Editing Triggers (section)

==== Example ====
Send the coversheeted version of the document rather than the original.
 $c->add_trigger( EP_TRIGGER_DOC_URL_REWRITE, sub
 {
     my( %args ) = @_;
 
     my( $request, $doc, $relations, $filename ) = @args{qw( request document relations filename )};
     return EP_TRIGGER_OK unless defined $doc && ref $doc eq "EPrints::DataObj::Document"; # To avoid missing field error with dark documents that do not get coversheeted.
 
     # check document is a pdf
     my $format = $doc->value( "format" ); # back compatibility
     my $mime_type = defined $doc->value( "mime_type" ) ? $doc->value( "mime_type" ) : "";
     return EP_TRIGGER_OK unless( $format eq "application/pdf" || $mime_type eq "application/pdf" || $filename =~ /\.pdf$/i );
 
     # ignore thumbnails e.g. http://.../8381/1.haspreviewThumbnailVersion/jacqueline-lane.pdf
     foreach my $rel ( @{$relations || []} )
     {
         return EP_TRIGGER_OK if( $rel =~ /^is\w+ThumbnailVersionOf$/ );
     }
 
     # ignore volatile documents
    return EP_TRIGGER_OK if $doc->has_relation( undef, "isVolatileVersionOf" );
 
     my $session = $doc->get_session;
     my $eprint = $doc->get_eprint;
 
     # search for a coversheet that can be applied to this document
     my $coversheet = EPrints::DataObj::Coversheet->search_by_eprint( $session, $eprint );
     return EP_TRIGGER_OK unless( defined $coversheet );
 
     # check whether there is an existing covered version and whether it needs to be regenerated
     my $current_cs_id = $doc->get_value( 'coversheetid' ) || -1; # coversheet used to cover document
     my $coverdoc; # existing covered version
 
     if( $coversheet->get_id == $current_cs_id )
     {
         # get the covered version of the document
         $coverdoc = $coversheet->get_coversheet_doc( $doc );
     }
 
     if( defined $coverdoc )
     {
         # return the covered version
         $coverdoc->set_value( "security", $doc->get_value( "security" ) );
         $request->pnotes( document => $coverdoc );
         $request->pnotes( dataobj => $coverdoc );
 
         # Only update request filename to coverdoc's filename if it is defined and the document filename matches that used in the request.  
         # If the document filename does not match that used in the request, then not updating the filename will mean requests with a
         # spurious filename will get a 404 error rather than returning the document under a spurious filename.
         $request->pnotes( filename => $coverdoc->get_main ) if defined $coverdoc->get_main && defined $doc->get_main && $filename eq $doc->get_main;
     }
  
     # return the uncovered document
 
     return EP_TRIGGER_DONE;
 
 }, priority => 100 );