=for rcs $Id: Object.pod,v 1.16 2007/09/13 15:12:25 dk Exp $

=head1 NAME

Prima::Object - Prima toolkit base classes

=head1 SYNOPSIS

    if ( $obj-> isa('Prima::Component')) {

        # set and get a property
	my $name = $obj-> name;
	$obj->name( 'an object' );

	# set a notification callback
	$obj-> onPostMessage( sub { 
	    shift;
	    print "hey! I've received this: @_\n";
	});

	# can set multiple properties. note, that 'name' and 'owner',
	# replace the old values, while onPostMessage are aggregated.
	$obj-> set(
	    name => 'AnObject',
	    owner => $new_owner,
	    onPostMessage => sub {
	       shift;
	       print "hey! me too!\n";
	    },
	);

        # de-reference by name
	$new_owner-> AnObject-> post_message(1,2);
    }

=head1 DESCRIPTION

Prima::Object and Prima::Component are the root objects of the Prima toolkit
hierarchy. All the other objects are derived from the Component class, which in
turn is the only descendant of Object class. Both of these classes are never
used for spawning their instances, although this is possible using

   Prima::Component-> create( .. parameters ... );

call. This document describes the basic concepts of the OO programming with
Prima toolkit. Although Component has wider functionality than Object, all
examples will be explained on Component, since Object has no descendant classes
and all the functionality of Object is present in Component.  Some of the
information here can be found in L<Prima::internals> as well, the difference is
that L<Prima::internals> considers the coding tasks from a C programmer's view,
whereas this document is wholly about perl programming.

=head1 Object base features

=head2 Creation

Object creation has fixed syntax: 

   $new_object = Class-> create(
     parameter => value,
     parameter => value,
     ...
   );

Parameters and values form a hash, which is passed to the create() method.
This hash is applied to a default parameter-value hash ( a I<profile> ),
specific to every Prima class. The object creation is performed in several
stages.

=over 4

=item create

create() calls profile_default() method that returns ( as its name states ) the
default profile, a hash with the appropriate default values assigned to its
keys.  The Component class defaults are ( see Classes.pm ):

     name        => ref $_[ 0],
     owner       => $::application,
     delegations => undef,

While the exact meaning of these parameters is described later, in
L<"Properties">, the idea is that a newly created object will have 'owner'
parameter set to '$::application' and 'delegations' to undef etc etc - unless
these parameters are explicitly passed to create(). Example:

     $a1 = Prima::Component-> create();

$a1's owner will be $::application

     $a2 = Prima::Component-> create( owner => $a1);

$a2's owner will be $a1.
The actual merging of the default and the parameter hashes
is performed on the next stage, in profile_check_in() method
which is called inside profile_add() method.

=item profile_check_in

A profile_check_in() method merges the default and the parameter profiles. By
default all specified parameters have the ultimate precedence over the default
ones, but in case the specification is incomplete or ambiguous, the
profile_check_in()'s task is to determine actual parameter values. In case of
Component, this method maintains a simple automatic naming of the newly created
objects. If the object name was not specified with create(), it is assigned to
a concatenated class name with an integer - Component1, Component2 etc. 

Another example can be taken from Prima::Widget::profile_check_in().
Prima::Widget horizontal position can be specified by using basic C<left> and
C<width> parameters, and as well by auxiliary C<right>, C<size> and C<rect>.
The default of both C<left> and C<width> is 100. But if only C<right>
parameter, for example, was passed to create() it is profile_check_in() job to
determine C<left> value, given that C<width> is still 100.

After profiles gets merged, the resulting hash is passed to the third stage,
init().

=item init

init() duty is to map the profile content into object, e.g., assign C<name>
property to C<name> parameter value, and so on - for all relevant parameters.
After that, it has to return the profile in order the overridden subsequent
init() methods can perform same actions. This stage along with the previous is
exemplified in almost all Prima modules. 

Note: usually init() attaches the object to its owner in order to keep the
newly-created object instance from being deleted by garbage-collection
mechanisms. More on that later ( see L<"Links between objects">).

After init() finishes, create() calls setup() method

=item setup

setup() method is a convenience function, it is used when some post-init
actions must be taken. It is seldom overloaded, primarily because the
Component::setup() method calls C<onCreate> notification, which is more
convenient to overload than setup(). 

=back

As can be noticed from the code pieces above, a successful create() call
returns a newly created object. If an error condition occurred, undef is
returned. It must be noted, that only errors that were generated via die()
during init() stage result in undef. Other errors raise an exception instead.
It is not recommended to frame create() calls in an C<eval{}> block, because
the error conditions can only occur in two situations. The first is a system
error, either inside perl or Prima guts, and not much can be done here, since
that error can very probably lead to an unstable program and almost always
signals an implementation bug. The second reason is a caller's error, when an
unexistent parameter key or invalid value is passed; such conditions are not
subject to a runtime error handling as are not the syntax errors.

After create(), the object is subject to the event flow.  As C<onCreate> event
is the first event the object receives, only after that stage other events can
be circulated. 

=head2 Destruction

Object destruction can be caused by many conditions, but all execution flow is
finally passed through destroy() method.  destroy(), as well as create()
performs several finalizing steps:

=over 4

=item cleanup

The first method called inside destroy() is cleanup().  cleanup() is the pair
to setup(), as destroy() is the pair to create(). cleanup() generates
C<onDestroy> event, which can be overridden more easily than cleanup() itself.

C<onDestroy> is the last event the object sees. After cleanup() no events are
allowed to circulate.

=item done

done() method is the pair to init(), and is the place where all object
resources are freed. Although it is as safe to overload done() as init(), it
almost never gets overloaded, primarily because overloading C<onDestroy> is
easier.

=back

The typical conditions that lead to object destructions are
direct destroy() call, garbage collections mechanisms, 
user-initiated window close ( on C<Prima::Window> only ), and
exception during init() stage. Thus, one must be careful implementing
done() which is called after init() throws an exception.

=head2 Methods

The class methods are declared and used with perl OO syntax, which allow both
method of object referencing:

  $object-> method();

and

  method( $object);

The actual code is a sub, located under the object class package.  The
overloaded methods that call their ancestor code use

  $object-> SUPER::method();

syntax. Most Prima methods have fixed number of parameters.

=head2 Properties

Properties are methods that combine functionality of two ephemeral "get" and
"set" methods. The idea behind properties is that many object parameters
require two independent methods, one that returns some internal state and
another that changes it.  For example, for managing the object name, set_name()
and get_name() methods are needed. Indeed, the early Prima implementation dealt
with large amount of these get's and set's, but later these method pairs were
deprecated in the favor of properties.  Currently, there is only one method
name() ( referred as C<::name> later in the documentation ). 

The property returns a value if no parameters ( except the object) are passed,
and changes the internal data to the passed parameters otherwise. Here's 
a sketch code for C<::name> property implementation:

 sub name
 {
    return $_[0]-> {name} unless $#_;
    $_[0]->{name} = $_[1];
 }

There are many examples of properties throughout the toolkit.  Not all
properties deal with scalar values, some accept arrays or hashes as well.
The properties can be set-called not only by name like

  $object-> name( "new name");

but also with set() method. The set() method accepts a hash,
that is much like to create(), and assigns the values to
the corresponding properties. For example, the code

  $object-> name( "new name");
  $object-> owner( $owner);

can be rewritten as

  $object-> set(
     name  => "new name",
     owner => $owner
  );

A minor positive effect of a possible speed-up is gained by eliminating
C-to-perl and perl-to-C calls, especially if the code called is implemented in
C. The negative effect of such technique is that the order in which the
properties are set, is undefined. Therefore, the usage of set() is recommended
either when the property order is irrelevant, or it is known beforehand that
such a call speeds up the code, or is an only way to achieve the result. An
example of the latter case from L<Prima::internals> shows that Prima::Image
calls

    $image-> type( $a);
    $image-> palette( $b);

and 

    $image-> palette( $b);
    $image-> type( $a);

produce different results. It is indeed the only solution
to call for such a change using

    $image-> set(
       type => $a,
       palette => $b
    );

when it is known beforehand that C<Prima::Image::set> is aware of such 
combinations and calls neither C<::type> nor C<::palette> but
performs another image conversion instead.

Some properties are read-only and some are write-only. Some methods that might
be declared as properties are not; these are declared as plain methods with
get_ or set_ name prefix. There is not much certainty about what methods are
better off being declared as properties and vice versa.

However, if get_ or set_ methods cannot be used in correspondingly write or
read fashion, the R/O and W/O properties can. They raise an exception on an
attempt to do so.

=head2 Links between objects

Prima::Component descendants can be used as containers, as objects that are on
a higher hierarchy level than the others. This scheme is implemented in a
child-owner relationship.  The 'children' objects have the C<::owner> property
value assigned to a reference to a 'owner' object, while the 'owner' object
conducts the list of its children. It is a one-to-many hierarchy scheme, as a
'child' object can have only one owner, but an 'owner' object can have many
children. The same object can be an owner and a child at the same time, so the
owner-child hierarchy can be viewed as a tree-like structure.

Prima::Component::owner property maintains this relation, and is writable - the
object can change its owner dynamically. There is no corresponding property
that manages children objects, but is a method get_components(), that returns
an array of the child references.

The owner-child relationship is used in several ways in the toolkit.  For
example, the widgets that are children of another widget appear ( usually, but
not always ) in the geometrical interior of the owner widget.  Some events (
keyboard events, for example ) are propagated automatically up and/or down the
object tree. Another important feature is that when an object gets destroyed,
its children are destroyed first.  In a typical program the whole object tree
roots in a Prima::Application object instance. When the application finishes,
this feature helps cleaning up the widgets and quitting gracefully.

Implementation note: name 'owner' was taken instead of initial 'parent',
because the 'parent' is a fixed term for widget hierarchy relationship
description. Prima::Widget relationship between owner and child is not the same
as GUI's parent-to-child.  The parent is the widget for the children widgets
located in and clipped by its inferior. The owner widget is more than that, its
children can be located outside its owner boundaries.

The special convenience variety of create(), the insert() method is used
to explicitly select owner of the newly created object. insert() can 
be considered a 'constructor' in OO-terms. It makes the construct

   $obj = Class-> create( owner => $owner, name => 'name);

more readable by introducing 

   $obj = $owner-> insert( 'Class', name => 'name');

scheme. These two code blocks are identical to each other. 

There is another type of relation, where objects can hold references to each
other. Internally this link level is used to keep objects from deletion by
garbage collection mechanisms.  This relation is many-to-many scheme, where
every object can have many links to other objects. This functionality is
managed by attach() and detach() methods.

=head1 Events

Prima::Component descendants employ a well-developed event propagation
mechanism, which allows handling events using several different schemes.  An
event is a condition, caused by the system or the user, or an explicit notify()
call. The formerly described events onCreate and onDestroy are triggered after
a new object is created or before it gets destroyed. These two events, and the
described below onPostMessage are present in namespaces of all Prima objects.
New classes can register their own events and define their execution flow,
using notification_types() method.  This method returns all available
information about the events registered in a class. 

Prima defines also a non-object event dispatching and filtering mechanism,
available through L<"event_hook"> static method.

=head2 Propagation

The event propagation mechanism has three layers of user-defined callback
registration, that are called in different order and contexts when an event is
triggered. The examples below show the usage of these layers. It is assumed
that an implicit 

  $obj-> notify("PostMessage", $data1, $data2);

call is issued for all these examples.

=over 4

=item Direct methods

As it is usual in OO programming, event callback routines
are declared as methods. 'Direct methods' employ such a paradigm,
so if a class method with name C<on_postmessage> is present,
it will be called as a method ( i.e., in the object context )
when C<onPostMessage> event is triggered. Example:

 sub on_postmessage 
 {
    my ( $self, $data1, $data2) = @_;
    ...
 }

The callback name is a modified lower-case event name: the name for Create
event is on_create, PostMessage - on_postmessage etc.  These methods can be
overloaded in the object's class descendants.  The only note on declaring these
methods in the first instance is that no C<::SUPER> call is needed, because
these methods are not defined by default.

Usually the direct methods are used for the internal object book-keeping,
reacting on the events that are not designed to be passed higher. For example,
a Prima::Button class catches mouse and keyboard events in such a fashion,
because usually the only notification that is interesting for the code that
employs push-buttons is C<Click>.  This scheme is convenient when an event
handling routine serves the internal, implementation-specific needs.

=item Delegated methods

The delegated methods are used when objects ( mostly widgets ) include other
dependent objects, and the functionality requires interaction between these.
The callback functions here are the same methods as direct methods, except that
they get called in context of two, not one, objects. If, for example, a $obj's
owner, $owner would be interested in $obj's PostMessage event, it would
register the notification callback by

   $obj-> delegations([ $owner, 'PostMessage']);

where the actual callback sub will be

 sub Obj_PostMessage 
 {
    my ( $self, $obj, $data1, $data2) = @_;
 }

Note that the naming style is different - the callback name is constructed from
object name ( let assume that $obj's name is 'Obj') and the event name. ( This
is one of the reasons why Component::profile_check_in() performs automatic
naming of newly created onbjects). Note also that context objects are $self (
that equals $owner ) and $obj.

The delegated methods can be used not only for the owner-child relations. Every
Prima object is free to add a delegation method to every other object. However,
if the objects are in other than owner-child relation, it is a good practice to
add Destroy notification to the object which events are of interest, so if it
gets destroyed, the partner object gets a message about that.

=item Anonymous subroutines

The two previous callback types are more relevant when a separate class is
developed, but it is not necessary to declare a new class every time the event
handling is needed.  It is possible to use the third and the most powerful
event hook method using perl anonymous subroutines ( subs ) for the easy
customization.

Contrary to the usual OO event implementations, when only one routine per class
dispatches an event, and calls inherited handlers when it is appropriate, Prima
event handling mechanism can accept many event handlers for one object ( it is
greatly facilitated by the fact that perl has I<anonymous subs>, however).

All the callback routines are called when an event is triggered, one by one in
turn. If the direct and delegated methods can only be multiplexed by the usual
OO inheritance, the anonymous subs are allowed to be multiple by the design.
There are three syntaxes for setting such a event hook; the example below
sets a hook on $obj using each syntax for a different situation:

- during create():

   $obj = Class-> create(
    ... 
    onPostMessage => sub {
       my ( $self, $data1, $data2) = @_;
    },
    ...
    );

- after create using set()

   $obj-> set( onPostMessage => sub {
       my ( $self, $data1, $data2) = @_;
   });

- after create using event name:

   $obj-> onPostMessage( sub {
       my ( $self, $data1, $data2) = @_;
   });

As was noted in L<Prima>, the events can be addressed as properties, with the
exception that they are not substitutive but additive.  The additivity is that
when the latter type of syntax is used, the subs already registered do not get
overwritten or discarded but stack in queue. Thus,

   $obj-> onPostMessage( sub { print "1" });
   $obj-> onPostMessage( sub { print "2" });
   $obj-> notify( "PostMessage", 0, 0);

code block would print

   21

as the execution result.

This, it is a distinctive feature of a toolkit is that two objects of same class may
have different set of event handlers.

=back

=head2 Flow

When there is more than one handler of a particular event type present on an
object, a question is risen about what are callbacks call priorities and when
does the event processing stop. One of ways to regulate the event flow is based
on prototyping events, by using notification_types() event type description.
This function returns a hash, where keys are the event names and the values are
the constants that describe the event flow. The constant can be a bitwise OR
combination of several basic flow constants, that control the three aspects of
the event flow. 

=over 4

=item Order

If both anonymous subs and direct/delegated methods are present, it must be
decided which callback class must be called first.  Both 'orders' are useful:
for example, if it is designed that a class's default action is to be
overridden, it is better to call the custom actions first. If, on the contrary,
the class action is primary, and the others are supplementary, the reverse
order is preferred. One of two C<nt::PrivateFirst> and C<nt::CustomFirst>
constants defines the order.

=item Direction

Almost the same as order, but for finer granulation of event flow, the
direction constants C<nt::FluxNormal> and C<nt::FluxReverse> are used. The
'normal flux' defines FIFO ( first in first out ) direction. That means, that
the sooner the callback is registered, the greater priority it would possess
during the execution.  The code block shown above 

   $obj-> onPostMessage( sub { print "1" });
   $obj-> onPostMessage( sub { print "2" });
   $obj-> notify( "PostMessage", 0, 0);

results in C<21>, not C<12> because PostMessage event type is prototyped
C<nt::FluxReverse>.

=item Execution control

It was stated above that the events are additive, - the callback storage is
never discarded  when 'set'-syntax is used.  However, the event can be told to
behave like a substitutive property, e.g. to call one and only one callback.
This functionality is governed by C<nt::Single> bit in execution control
constant set, which consists of the following constants:

  nt::Single
  nt::Multiple
  nt::Event

These constants are mutually exclusive, and may not appear together in an event
type declaration.  A C<nt::Single>-prototyped notification calls only the first
( or the last - depending on order and direction bits ) callback. The usage of
this constant is somewhat limited.

In contrary of C<nt::Single>, the C<nt::Multiple> constant sets the execution
control to call all the available callbacks, with respect to direction and
order bits. 

The third constant, C<nt::Event>, is the  impact as C<nt::Multiple>, except
that the event flow can be stopped at any time by calling clear_event() method.

=back

Although there are 12 possible event type combinations, a half of them are not
viable. Another half were assigned to unique more-less intelligible names:

  nt::Default       ( PrivateFirst | Multiple | FluxReverse)
  nt::Property      ( PrivateFirst | Single   | FluxNormal )
  nt::Request       ( PrivateFirst | Event    | FluxNormal )
  nt::Notification  ( CustomFirst  | Multiple | FluxReverse )
  nt::Action        ( CustomFirst  | Single   | FluxReverse )
  nt::Command       ( CustomFirst  | Event    | FluxReverse )

=head2 Success state

Events do not return values, although the event generator, the notify() method
does - it returns either 1 or 0, which is the value of event success state.
The 0 and 1 results in general do not mean either success or failure, they
simply reflect the fact whether clear_event() method was called during the
processing - 1 if it was not, 0 otherwise. The state is kept during the whole
processing stage, and can be accessed from Component::eventFlag property. Since
it is allowed to call notify() inside event callbacks, the object maintains a
stack for those states.  Component::eventFlags always works with the topmost
one, and fails if is called from outside the event processing stage. Actually,
clear_event() is an alias for ::eventFlag(0) call. The state stack is operated
by push_event() and pop_event() methods.

Implementation note: a call of clear_event() inside a C<nt::Event>-prototyped
event call does not automatically stops the execution. The execution stops if
the state value equals to 0 after the callback is finished.  A ::eventFlag(1)
call thus cancels the effect of clear_event().

A particular coding style is used when the event is C<nt::Single>-prototyped
and is called many times in a row, so overheads of calling notify() become a
burden. Although notify() logic is somewhat complicated, it is rather simple
with C<nt::Single> case. The helper function get_notify_sub() returns the context
of callback to-be-called, so it can be used to emulate notify() behavior.
Example:

  for ( ... ) {
     $result = $obj-> notify( "Measure", @parms);
  }

can be expressed in more cumbersome, but efficient code if
C<nt::Single>-prototyped event is used:

   my ( $notifier, @notifyParms) = $obj-> get_notify_sub( "Measure" );
   $obj-> push_event;
   for ( ... ) {
       $notifier-> ( @notifyParms, @parms);
       # $result = $obj-> eventFlag; # this is optional
   }
   $result = $obj-> pop_event;

=head1 API

=head2 Prima::Object methods

=over 4

=item alive

Returns the object 'vitality' state - true if the object is alive and usable,
false otherwise.  This method can be used as a general checkout if the scalar
passed is a Prima object, and if it is usable.  The true return value can be 1
for normal and operational object state, and 2 if the object is alive but in
its init() stage. Example:

  print $obj-> name if Prima::Object::alive( $obj);

=item can NAME, CACHE = 1

Checks if an object namespace contains a NAME method.  Returns the code
reference to it, if found, and undef if not.  If CACHE is true, caches the
result to speed-up subsequent calls.

=item cleanup

Called right after destroy() started. Used to initiate C<cmDestroy>
event. Is never called directly.

=item create CLASS, %PARAMETERS

Creates a new object instance of a given CLASS
and sets its properties corresponding to the passed parameter hash. Examples:

   $obj = Class-> create( PARAMETERS);
   $obj = Prima::Object::create( "class" , PARAMETERS);

Is never called in an object context.

Alias: new()

=item destroy

Initiates the object destruction. Perform in turn cleanup() and done() calls.
destroy() can be called several times and is the only Prima re-entrant
function, therefore may not be overloaded.

=item done

Called by destroy() after cleanup() is finished. Used to free the object
resources, as a finalization stage.  During done() no events are allowed to
circulate, and alive() returns 0. The object is not usable after done()
finishes. Is never called directly.

Note: the eventual child objects are destroyed inside done() call.

=item get @PARAMETERS

Returns hash where keys are @PARAMETERS and values are
the corresponding object properties.

=item init %PARAMETERS

The most important stage of object creation process.  %PARAMETERS is the
modified hash that was passed to create().  The modification consists of
merging with the result of profile_default() class method inside
profile_check_in() method. init() is responsible for applying the relevant data
into PARAMETERS to the object properties. Is never called directly.

=item insert CLASS, %PARAMETERS

A convenience wrapper for create(), that explicitly sets
the owner property for a newly created object. 

   $obj = $owner-> insert( 'Class', name => 'name');

is adequate to

   $obj = Class-> create( owner => $owner, name => 'name);

code. insert() has another syntax that allows simultaneous
creation of several objects:

   @objects = $owner-> insert( 
     [ 'Class', %parameters],
     [ 'Class', %parameters],
     ...
   );

With such syntax, all newly created objects would have $owner
set to their 'owner' properties.

=item new CLASS, %PARAMETERS

Same as L<create>.

=item profile_add PROFILE

The first stage of object creation process.  PROFILE is a reference to a
PARAMETERS hash, passed to create().  It is merged with profile_default() after
passing both to profile_check_in(). The merge result is stored back in PROFILE.
Is never called directly.

=item profile_check_in CUSTOM_PROFILE, DEFAULT_PROFILE

The second stage of object creation process.  Resolves eventual ambiguities in
CUSTOM_PROFILE, which is the reference to PARAMETERS passed to create(), by
comparing to and using default values from DEFAULT_PROFILE, which is the result
of profile_default() method. Is never called directly.

=item profile_default

Returns hash of the appropriate default values for all properties of a class.
In object creation process serves as a provider of fall-back values, and is
called implicitly. This method can be used directly, contrary to the other
creation process-related functions.

Can be called in a context of class.

=item raise_ro TEXT

Throws an exception with text TEXT when a read-only property is called in
a set- context.

=item raise_wo TEXT

Throws an exception with text TEXT when a write-only property is called in
a get- context.

=item set %PARAMETERS

The default behavior is an equivalent to

  sub set
  {
     my $obj = shift;
     my %PARAMETERS = @_;
     $obj-> $_( $PARAMETERS{$_}) for keys %PARAMETERS;
  }

code. Assigns object properties correspondingly to PARAMETERS hash.  Many
Prima::Component descendants overload set() to make it more efficient for
particular parameter key patterns. 

As the code above, raises an exception if the key in PARAMETERS has no
correspondent object property.

=item setup

The last stage of object creation process.  Called after init() finishes. Used
to initiate C<cmCreate> event. Is never called directly.

=back

=head2 Prima::Component methods

=over 4

=item add_notification NAME, SUB, REFERER = undef, INDEX = -1

Adds SUB to the list of notification of event NAME.  REFERER is the object
reference, which is used to create a context to SUB and is passed as a
parameter to it when called.  If REFERER is undef ( or not specified ), the
same object is assumed. REFERER also gets implicitly attached to the object, -
the implementation frees the link between objects when one of these gets
destroyed.

INDEX is a desired insert position in the notification list.  By default it is
-1, what means 'in the start'. If the notification type contains nt::FluxNormal
bit set, the newly inserted SUB will be called first. If it has
nt::FluxReverse, it is called last, correspondingly.

Returns positive integer value on success, 0 on failure.  This value can be
later used to refer to the SUB in remove_notification(). 

See also: C<remove_notification>, C<get_notification>.

=item attach OBJECT 

Inserts OBJECT to the attached objects list and increases OBJECT's reference
count. The list can not hold more than one reference to the same object. The
warning is issued on such an attempt. 

See also: C<detach>.

=item bring NAME

Looks for a immediate child object that has name equals to NAME.  Returns its
reference on success, undef otherwise. It is a convenience method, that makes
possible the usage of the following constructs:

   $obj-> name( "Obj");
   $obj-> owner( $owner);
   ...
   $owner-> Obj-> destroy;

=item can_event

Returns true if the object event circulation is allowed.  In general, the same
as C<alive() == 1>, except that can_event() fails if an invalid object
reference is passed.

=item clear_event

Clears the event state, that is set to 1 when the event processing begins.
Signals the event execution stop for nt::Event-prototyped events.

See also: L<"Events">, C<push_event>, C<pop_event>, C<::eventFlag>, C<notify>.

=item detach OBJECT, KILL 

Removes OBJECT from the attached objects list and decreases OBJECT's reference
count. If KILL is true, destroys OBJECT.

See also: C<attach>

=item event_error

Issues a system-dependent warning sound signal.

=item event_hook [ SUB ]

Installs a SUB to receive all events on all Prima objects.  SUB receives same
parameters passed to L<notify>, and must return an integer, either 1 or 0, to
pass or block the event respectively.

If no SUB is set, returns currently installed event hook pointer.  If SUB is
set, replaces the old hook sub with SUB. If SUB is C<'undef'>, event filtering
is not used.

Since the C<'event_hook'> mechanism allows only one hook routine to be
installed at a time, direct usage of the method is discouraged.  Instead, use
L<Prima::EventHook> for multiplexing of the hook access.

The method is static, and can be called either with or without class or
object as a first parameter.

=item get_components

Returns array of the child objects.

See: C<create>, L<"Links between objects">.

=item get_handle

Returns a system-dependent handle for the object.  For example, Prima::Widget
return its system WINDOW/HWND handles, Prima::DeviceBitmap - its system
PIXMAP/HBITMAP handles, etc.

Can be used to pass the handle value outside the program, for an eventual
interprocess communication scheme.

=item get_notification NAME, @INDEX_LIST

For each index in INDEX_LIST return three scalars, bound at the index position
in the NAME event notification list.  These three scalars are REFERER, SUB and
ID. REFERER and SUB are those passed to C<add_notification>, and ID is its
result.  

See also: C<remove_notification>, C<add_notification>.

=item get_notify_sub NAME

A convenience method for nt::Single-prototyped events.  Returns code reference
and context for the first notification sub for event NAME.

See L<"Success state"> for example.

=item notification_types

Returns a hash, where the keys are the event names and the values are the
C<nt::> constants that describe the event flow.

Can be called in a context of class.

See L<"Events"> and L<"Flow"> for details.

=item notify NAME, @PARAMETERS

Calls the subroutines bound to the event NAME with parameters @PARAMETERS in
context of the object.  The calling order is described by C<nt::> constants,
contained in the notification_types() result hash.

notify() accepts variable number of parameters, and while it is possible, it is
not recommended to call notify() with the exceeding number of parameters; the
call with the deficient number of parameters results in an exception.

Example:

   $obj-> notify( "PostMessage", 0, 1);

See L<"Events"> and L<"Flow"> for details.

=item pop_event

Closes event processing stage brackets.

See C<push_event>, L<"Events">

=item post_message SCALAR1, SCALAR2

Calls C<PostMessage> event with parameters SCALAR1 and SCALAR2 once during idle
event loop. Returns immediately.  Does not guarantee that C<PostMessage> will
be called, however.

See also L<Prima::Utils/post>

=item push_event

Opens event processing stage brackets.

See C<pop_event>, L<"Events">

=item remove_notification ID

Removes a notification subroutine that was registered before with
C<add_notification>, where ID was its result. After successful removal, the
eventual context object gets implicitly detached from the storage object.

See also: C<add_notification>, C<get_notification>.

=item set_notification NAME, SUB 

Adds SUB to the event NAME notification list. Almost never used directly, but
is a key point in enabling the following notification add syntax

   $obj-> onPostMessage( sub { ... });

or

   $obj-> set( onPostMessage => sub { ... });

that are shortcuts for

   $obj-> add_notification( "PostMessage", sub { ... });


=item unlink_notifier REFERER

Removes all notification subs from all event lists bound to REFERER object. 

=back

=head2 Prima::Component properties

=over 4

=item eventFlag STATE

Provides access to the last event processing state in the object event state
stack.

See also: L<"Success state">, C<clear_event>, L<"Events">.

=item delegations [ <REFERER>, NAME, <NAME>, < <REFERER>, NAME, ... > ]

Accepts an anonymous array in I<set-> context, which consists of a list of event NAMEs,
that a REFERER object ( the caller object by default ) is interested in.
Registers notification entries for routines if subs with naming scheme
REFERER_NAME are present on REFERER name space.  The example code

   $obj-> name("Obj");
   $obj-> delegations([ $owner, 'PostMessage']);

registers Obj_PostMessage callback if it is present in $owner namespace.

In I<get-> context returns an array reference that reflects the object's delegated
events list content.

See also: L<"Delegated methods">.

=item name NAME

Maintains object name. NAME can be an arbitrary string, however it is
recommended against usage of special characters and spaces in NAME, to
facilitate the indirect object access coding style:

   $obj-> name( "Obj");
   $obj-> owner( $owner);
   ...
   $owner-> Obj-> destroy;

and to prevent system-dependent issues. If the system provides capabilities
that allow to predefine some object parameters by its name ( or class), then it
is impossible to know beforehand the system naming restrictions.  For example,
in X window system the following resource string would make all Prima toolkit
buttons green:

  Prima*Button*backColor: green

In this case, using special characters such as C<:> or C<*> in the name of an object
would make the X resource unusable.

=item owner OBJECT

Selects an owner of the object, which may be any Prima::Component descendant.
Setting an owner to a object does not alter its reference count. Some classes
allow OBJECT to be undef, while some do not. All widget objects can not exist
without a valid owner; Prima::Application on the contrary can only exist with
owner set to undef. Prima::Image objects are indifferent to the value of the
owner property.

Changing owner dynamically is allowed, but it is a main source of
implementation bugs, since the whole hierarchy tree is needed to be recreated.
Although this effect is not visible in perl, the results are deeply
system-dependent, and the code that changes owner property should be thoroughly
tested.

Changes to C<owner> result in up to three notifications: C<ChangeOwner>, which
is called to the object itself, C<ChildLeave>, which notifies the previous
owner that the object is about to leave, and C<ChildEnter>, telling the new
owner about the new child.

=back

=head2 Prima::Component events

=over 4

=item ChangeOwner OLD_OWNER

Called at runtime when the object changes its owner. 

=item ChildEnter CHILD

Triggered when a child object is attached, either as
a new instance or as a result of runtime owner change.

=item ChildLeave CHILD

Triggered when a child object is detached, either because
it is getting destroyed or as a result of runtime owner change.

=item Create

The first event an event sees. Called automatically after init() is finished.
Is never called directly.

=item Destroy

The last event an event sees. Called automatically before done() is started.
Is never called directly.

=item PostMessage SCALAR1, SCALAR2

Called after post_message() call is issued, not inside post_message() but at
the next idle event loop.  SCALAR1 and SCALAR2 are the data passed to
post_message().

=back

=head1 AUTHOR

Dmitry Karasik, E<lt>dmitry@karasik.eu.orgE<gt>.

=head1 SEE ALSO

L<Prima>, L<Prima::internals>, L<Prima::EventHook>.
