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ObjectCache cpan:ELIZABETH last updated on 2021-01-30

use v6.d;

# Using nqp for optimal performance
use nqp;

role ObjectCache:ver<0.0.4>:auth<cpan:ELIZABETH>[&args2str] {
    has $!WHICH;

    my $cache := nqp::hash;
    my $lock  :=;
    my $prefix = nqp::concat(::?CLASS.^name,'|');

    method !SET-WHICH(\WHICH) {
        $!WHICH :=;
    multi method WHICH(::?CLASS:D:) { $!WHICH }

    method bless(*%_) {
        my str $WHICH = nqp::concat($prefix,args2str(%_).Str);
        $lock.protect: {

    method !EVICT() {
        $lock.protect: {
              nqp::isnull(my \value := nqp::atkey($cache,$!WHICH)),

    method !CLEAR(--> Int:D) {
        $lock.protect: {
            my int $elems = nqp::elems($cache);
            $cache := nqp::hash;

=begin pod

=head1 NAME

ObjectCache - A role to cache objects


  use ObjectCache;

  sub id(%h --> Int:D) {
      %h<id> or die "must have an id";

  class Article does ObjectCache[&id] {
      has $.id;
      # has many more attributes

  say => 42666789).WHICH;  # Article|42666789


The ObjectCache role mixes the logic of creating a proper ObjectType class
B<and> making sure that each unique ObjectType only exists once in memory,
into a class.

The role takes a C<Callable> parameterization to indicate how a unique ID
should be created from the parameters given (as a hash) with the call to the
C<new> method.  You can also adapt the given hash making sure any default
values are properly applied.  If there's already an object created with the
returned ID, then no new object will be created but the one from the cache
will be returned.

A class is considered to be an object type if the C<.WHICH> method returns an
object of the C<ObjAt> class.

This is specifically important when using set operators (such as C<(elem)>,
or C<Set>s, C<Bag>s or C<Mix>es, or any other functionality that is based
on the C<===> operator functionality, such as C<unique> and C<squish>.

The format of the value that is being returned by C<WHICH> is only valid
during a run of a process.  So it should B<not> be stored in any permanent

=head2 Removing objects from cache

The C<ObjectCache> role contains a private method C<!EVICT>.  If you'd like
to have the ability to remove an object from the cache, you should create
a method in your class that will call this method:

  class Article does ObjectCache[&id] {
      method remove-from-cache() { self!EVICT }
      # more stuff

The C<!EVICT> method returns the object that is removed from the cache,
or C<Nil> if the object was not in the cache.

=head2 Clearing the cache completely

The C<ObjectCache> role contains a private method C<!CLEAR>.  If you'd like
to have the ability to cleare the cache completely, you should create a
method in your class that will call this method:

  class Article does ObjectCache[&id] {
      method clear-cache() { self!CLEAR }
      # more stuff

The C<!CLEAR> method returns the number of objects that have been removed
from the cache.

=head1 AUTHOR

Elizabeth Mattijsen <[email protected]>

Source can be located at: . Comments
and Pull Requests are welcome.


Copyright 2020,2021 Elizabeth Mattijsen

This library is free software; you can redistribute it and/or modify it under
the Artistic License 2.0.

=end pod

# vim: expandtab shiftwidth=4