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Prima::Object - Prima toolkit base classes |
Prima::Object - Prima toolkit base classes
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 the Prima::internals manpage as well, the difference is that the Prima::internals manpage considers the coding tasks from a C programmer's view, whereas this document is wholly about perl programming.
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 profile ),
specific to every Prima class. The object creation is performed in several stages.
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 => $::application,
delegations => undef,
While the exact meaning of these parameters is described later, in Properties, the idea is that a newly created object will have 'owner' parameter set to '$::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.
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 left and width parameters, and
as well by auxiliary right, size and rect. The default of both
left and width is 100. But if only right parameter, for
example, was passed to create() it is profile_check_in() job to determine
left value, given that width is still 100.
After profiles gets merged, the resulting hash is passed to the third stage, init().
init() duty is to map the profile content into object, e.g., assign
name property to 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 Links between objects).
setup() method is a convenience function, it is used when
some post-init actions must be taken. It gets seldom overloaded,
primarily because the Component::setup() method calls onCreate
notification, which is more convenient to overload than setup().
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 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 events flow.
As onCreate event is the first event the object receives, only after
that stage other events can be circulated.
Object destruction can be caused by many conditions, but
all execution flow is passed through destroy() method.
destroy(), as well as create() performs several stages,
and has its own considerations.
destroy() is cleanup().
cleanup() is the pair to setup(), as destroy() is the pair
for create(). cleanup() generates onDestroy event,
which can be overridden more easily than cleanup() itself.
onDestroy is the last event the object sees. After cleanup()
no events are allowed to circulate.
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 onDestroy overloading can be more easily performed.
The typical conditions that lead to object destructions are
direct destroy() call, garbage collections mechanisms,
user-initiated window close ( on Prima::Window only ),
exception during init() stage. One must be careful implementing
done() if init() throws an exception therefore.
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.
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 would be 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.
For 'name' there is only one method name() ( or ::name later
in the documentation ).
The property returns a value if no parameters ( except the object)
are passed, or otherwise changes the internal data to the passed
parameters. As an example a theoretical code for ::name is exemplified:
sub name
{
return $_[0]-> {name} unless $#_;
$_[0]->{name} = $_[1];
}
Examples of properties are many throughout the toolkit. Not all properties deal only 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 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 calling order
is uncertain. Therefore, the usage of set() is recommended either
when the calling 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 from the Prima::internals manpage discovers
that Prima::Image functionality
$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 Prima::Image::set is aware of such
combinations and calls neither ::type nor ::palette but
performs another conversion instead.
Some properties are read-only and some are write-only. Some methods that could be declared as properties are not, and instead being declared as read-only or write-only properties, they are still declared as plain methods with get_ or set_ name prefix. There is not much certainty about what methods should be 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.
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 ::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 many ways under the toolkit. For example, widgets that are children of another widget appear ( usually, but far 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. An important feature is that when an object gets destroyed, its children get destroyed first. In a typical program the whole object tree has a root in Prima::Application object. When the application finishes, this feature helps 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.
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
creates or before it gets destroyed - these two events, along with
onPostMessage ( which is described later ) are intrinsic to all
Prima toolkit objects. New classes can register their own events
and define their execution flow, using notification_types() method.
This method is as well a good source for searching information about
events that a class generates. The events description for the built-in
classes is preserved in Classes.pm.
The propagation mechanism has three layers of user-defined callback registration, that are called in different order and context 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.
on_postmessage is present,
it will be called as a method ( i.e., in the object context )
when 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 ::SUPER call is needed, because these methods are not defined by default.
Usually the direct methods are used for the internal object realization,
for the events that are not designed or not interested for the higher level
usage.
For example, a Prima::Button class catches mouse and keyboard events in such
a fashion, because the only notification that is interesting for the
code that employs push-buttons is Click.
This scheme is convenient when an event handling routine
serves the internal, implementation-specific needs.
$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. ( That is one of the reasons why Component::profile_check_in() bothers if about auto naming ). Note also that context objects are $self ( that equals $owner ) and $obj.
The delegated methods can be used not for owner-child relations only. 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, a good practice is to add Destroy notification to the object which events are of interest, so if it quits, the counterpart will get notified.
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 accepts multiple event handlers that coexist in one object ( it is greatly facilitated by the fact that perl has 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. The example of setting such a event hook to a $obj is as follows and has three forms:
- 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 Prima, the events can be addressed as properties, with the exception that they are not substitutive but additive. The additivity is that when such 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.
So, the distinctive feature of a toolkit is that two objects of same class may have different set of event handlers.
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.
nt::PrivateFirst and nt::CustomFirst constants defines the order.
nt::FluxNormal and nt::FluxReverse used. 'Normal' is set for
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 21, not 12 because PostMessage event
type is prototyped nt::FluxReverse.
nt::Single bit in execution control constant set,
which is
nt::Single nt::Multiple nt::Event
These constants are exclusive and can not appear together in one event type.
A 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, but it is used nevertheless.
In contrary of nt::Single, the nt::Multiple constant
sets the execution control to call all the callbacks present,
with respect to direction and order bits.
The third constant, nt::Event, has the same impact as
nt::Multiple, except that the processing can be stopped
at any time by calling clear_event() method.
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 )
Events do not return values, although the event-generating
notify() 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 just reflect the fact if 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 the 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 nt::Event-prototyped
event call does not automatically stops the execution. The execution
stops if the state value equals to 0 after a callback is finished.
A ::eventFlag(1) call thus cancels the clear_event() effect.
A particular coding style is used when the event is 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 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 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;
init()
stage. Example:
print $obj-> name if Prima::Object::alive( $obj);
destroy() started. Used to initiate cmDestroy
event. Never called directly.
$obj = Class-> create( PARAMETERS); $obj = Prima::Object::create( "class" , PARAMETERS);
Never called in an object context.
cleanup()
and done() calls. destroy() can be called several times
and is the only Prima re-entrant function, therefore may not
be overloaded.
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. Never called directly.
Note: the eventual child objects are destroyed inside done()
call.
profile_default() class method inside profile_check_in()
method. init() is responsible for applying the relevant data
into PARAMETERS to the object properties. Never called directly.
$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.
profile_default() after passing both
to profile_check_in(). The merge result is stored back in PROFILE.
Never called directly.
profile_default() method. Never called directly.
Can be called in a context of class.
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.
init() finishes. Used to initiate cmCreate event.
Never called directly.
Methods
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.
Returns positive integer value on success, 0 on failure. This value is used to refer to the SUB in remove_notification().
See also: remove_notification, get_notification.
See also: detach.
$obj-> name( "Obj"); $obj-> owner( $owner); ... $owner-> Obj-> destroy;
alive() == 1, except that can_event()
fails if an invalid object reference is passed.
See also: Events, push_event, pop_event, ::eventFlag, notify.
See also: attach
See: create, Links between objects.
Uses to pass the handle value outside the program, for an eventual interprocess communication scheme.
add_notification, and ID is its result.
See also: remove_notification, add_notification.
See Success state for example.
nt:: constants that describe the event flow.
Can be called in a context of class.
nt:: constants, contained
in the notification_types() result hash.
notify() accepts variable number of parameters, however
every event handler excepts a particular number. While
it is possible, although 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 push_event, Events
PostMessage event with parameters SCALAR1 and SCALAR2
once during idle event loop. Returns immediately.
Does not guarantee that PostMessage will be called, however.
add_notification, where ID was its result. After
successful removal, the eventual context object gets implicitly
detached from the storage object.
See also: add_notification, get_notification.
$obj-> onPostMessage( sub { ... });
or
$obj-> set( onPostMessage => sub { ... });
that are shortcuts for
$obj-> add_notification( "PostMessage", sub { ... });
Properties
See also: Success state, clear_event, Events.
$obj-> name("Obj");
$obj-> delegations([ $owner, 'PostMessage']);
registers Obj_PostMessage callback if it is present in $owner namespace.
On get returns an array reference that reflects the object's delegated events list content.
See also: Delegated methods.
$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
If, for example, a class would contain ':' in its name the result would be different.
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 system-dependent, and the code that changes owner property should be tested especially thorough.
Events
init() is finished. Never called directly.
done() is started. Never called directly.
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().
Dmitry Karasik, <dmitry@karasik.eu.org>.
Prima, the Prima::internals manpage
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Prima::Object - Prima toolkit base classes |