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 # By default this just returns the value of $ENV{ALIEN_INSTALL_TYPE}
 sub override {
   return $ENV{ALIEN_INSTALL_TYPE};
 }
 
 # Default download implementation; can be
 # replaced by specifying a different download
 # hook.  See Alien::Build::Plugin::Core::Download
 # for detailed implementation.
 sub download {
 
   my $response = fetch();
 
   if($response->{type} eq 'html' || $response->{type} eq 'dir_listing') {
     # decode will transform an HTML listing (html) or a FTP directory
     # listing (dir_listing) into a regular list
     $response = decode($response);
   }
 
   if($response->{type} eq 'list') {
 
     # prefer will filter bad entries in the list
     # and sort them so that the first one is
     # the one that we want
     $response = prefer($response);
 
     my $first_preferred = $res->{list}->[0];
 
     # prefer can sometimes infer the version from the
     # filename.
     if(defined $first_preferred->{version}) {
       # not a hook
       runtime_prop->{version} = $first_preferred->{version};
     }
 
     $response = fetch($first_preferred);
 
   }
 
   if($response->{type} eq 'file') {
     # not a hook
     write_file_to_disk $response;
   }
 
 }

=head1 DESCRIPTION

This document explains how to write L<Alien::Build> plugins using the
L<Alien::Build::Plugin> base class.

=head2 Writing plugins

Plugins use L<Alien::Build::Plugin>, which sets the appropriate base
class, and provides you with the C<has> property builder.  C<has> takes
two arguments, the name of the property and the default value.  (As
with L<Moose> and L<Moo>, you should use a code reference to specify
default values for non-string defaults).  No B<not> set this as your
plugin's base class directly:

 use parent qw( Alien::Build::Plugin );  # wrong
 use Alien::Build::Plugin;               # right

The only method that you need to implement is C<init>.  From this method
you can add hooks to change the behavior of the L<alienfile> recipe.
This is a very simple example of a probe hook, with the actual probe
logic removed:

 sub init
 {
   my($self, $meta) = @_;
   $meta->register_hook(
     probe => sub {
       my($build) = @_;
       if( ... )
       {
         return 'system';
       }
       else
       {
         return 'share';
       }
     },
   );
 }

Hooks get the L<Alien::Build> instance as their first argument, and depending
on the hook may get additional arguments.

=head2 Modifying hooks

You can also modify hooks using C<before_hook>, C<around_hook> and C<after_hook>,
similar to L<Moose> modifiers:

 sub init
 {
   my($self, $meta) = @_;
 
   $meta->before_hook(
     build => sub {
       my($build) = @_;
       $build->log('this runs before the build');
     },
   );
 
   $meta->after_hook(
     build => sub {
       my($build) = @_;
       $build->log('this runs after the build');
     },
   );
 
   $meta->around_hook(
     build => sub {
       my $orig = shift;
 
       # around hooks are useful for setting environment variables
       local $ENV{CPPFLAGS} = '-I/foo/include';
 
       $orig->(@_);
     },