=encoding utf8

=head1 NAME

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

=head1 SYNOPSIS

 package MyLazyObject;

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

=head1 DESCRIPTION

The C<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.

=head1 METHODS

=head2 Construction

=over 4

=item B<use>(Object::Realize::Later %options)

When you invoke (C<use>) the C<Object::Realize::Later> package, it will
add a set of methods to your package (see section L</Added to YOUR class>).

 -Option            --Default
  becomes             <required>
  believe_caller      <false>
  realize             <required>
  source_module       <becomes>
  warn_realization    <false>
  warn_realize_again  <false>

=over 2

=item becomes => CLASS

Which type will this object become after realization.

=item believe_caller => BOOLEAN

When a method is called on the un-realized object, the AUTOLOAD
checks whether this resolves the need.  If not, the realization is
not done.  However, when realization may result in an object that
extends the functionality of the class specified with C<becomes>,
this check must be disabled.  In that case, specify true for
this option.

=item realize => METHOD|CODE

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 L<Memoize>.  See examples below.

=item source_module => CLASS

if the class (a package) is included in a file (module) with a different
name, then use this argument to specify the file name. The name is
expected to be the same as in the C<require> call which would load it.

=item warn_realization => BOOLEAN

Print a warning message when the realization starts.  This is for
debugging purposes.

=item 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 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.

=back

=back

=head2 Added to YOUR class

=over 4

=item $obj-E<gt>B<AUTOLOAD>()

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

=item $obj-E<gt>B<can>($method)

=item Object::Realize::Later-E<gt>B<can>($method)

Is the specified $method available for the lazy or the realized version
of this object?  It will return the reference to the code.

example: 

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

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

=item $obj-E<gt>B<forceRealize>()

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

=item Object::Realize::Later-E<gt>B<isa>($class)

Is this object a (sub-)class of the specified $class or can it become a
(sub-)class of $class.

example: 

 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

=item $obj-E<gt>B<willRealize>()

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

=back

=head2 Object::Realize::Later internals

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

=over 4

=item Object::Realize::Later-E<gt>B<import>(%options)

The %options used for C<import> are the values after the class name
with C<use>.  So this routine implements the actual option parsing.
It generates code dynamically, which is then evaluated in the
callers name-space.

=item Object::Realize::Later-E<gt>B<realizationOf>( $object, [$realized] )

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

=item Object::Realize::Later-E<gt>B<realize>(%options)

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

=back

=head1 DETAILS

=head2 About lazy loading

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 L<realize()|Object::Realize::Later/"Object::Realize::Later internals">.  The tests cannot cover all ordenings of method-calls to
detect the mistakes.

The I<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.

C<Object::Realize::Later> solves the inheritance problems (especially
the L<isa()|Object::Realize::Later/"Added to YOUR class"> and L<can()|Object::Realize::Later/"Added to YOUR class"> methods) and supplies the AUTOLOAD method.
Class methods which are not defined in the stub object are forwarded
as class methods without realization.

=head2 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:

=over 4

=item * 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.

=item * 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; $self->{label} = shift}
 sub load()      {$_[0] = Realized->new($_[0]->{label}) }

 package Realized;  # file Realized.pm or use use(source_module)
 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 ref $original;           # prints 'Realized'
 print ref $copy;               # prints 'Delayed'
 $original->setLabel('changed');
 print $original->getLabel;     # prints 'changed'
 print $copy->getLabel;         # prints 'original'

=back

=head2 Examples

=head3 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.

 package 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.

=head3 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 I<reblessed>.
There is no danger that the un-realized version of the object is kept
somewhere: all variable which know about this partical I<deer> see the
change.

=head3 Example 3

This module is especially useful 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 MailBox modules:

The L<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, MailBox 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 MailBox' case, the L<Mail::Message>-object has the choice
between a number of L<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) = @_;
     my $self = bless {head => $head, body => $body}, $class;
     $body->message($self);          # tell body about the message
 }
 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, $self);

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

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

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

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

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

 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 yet.
 print $message->lines;        # realizes automatically.

=head1 SEE ALSO

This module is part of Object-Realize-Later distribution version 0.21,
built on January 24, 2018. Website: F<http://perl.overmeer.net/CPAN/>

=head1 LICENSE

Copyrights 2001-2018 by [Mark Overmeer]. For other contributors see ChangeLog.

This program is free software; you can redistribute it and/or modify it
under the same terms as Perl itself.
See F<http://dev.perl.org/licenses/>