Refactoring Dispatch Tables into Objects

One of the cool ways of doing things in Perl is to use a dispatch table. The most obvious dispatch table is a hash of subroutines:

my $x;

my $table = {
   GET => sub { return $x  },
   PUT => sub { $x = $_[0] },
};

sub dispatch {
   my ($method, $data) = @_;

   if (my $fn = $table->{$method}) {
      $fn->($data)
   } else {
      die 'METHOD NOT ALLOWED!'
   }
}

This is a pretty cool thing to be able to do easily. But what’s even cooler is that we can refactor the dispatch table into a package, which allows us to make objects that can override bits of the dispatch table:

package Table {
   sub new { bless {}, $_[0] }
   sub GET { return $_[0]->{x}  }
   sub PUT { $_[0]->{x} = $_[1] }
}

package SubTable {
   use parent 'Table';
   sub DELETE { delete $_[0]->{x} }
}

my $table = SubTable->new;
sub dispatch {
   my ($method, $data) = @_;

   if (my $fn = $table->can($method)) {
      $table->$method($data)

      # the following would also work and would be
      # marginally faster
      # $table->$fn($data)
   } else {
      die 'METHOD NOT ALLOWED!'
   }
}

Note that one thing you might consider is prefixing the methods with “public_” or something like that; just in case your dispatcher object as private methods you don’t want web browsers executing. Generally though I’d just not put such methods in my dispatcher, but I haven’t yet made anything super complex using this pattern. I am using the pattern for a pluggable dashboard system at work, but the methods there are all called GET_foo or POST_bar, so users can’t run methods I didn’t specifically make for HTTP.

Posted Thu, Aug 25, 2011

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