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Object::Realize::LaterUser Contributed Perl DocumentaObject::Realize::Later(3)

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

	package	MyLazyObject;

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

       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

       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.

       use(Object::Realize::Later %options)
	   When	you invoke ("use") the "Object::Realize::Later"	package, it
	   will	add a set of methods to	your package (see section "Added to
	   YOUR	class").

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

	   becomes => CLASS
	     Which type	will this object become	after realization.

	   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
	     "becomes",	this check must	be disabled.  In that case, specify
	     true for this option.

	   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	Memoize.  See examples

	   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 "require" call
	     which would load it.

	   warn_realization => BOOLEAN
	     Print a warning message when the realization starts.  This	is for
	     debugging purposes.

	   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.

   Added to YOUR class
	   When	a method is called which is not	available for the lazy object,
	   the AUTOLOAD	is called.

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


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

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

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

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


	    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

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

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

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

       Object::Realize::Later->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

	   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)

   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;

	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.  Class
       methods which are not defined in	the stub object	are forwarded as class
       methods without realization.

       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:

       o   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.

       o   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 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'
	    print $original->getLabel;	   # prints 'changed'
	    print $copy->getLabel;	   # prints 'original'

       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;

       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 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 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 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) =	@_;
	    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.

       This module is part of Object-Realize-Later distribution	version	0.21,
       built on	January	24, 2018. Website:

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

       This program is free software; you can redistribute it and/or modify it
       under the same terms as Perl itself.  See

perl v5.32.1			  2018-01-24	     Object::Realize::Later(3)


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