in reply to Re: Avoiding circular references
in thread Avoiding circular references

it's a good idea for it to avoid closing over $self.

[I didn't study the OP's code, so this may or may not be pertinent, but I think it is.]

When you have an object that calls a callback, it's a good idea to pass the object to the callback as an argument.

For example, say there's a class that calls a function periodically.

The following exhibits a circular reference: 

my $timer;
$timer = Timer->new(
   callback => sub {
      ...
      if (...) {
         $timer->cancel;
      }
      ...
   },
   period => 5,
)

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The following doesn't exhibit a circular reference: 

my $timer = Timer->new(
   callback => sub {
      my ($timer) = @_;
      ...
      if (...) {
         $timer->cancel;
      }
      ...
   },
   period => 5,
)

[download]

The latter requires that Timer passes a reference to itself to the callback.

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Re^3: Avoiding circular references
by ikegami (Patriarch) on Dec 06, 2019 at 22:27 UTC

    Your problem can be solved using something similar to the aforementioned "trick", as shown below.

    I've identified the three changes I've made using "# <-----". 

    use strict;
use warnings;
use Devel::Cycle;

package A;

sub new {
    my ( $class, $href ) = @_;
    my $self = { dbi => $$href{dbi} };
    $self->{helper} = Helper->new(
        helper_function => sub {
            #$self->this_is_from_A(@_)
            shift->this_is_from_A(@_)   # <-----
        },
    );
    bless $self, $class;
  
}

sub do_something {
    my $self = shift;
    #$self->{helper}->called_from_helper();
    $self->{helper}->called_from_helper($self);   # <-----
}

sub this_is_from_A {
    my ( $self, $arg ) = @_;
    print "$arg\nthis_is_from_A using $self->{dbi}\n";
}

package Helper;
sub new {
    my $class = shift;
    my %def   = ( a => "something" );
    my %arg   = ( %def, ref $_[0] eq "HASH" ? %{ $_[0] } : @_ );
    my $self  = \%arg;
    bless $self, $class;
}

sub called_from_helper {
    my $self = shift;
    print "called_from_helper\n";
    #$self->{helper_function}->("param from Helper");
    $self->{helper_function}->(@_, "param from Helper");   # <-----
}

package main;
my $a = A->new( { dbi => "some dbi object" } );
$a->do_something();
$a->do_something();
find_cycle($a);

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      In this case, however, it's easier to use weaken.

      Simply change 

      sub new {
    my ( $class, $href ) = @_;
    my $self = { dbi => $$href{dbi} };
    $self->{helper} = Helper->new(
        helper_function => sub {
            $self->this_is_from_A(@_)
        },
    );
    bless $self, $class;
}

      [download]
      to 
      use Scalar::Util qw( weaken );

sub new {
    my ( $class, $href ) = @_;
    my $self = { dbi => $$href{dbi} };

    {
        weaken( my $self = $self );
        $self->{helper} = Helper->new(
            helper_function => sub {
                $self->this_is_from_A(@_)
            },
        );
    }

    bless $self, $class;
}

      [download]

      (By using the same name for the weakened variable as the unweakened one, we avoid using the wrong one by accident.)

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