package Class::Workflow::Transition::Validate;
use Moose::Role;

requires "validate";

around apply => sub {
	my $next = shift;
	my ( $self, $instance, @args ) = @_;

	my $error;
		local $@;
		eval { $self->validate( $instance, @args ) };
		$error = $@;

	if ( $error ) {
		return $self->validation_error( $error, $instance, @args );
	} else {
		return $self->$next( $instance, @args );

sub validation_error {
	my ( $self, $error, $instance, @args ) = @_;
	die $error;




=head1 NAME

Class::Workflow::Transition::Validate - Provide a hook for validating a
transition (conditionals, input validators, etc).


	package MyTransition;
	use Moose;

	with qw/

	sub validate {
		my ( $self, $instance, %args ) = @_;

		die "only the owner can apply this transition"
			unless $args{user} eq $instance->owner;


This role will call the C<validate> method at the appropriate time.

C<validate> receives the same arguments as C<apply>, and is expected to die if
any of the parameters for the transition are invalid.

Technically, this role doesn't do much more than adding syntactic sugar for
C<before 'apply'>. However, it's value is in the convention that you can call
C<validate> without applying the body. This eases writing side effect free
introspection of transitions.