set the value in validation methods

In a validate method, you can change the format of the value, with $field->value(...);


=head3 Deflation methods

=over 4

=item deflate_method

Most general purpose deflation method. Provide a coderef which is a method
on the field:

   has_field => 'foo' => ( deflate_method => \&deflate_foo );
   sub deflate_foo {
       my ( $self, $value ) = @_;  # $self is the 'foo' field
       <perform conversion>
       return $value;

=item deflation

This is a coderef that performs deflation.

   has_field => 'bar' => ( deflation => sub { shift $value; ... return $value } );

=item set the value in validation methods

Just like for inflation, you can change the value in a validation method; however,
it won't be used for fill-in-form unless you set the 'fif_from_value' flag to true.


=head3 fif_from_value

Normally the fill-in-form value will be the param value that was submitted.
If you want to change the format of the input when re-presenting the
form, you can set 'fif_from_value'.

=head3 deflate_to

Earlier versions of FormHandler provided a 'deflate_to' attribute which
allowed the deflation methods to be used for multiple, confusing purposes. This
flag has been removed, since it made the process hard to understand and was
mixing apples and oranges. The new inflation/deflation methods can handle
all of the previous situations.

=head1 AUTHOR

FormHandler Contributors - see HTML::FormHandler


This software is copyright (c) 2017 by Gerda Shank.

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