3.1. Programming Watch Points in catalog.cfg

Almost any configuration variable can be set up to be tied to a subroutine if the Tie::Watch module installed. It uses a notation like the <<HERE document, but <&HERE is the notation. Here is a simple case:

   MailOrderTo orders@akopia.com
   MailOrderTo <&EOF
   sub {
       my($self, $default) = @_;
       if($Values->{special_handling}) {
           return 'vip@akopia.com';
       else {
           return $default;

When the order is mailed out, if the user has a variable called special_handling set in their session (from UserDB, perhaps), the order will be sent to 'vip@akopia.com.' Note the single quotes to prevent problems with the @ sign. Otherwise, the order will get sent to the previously defined value of orders@akopia.com.

If the configuration value being watched is a SCALAR, the subroutine gets the following call:

   &{$subref}(SELF, PREVIOUS_VALUE)

The subroutine should simply return the proper value.

SELF is a reference to the Tie::Watch object (read its documentation for what all it can do) and PREVIOUS_VALUE is the previously set value for the directive. If set after the watch is set up, it will simply have the effect of destroying the watch and having unpredictable effects. (In the future, a "Store" routine may be able to be set up that can subsequently set values).

If the configuration value being watched is an ARRAY, the subroutine gets the following call:


INDEX is the index of the array element being accessed. Setting up watch points on array values is not recommended. Most Interchange subroutines call arrays in their list context, and no access method is provided for that.

If the configuration value being watched is a HASH, the subroutine gets the following call:

   &{$subref}(SELF, KEY, PREVIOUS_VALUE)

KEY is the index into the hash, an example of HASH type Interchange configuration values. NOTE: The following is not recommended for performance reasons. The Variable is a commonly used thing and should not bear the extra overhead of tieing, but it illustrates the power of this operation:

   Variable TESTIT Unwatch worked.

   Variable <&EOV
   sub {
       my ($self, $key, $orig) = @_;
       if($key eq 'TESTIT') {
           # only the first time
           if($Scratch->{$key}++) {
               return $orig->{TESTIT};
           else {
               return "Tie::Watch works! -- name=$Values->{name}";
       else {
           return $orig->{$key};

The first time __TESTIT__ is called for a particular user, it will return the string "Tie::Watch works! -- name=" along with their name set in the session (if that exists). Any other variables will receive the value that they were set to previously. Once the TESTIT key has been accessed for that user, the watch is dropped upon the next access.