Provided by: libobject-realize-later-perl_0.21-1_all bug

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.

METHODS

   Construction
       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 below.

           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
       $obj->AUTOLOAD()
           When a method is called which is not available for the lazy object, the AUTOLOAD is
           called.

       $obj->can($method)
       Object::Realize::Later->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

       $obj->forceRealize()
           You can force the load by calling this method on your object.  It returns the realized
           object.

       Object::Realize::Later->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

       $obj->willRealize()
           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.

       Object::Realize::Later->import(%options)
           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 again.

       Object::Realize::Later->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)

DETAILS

   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 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.

   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; $self->{label} = shift}
            sub load()      {$_[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'

   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.

        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.

       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.

SEE ALSO

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

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 http://dev.perl.org/licenses/