Defining behaviour of 'configure' for composite widgets.

## NAME

ConfigSpecs - Defining behaviour of 'configure' for composite widgets.

## SYNOPSIS

      sub Populate
      {
       my ($composite,$args) = @_;
       ....
       $composite->ConfigSpecs(-attribute => [ where, dbName, dbClass, default ]);
       $composite->ConfigSpecs(-alias     => '-otherattribute');
       $composite->ConfigSpecs(  DEFAULT  => [ where ]);
       ....
      }

      $composite->configure( -attribute => value );

## DESCRIPTION

The aim is to make the composite widget configure method look as much like a regular Tk widget's configure as possible. (See the configure manual entry for a description of this behaviour.) To enable this the attributes that the composite as a whole accepts needs to be defined.

Defining the ConfigSpecs for a class.
  Typically a widget will have one or more calls like the following

$composite->ConfigSpecs(-attribute => [ where, dbName, dbClass, default ]);
in its Populate method. When ConfigSpecs is called this way (with arguments) the arguments are used to construct or augment/replace a hash table for the widget. (More than one key => value pair can be specified to a single call.)

dbName, dbClass and default are only used by ConfigDefault described below, or to respond to 'inquiry' configure commands.

It may be either one of the values below, or a list of such values enclosed in [ ].

The currently permitted values of where are:

'DESCENDANTS'
  apply 'configure' recursively to all descendants
'ADVERTISED'
  apply 'configure' to 'advertised' subwidgets.
'SELF'
  apply 'configure' to the core widget (e.g. Frame) that is the basis of the composite. (This is the default behaviour for most attributes which makes a simple Frame behave the way you would expect.) Note that once you have specified ConfigSpecs for an attribute you must explicitly include SELF in the list if you want the attribute to apply to the composite itself (this avoids nasty infinite recursion problems).
'CHILDREN'
  apply 'configure' to all children.
'PASSIVE'
  simply store value in $composite->{Configure}{-attribute}.

  This form is also a useful placeholder for attributes which you currently only handle at create time. IP "'CALLBACK'" 4 Setting the attribute does Tk::Callback->new\|($value) before storing in $composite->{Configure}{-attribute}. This is appropriate for -command => ... attributes that are handled by the composite and not forwarded to a subwidget. (e.g. Tk::Tiler has -yscrollcommand to allow it to have scrollbar attached.)

  This may be the first of several 'validating' keywords (e.g. font, cursor, anchor etc.) that core Tk makes special for C code.

'METHOD'
  Call $cw->attribute\|(value)

  This is the most general case. Simply have a method of the composite class with the same name as the attribute. The method may do any validation and have whatever side-effects you like. (It is probably worth 'queueing' using afterIdle for more complex side-effects.)

$reference
  Call $reference->configure(-attribute => value)

  A common case is where $reference is a subwidget.

  $reference may also be result of

  Tk::Config->new(setmethod,getmethod,args,...)
  Tk::Config class is used to implement all the above keyword types. The class has 'configure' and 'cget' methods so allows higher level code to always just call one of those methods on an 'object' of some kind.

'otherstring'
  Call $cw->subwidget\|('otherstring')->configure(-attribute => value)

  While this is here for backward compatibility with Tk-b5, it is probably better just to use the subwidget reference directly. The only case for retaining this form is to allow an additional layer of abstraction - perhaps having a 'current' subwidget - this is unproven.

Aliases
  ConfigSpecs(-alias => '-otherattribute') is used to make -alias equivalent to '-otherattribute'. For example the aliases -fg => -'-foreground', -bg => -background are provided automatically (if not already specified).

Default Values
  When the Populate method returns ConfigDefault is called. This calls

$composite->ConfigSpecs;
(with no arguments) to return a reference to a hash. Entries in the hash take the form:

'-attribute' => [ where, dbName, dbClass, default ]
ConfigDefault ignores 'where' completely (and also the DEFAULT entry) and checks the 'options' database on the widget's behalf, and if an entry is present matching dbName/dbClass

-attribute => value
is added to the list of options that new will eventually apply to the widget. Likewise if there is not a match and default is defined this default value will be added.

Alias entries in the hash are used to convert user-specified values for the alias into values for the real attribute.

new-time configure
  Once control returns to new, the list of user-supplied options augmented by those from ConfigDefault are applied to the widget using the configure method below.

Widgets are most flexible and most Tk-like if they handle the majority of their attributes this way.

Configuring composites
  Once the above have occurred calls of the form:

$composite->configure( -attribute => value );
should behave like any other widget as far as end-user code is concerned. configure will be handled by Tk::Derived::configure as follows:

$composite->ConfigSpecs;
is called (with no arguments) to return a reference to a hash -attribute is looked up in this hash, if -attribute is not present in the hash then 'DEFAULT' is looked for instead. (Aliases are tried as well and cause re-direction to the aliased attribute.) The result should be a reference to a list like :

[ where, dbName, dbClass, default ]
at this stage only 'where' is of interest, it maps to a list of object references (maybe only one) foreach one

$object->configure( -attribute => value );
is evaled.

Inquiring attributes of composites

$composite->cget( '-attribute' );
This is handled by Tk::Derived::cget in a similar manner to configure. At present if where is a list of more than one object it is ignored completely and the "cached" value in $composite->{Configure}{-attribute}.

is returned.

## CAVEATS

It is the author's intention to port as many of the "Tix" composite widgets as make sense. The mechanism described above may have to evolve in order to make this possible, although now aliases are handled I think the above is sufficient.

## SEE ALSO

configure