NAME

Sentinel - create lightweight SCALARs with get/set callbacks

SYNOPSIS

package Some::Class;

use Sentinel;

sub attribute_name :lvalue
{
   my $self = shift;
   sentinel get => sub { return $self->get_attribute_name },
            set => sub { $self->set_attribute_name( $_[0] ) };
}

sub another_attribute :lvalue
{
   my $self = shift;
   sentinel value => $self->get_another_attribute,
            set   => sub { $self->set_attribute_name( $_[0] ) };
}

sub yet_another_attribute :lvalue
{
   my $self = shift;
   sentinel obj => $self,
            get => \&get_another_attribute,
            set => \&set_another_attribute;
}

DESCRIPTION

This module provides a single lvalue function, sentinel, which yields a scalar that invoke callbacks to get or set its value. Primarily this is useful to create lvalue object accessors or other functions, to invoke actual code when a new value is set, rather than simply updating a scalar variable.

FUNCTIONS

$scalar = sentinel %args

Returns (as an lvalue) a scalar with magic attached to it. This magic is used to get the value of the scalar, or to inform of a new value being set, by invoking callback functions supplied to the sentinel. Takes the following named arguments:

get => CODE

A CODE reference to invoke when the value of the scalar is read, to obtain its value. The value returned from this code will appear as the value of the scalar.

set => CODE

A CODE reference to invoke when a new value for the scalar is written. The code will be passed the new value as its only argument.

value => SCALAR

If no get callback is provided, this value is given as the initial value of the scalar. If the scalar manages to survive longer than a single assignment, its value on read will retain the last value set to it.

obj => SCALAR

Optional value to pass as the first argument into the get and set callbacks. If this value is provided, then the get and set callbacks may be given as direct sub references to object methods, rather than closures that capture the referent object. This avoids the runtime overhead of creating lots of small one-use closures around the object.

Important note

The syntax used in the SYNOPSIS only works on perl version 5.14 and above. Before version 5.14, the lvalue context is not properly propagated through nested lvalue functions and instead it dies at runtime with an exception

Can't return a temporary from lvalue subroutine at ...

To be compatible with prior versions of perl, you must instead write a slightly more awkward syntax, taking a SCALAR ref to the sentinel return value then immediately dereferencing it again:

sub attribute_name :lvalue
{
   my $self = shift;
   ${ \sentinel get => sub { return $self->get_attribute_name },
                set => sub { $self->set_attribute_name( $_[0] ) } };
}

This is purely a workaround for older perl behaviour; if you do not need backward compatibility before perl 5.14, then you can yield sentinel directly from an :lvalue function.

AUTHOR

Paul Evans <leonerd@leonerd.org.uk>