| Class::Tiny - Minimalist class construction |
Class::Tiny - Minimalist class construction
version 1.004
In Person.pm:
package Person;
use Class::Tiny qw( name );
1;
In Employee.pm:
package Employee; use parent 'Person';
use Class::Tiny qw( ssn ), {
timestamp => sub { time } # attribute with default
};
1;
In example.pl:
use Employee;
my $obj = Employee->new( name => "Larry", ssn => "111-22-3333" );
# unknown attributes are ignored
my $obj = Employee->new( name => "Larry", OS => "Linux" );
# $obj->{OS} does not exist
This module offers a minimalist class construction kit in around 120 lines of code. Here is a list of features:
new constructor
new takes a hash reference or list of key/value pairs
new supports providing BUILDARGS to customize constructor options
new calls BUILD for each class from parent to child
superclass provides a DESTROY method
DESTROY calls DEMOLISH for each class from child to parent
Multiple-inheritance is possible, with superclass order determined via mro::get_linear_isa.
It uses no non-core modules for any recent Perl. On Perls older than v5.10 it requires the MRO::Compat manpage. On Perls older than v5.14, it requires the Devel::GlobalDestruction manpage.
Define attributes as a list of import arguments:
package Foo::Bar;
use Class::Tiny qw(
name
id
height
weight
);
For each attribute, a read-write accessor is created unless a subroutine of that name already exists:
$obj->name; # getter
$obj->name( "John Doe" ); # setter
Attribute names must be valid subroutine identifiers or an exception will be thrown.
You can specify lazy defaults by defining attributes with a hash reference. Keys define attribute names and values are constants or code references that will be evaluated when the attribute is first accessed if no value has been set. The object is passed as an argument to a code reference.
package Foo::WithDefaults;
use Class::Tiny qw/name id/, {
title => 'Peon',
skills => sub { [] },
hire_date => sub { $_[0]->_build_hire_date },
};
When subclassing, if multiple accessors of the same name exist in different classes, any default (or lack of default) is determined by standard method resolution order.
To make your own custom accessors, just pre-declare the method name before loading Class::Tiny:
package Foo::Bar;
use subs 'id';
use Class::Tiny qw( name id );
sub id { ... }
Even if you pre-declare a method name, you must include it in the attribute list for Class::Tiny to register it as a valid attribute.
If you set a default for a custom accessor, your accessor will need to retrieve the default and do something with it:
package Foo::Bar;
use subs 'id';
use Class::Tiny qw( name ), { id => sub { int(rand(2*31)) } };
sub id {
my $self = shift;
if (@_) {
return $self->{id} = shift;
}
elsif ( exists $self->{id} ) {
return $self->{id};
}
else {
my $defaults =
Class::Tiny->get_all_attribute_defaults_for( ref $self );
return $self->{id} = $defaults->{id}->();
}
}
If your class does not already inherit from some class, then
Class::Tiny::Object will be added to your @ISA to provide new and
DESTROY.
If your class does inherit from something, then no additional inheritance is set up. If the parent subclasses Class::Tiny::Object, then all is well. If not, then you'll get accessors set up but no constructor or destructor. Don't do that unless you really have a special need for it.
Define subclasses as normal. It's best to define them with the base manpage, parent
or superclass before defining attributes with Class::Tiny so the @ISA
array is already populated at compile-time:
package Foo::Bar::More;
use parent 'Foo::Bar';
use Class::Tiny qw( shoe_size );
If your class inherits from Class::Tiny::Object (as it should if you followed
the advice above), it provides the new constructor for you.
Objects can be created with attributes given as a hash reference or as a list of key/value pairs:
$obj = Foo::Bar->new( name => "David" );
$obj = Foo::Bar->new( { name => "David" } );
If a reference is passed as a single argument, it must be able to be dereferenced as a hash or an exception is thrown.
Unknown attributes in the constructor arguments will be ignored. Prior to version 1.000, unknown attributes were an error, but this made it harder for people to cleanly subclass Class::Tiny classes so this feature was removed.
You can define a BUILDARGS method to change how arguments to new are
handled. It will receive the constructor arguments as they were provided and
must return a hash reference of key/value pairs (or else throw an
exception).
sub BUILDARGS {
my $class = shift;
my $name = shift || "John Doe";
return { name => $name };
};
Foo::Bar->new( "David" );
Foo::Bar->new(); # "John Doe"
Unknown attributes returned from BUILDARGS will be ignored.
If your class or any superclass defines a BUILD method, it will be called
by the constructor from the furthest parent class down to the child class after
the object has been created.
It is passed the constructor arguments as a hash reference. The return value
is ignored. Use BUILD for validation, checking required attributes or
setting default values that depend on other attributes.
sub BUILD {
my ($self, $args) = @_;
for my $req ( qw/name age/ ) {
croak "$req attribute required" unless defined $self->$req;
}
croak "Age must be non-negative" if $self->age < 0;
$self->msg( "Hello " . $self->name );
}
The argument reference is a copy, so deleting elements won't affect data in the
original (but changes will be passed to other BUILD methods in @ISA).
Class::Tiny provides a DESTROY method. If your class or any superclass
defines a DEMOLISH method, they will be called from the child class to the
furthest parent class during object destruction. It is provided a single
boolean argument indicating whether Perl is in global destruction. Return
values and errors are ignored.
sub DEMOLISH {
my ($self, $global_destruct) = @_;
$self->cleanup();
}
You can retrieve an unsorted list of valid attributes known to Class::Tiny
for a class and its superclasses with the get_all_attributes_for class
method.
my @attrs = Class::Tiny->get_all_attributes_for("Employee");
# returns qw/name ssn timestamp/
Likewise, a hash reference of all valid attributes and default values (or code
references) may be retrieved with the get_all_attribute_defaults_for class
method. Any attributes without a default will be undef.
my $def = Class::Tiny->get_all_attribute_defaults_for("Employee");
# returns {
# name => undef,
# ssn => undef
# timestamp => $coderef
# }
The import method uses two class methods, prepare_class and
create_attributes to set up the @ISA array and attributes. Anyone
attempting to extend Class::Tiny itself should use these instead of mocking up
a call to import.
When the first object is created, linearized @ISA, the valid attribute list
and various subroutine references are cached for speed. Ensure that all
inheritance and methods are in place before creating objects. (You don't want
to be changing that once you create objects anyway, right?)
I wanted something so simple that it could potentially be used by core Perl modules I help maintain (or hope to write), most of which either use the Class::Struct manpage or roll-their-own OO framework each time.
the Object::Tiny manpage and the Object::Tiny::RW manpage were close to what I wanted, but lacking some features I deemed necessary, and their maintainers have an even more strict philosophy against feature creep than I have.
I also considered the Class::Accessor manpage, which has been around a long time and is heavily used, but it, too, lacked features I wanted and did things in ways I considered poor design.
I looked for something else on CPAN, but after checking a dozen class creators I realized I could implement exactly what I wanted faster than I could search CPAN for something merely sufficient.
In general, compared to most things on CPAN (other than Object::Tiny), Class::Tiny is smaller in implementation and simpler in API.
Specifically, here is how Class::Tiny (``C::T'') compares to Object::Tiny (``O::T'') and Class::Accessor (``C::A''):
FEATURE C::T O::T C::A -------------------------------------------------------------- attributes defined via import yes yes no read/write accessors yes no yes lazy attribute defaults yes no no provides new yes yes yes provides DESTROY yes no no new takes either hashref or list yes no (list) no (hash) Moo(se)-like BUILD/DEMOLISH yes no no Moo(se)-like BUILDARGS yes no no no extraneous methods via @ISA yes yes no
Moose and Moo are both excellent OO frameworks. Moose offers a powerful meta-object protocol (MOP), but is slow to start up and has about 30 non-core dependencies including XS modules. Moo is faster to start up and has about 10 pure Perl dependencies but provides no true MOP, relying instead on its ability to transparently upgrade Moo to Moose when Moose's full feature set is required.
By contrast, Class::Tiny has no MOP and has zero non-core dependencies for Perls in the support window. It has far less code, less complexity and no learning curve. If you don't need or can't afford what Moo or Moose offer, this is intended to be a reasonable fallback.
That said, Class::Tiny offers Moose-like conventions for things like BUILD
and DEMOLISH for some minimal interoperability and an easier upgrade path.
Please report any bugs or feature requests through the issue tracker at https://github.com/dagolden/Class-Tiny/issues. You will be notified automatically of any progress on your issue.
This is open source software. The code repository is available for public review and contribution under the terms of the license.
https://github.com/dagolden/Class-Tiny
git clone https://github.com/dagolden/Class-Tiny.git
David Golden <dagolden@cpan.org>
This software is Copyright (c) 2013 by David Golden.
This is free software, licensed under:
The Apache License, Version 2.0, January 2004
| Class::Tiny - Minimalist class construction |