NAME

Object::Realize::Later - Delayed creation of objects

SYNOPSIS

package MyLazyObject;

use Object::Realize::Later
   becomes => 'MyRealObject',
   realize => 'load';

DESCRIPTION

The Object::Realize::Later class helps with implementing transparent on demand realization of object data. This is related to the tricks on autoloading of data, the lesser known cousin of autoloading of functionality.

On demand realization is all about performance gain. Why should you spent costly time on realizing an object, when the data on the object is never (or not yet) used? In interactive programs, postponed realization may boost start-up: the realization of objects is triggered by the use, so spread over time.

Now there are two ways to implement lazy behaviour: you may choose to check whether you have realized the data in each method which accesses the data, or use the autoloading of data trick.

An implementation of the first solution is:

sub realize {
    my $self = shift;
    return $self unless $self->{_is_realized};

    # read the data from file, or whatever
    $self->{data} = ....;

    $self->{_is_realized} = 1;
    $self;
}

sub getData() {
    my $self = shift;
    return $self->realize->{data};
}

The above implementation is error-prone, where you can easily forget to call realize. The tests cannot cover all ordenings of method-calls to detect the mistakes.

The second approach uses autoloading, and is supported by this package. First we create a stub-object, which will be transformable into a realized object later. This transformation is triggered by AUTOLOAD.

This stub-object may contain some methods from the realized object, to reduce the need for realization. The stub will also contain some information which is required for the creation of the real object.

Object::Realize::Later solves the inheritance problems (especially the isa() and can() methods) and supplies the AUTOLOAD method.

USE

When you invoke (use) the Object::Realize::Later package, it will add set of methods to your package (see the EXPORTS section below).

Specify the following arguments:

• becomes => CLASS

  (required) Which type will this object become after realization.

• realize => METHOD|CODE

  (required) How will transform. If you specify a CODE-reference, then this will be called with the lazy-object as first argument, and the requested method as second.

  After realization, you may still have your hands on the lazy object on various places. Be sure that your realization method is coping with that, for instance by using Memoize. See examples below.

• warn_realization => BOOLEAN

  Print a warning message when the realization starts. This is for debugging purposes. By default this is FALSE.

• warn_realize_again => BOOLEAN

  When an object is realized, the original object -which functioned as a stub- is reconstructed to work as proxy to the realized object. This option (default FALSE) will issue a warning when that proxy is used, which means that somewhere in your program there is a variable still holding a reference to the stub. This latter is not problematic at all, although it slows-down each method call.

See further down in this manual-page about EXAMPLES.

TRAPS

Be aware of dangerous traps in the current implementation. These problems appear by having multiple references to the same delayed object. Depending on how the realization is implemented, terrible things can happen.

The two versions of realization:

• by reblessing

  This is the safe version. The realized object is the same object as the delayed one, but reblessed in a different package. When multiple references to the delayed object exists, they will all be updated at the same, because the bless information is stored within the refered variable.

• by new instance

  This is the nicest way of realization, but also quite more dangerous. Consider this:

  package Delayed;
  use Object::Realize::Later
       becomes => 'Realized',
       realize => 'load';
  
  sub new($)      {my($class,$v)=@_; bless {label=>$v}, $class}
  sub setLabel($) {my $self = shift; $self->{label} = shift}
  sub load()      {my $self = shift; Realize->new($self->{label});
  
  package Realized;
  sub new($)      {my($class,$v)=@_; bless {label=>$v}, $class}
  sub setLabel($) {my $self = shift; $self->{label} = shift}
  sub getLabel()  {my $self = shift; $self->{label}}
  
  package main;
  my $original = Delayed->new('original');
  my $copy     = $original;
  print $original->getLabel;     # prints 'original'
  print $copy->getLabel;         # prints 'original'
  # $original gets realized, but $copy is not informed!
  
  print $copy->setLabel('copy'); # prints 'copy'
  print $original->getLabel;     # still prints 'original'
  print $copy->getLabel;         # prints 'original'
  # Now also copy is realized to the same object.

EXPORTS

The following methods are added to your package:

isa CLASS

(Class and instance method) Is this object a (sub-)class of the specified CLASS or can it become a (sub-)class of CLASS.

Examples:

MyLazyObject->isa('MyRealObject')      # true
MyLazyObject->isa('SuperClassOfLazy'); # true
MyLazyObject->isa('SuperClassOfReal'); # true

my $lazy = MyLazyObject->new;
$lazy->isa('MyRealObject');            # true
$lazy->isa('SuperClassOfLazy');        # true
$lazy->isa('SuperClassOfReal');        # true

can METHOD

(Class and instance method) Is the specified METHOD available for the lazy or the realized version of this object? It will return the reference to the code.

Examples:

MyLazyObject->can('lazyWork')      # true
MyLazyObject->can('realWork')      # true

my $lazy = MyLazyObject->new;
$lazy->can('lazyWork');            # true
$lazy->can('realWork');            # true

AUTOLOAD

When a method is called which is not available for the lazy object, the AUTOLOAD is called.

forceRealize

You can force the load by calling this method on your object. It returns the realized object.

willRealize

Returns which class will be the realized to follow-up this class.

Own METHODS

The next methods are not exported to the class where the `use' took place. These methods implement the actual realization.

realize OPTIONS

This method is called when a $object-forceRealize()> takes place. It checks whether the realization has been done already (is which case the realized object is returned)

realizationOf OBJECT [,REALIZED]

Returns the REALIZED version of OBJECT, optionally after setting it first. When the method returns undef, the realization has not yet taken place or the realized object has already been removed again.

EXAMPLES

Example 1

In the first example, we delay-load a message. On the moment the message is defined, we only take the location. When the data of the message is taken (header or body), the data is autoloaded.

use Mail::Message::Delayed;

use Object::Realize::Later
  ( becomes => 'Mail::Message::Real'
  , realize => 'loadMessage'
  );

sub new($) {
    my ($class, $file) = @_;
    bless { filename => $file }, $class;
}

sub loadMessage() {
    my $self = shift;
    Mail::Message::Real->new($self->{filename});
}

In the main program:

package main;
use Mail::Message::Delayed;

my $msg    = Mail::Message::Delayed->new('/home/user/mh/1');
$msg->body->print;     # this will trigger autoload.

Example 2

Your realization may also be done by reblessing. In that case to change the type of your object into a different type which stores the same information. Is that right? Are you sure? For simple cases, this may be possible:

package Alive;
use Object::Realize::Later
     becomes => 'Dead',
     realize => 'kill';

sub new()         {my $class = shift; bless {@_}, $class}
sub jump()        {print "Jump!\n"}
sub showAntlers() {print "Fight!\n"}
sub kill()        {bless(shift, 'Dead')}

package Dead;
sub takeAntlers() {...}

In the main program:

my $deer   = Alive->new(Animal => 'deer');
my $trophy = $deer->takeAntlers();

In this situation, the object (reference) is not changed but is reblessed. There is no danger that the un-realized version of the object is kept somewhere: all variable which know about this partical deer see the change.

Example 3

This module is especially usefull for larger projects, which there is a need for speed or memory reduction. In this case, you may have an extra overview on which objects have been realized (transformed), and which not. This example is taken from the Mail::Box modules:

The Mail::Box module tries to boost the access-time to a folder. If you only need the messages of the last day, why shall all be read? So, Mail::Box only creates an invertory of messages at first. It takes the headers of all messages, but leaves the body (content) of the message in the file.

In Mail::Box' case, the Mail::Message-object has the choice between a number of Mail::Message::Body's, one of which has only be prepared to read the body when needed. A code snippet:

package Mail::Message;
sub new($$)
{   my ($class, $head, $body) = @_;
    bless {head => $head, body => $body}, $class;
}
sub head()     { shift->{head} }
sub body()     { shift->{body} }

sub loadBody()
{   my $self = shift;
    my $body = $self->body;

    # Catch re-invocations of the loading.  If anywhere was still
    # a reference to the old (unrealized) body of this message, we
    # return the new-one directly.
    return $body unless $body->can('forceRealize');

    # Load the body (change it to anything which really is of
    # the promised type, or a sub-class of it.
    my ($lines, $size) = .......;    # get the data
    $self->{body} = Mail::Message::Body::Lines->new($lines, $size);

    # Return the realized object.
    return $self->{body};
}


package Mail::Message::Body::Lines;
use base 'Mail::Message::Body';

sub new($$)
{   my ($class, $lines, $size) = @_;
    bless { lines => $lines, size => $size }, $class;
}
sub size()  { shift->{size} }
sub lines() { shift->{lines} }


package Mail::Message::Body::Delayed;
use Object::Realize::Later
    becomes => 'Mail::Message::Body',
    realize => sub {shift->message->loadBody};

sub new() {
    my ($class, $message, $size) = @_;
    bless {message => $message, size => $size}, $class;
}
sub size() { shift->{size} }


package main;
use Mail::Message;
use Mail::Message::Body::Delayed;

my $body    = Mail::Message::Body::Delayed->new(42);
my $message = Mail::Message->new($head, $body);

print $message->size;         # will not trigger realization!
print $message->can('lines'); # true, but no realization
print $message->lines;        # realizes automatically.

SEE ALSO

L<Object::Realize::Proxy>

AUTHOR

Mark Overmeer (Mark@Overmeer.net). All rights reserved. This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself.

cpanm Object::Realize::Later

perl -MCPAN -e shell
install Object::Realize::Later