Ok, now I’m a little less confused. :-) “Parent/child” refers to an inheritance (ISA) relationship; but the design you are describing uses HASA relationships between objects. So far, so good.

Now, let’s look at the simple example:

sub some_method {
    my $self = shift;
    $self->get_some_framework->get_logger->log_trace("Hello, world!");
}
[download]

The problem with this is that it breaks encapsulation. Specifically, SomeFramework::SomeSubsystem::some_method has to know not only the public interface of the SomeFramework object (which it owns), but also the private implementation detail that a SomeFramework object provides logging via a SomeFramework::Logger object. (Otherwise it won’t know how to call the log_trace method.)

What if you later decide to change the way SomeFramework implements trace logging? You will have to change the method calls in classes such as SomeFramework::Subsystem, which ought not to be affected by such details. That’s a poor design, because it makes the code brittle. Better to access a single SomeFramework method and let the SomeFramework class delegate its implementation. For example, if file “SomeFramework.pm” is changed as follows:

# SomeFramework.pm

package SomeFramework;

use Moose;
use MooX::ProtectedAttributes;

protected_has logger =>
(
    is  => 'ro',
    isa => 'SomeFramework::Logger',
);

sub log_trace
{
    my ($self, $msg) = @_;
    $self->logger->log_trace($msg);
}

1;
[download]

then SomeFramework::Subsystem::some_method can be defined like this:

sub some_method {
    my $self = shift;
    $self->get_some_framework->log_trace("Hello, world!");
}
[download]

The goal is for each class to keep its implementation details private and to expose only a public interface to its “clients.” (In a HASA relationship, the “owner” object is a client of the objects it owns.) This promotes encapsulation by de-coupling classes as much as possible. The end result is a design which is simpler, clearer, more flexible, and easier to maintain.

Hope that helps,

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