



Annotate this POD


View/Report Bugs
Module Version: 19   Source  


Glib::Ex::ConnectProperties::Element -- special property handling


 package Glib::Ex::ConnectProperties::Element::some_thing;
 use base 'Glib::Ex::ConnectProperties::Element';


This is the base class for special properties in Glib::Ex::ConnectProperties. A subclass for a property such as


should be a class


An Element object is a hashref created from Glib::Ex::ConnectProperties->new() etc with property name and options.

    { object  => $obj,       # Glib::Object
      pname   => $string,    # property name "foo"
      ids     => $signalids, # signal connections
      ... further options

A subclass should as a minimum implement the following methods, described below.

    find_property() or pspec_hash()
    get_value()            # if readable
    set_value()            # if writable

As an example, the simplest subclass is Glib::Ex::ConnectProperties::Element::object which is the code for plain object get_property() etc. It can be seen how find_property() looks at the object property pspecs and read_signal() is the notify, then simply get_property() or set_property() to manipulate the value.

Glib::Ex::ConnectProperties::Element::textbuffer is an example of read-only access to non-property characteristics of a text buffer. It has a fixed set of property names in pspec_hash(), and in fact shares the boolean for "empty" and "not-empty" because the ParamSpec name field doesn't matter, only the type etc.

Glib::Ex::ConnectProperties::Element::screen_size is another read-only, with some runtime adaption to notice when Gtk2 is new enough to have the monitors-changed signal for screen size in millimetres changing.


Mandatory Methods

$pspec = $elem->find_property()
$hashref = $elem->pspec_hash()

find_property() should return a Glib::ParamSpec object for the property name $elem->{'pname'}, or return undef if no such property.

The default find_property() implementation calls pspec_hash(). That method should return a hashref name to ParamSpec

   { pname => $pspec,

in which to lookup $elem->{'pname'}. pspec_hash() suits subclasses with a fixed set of property names. Subclasses where it varies should implement some sort of find_property().

In each ParamSpec the following fields should be set

    flags       "readable" and/or "writable"
    type        for object, boxed, enum, flags

and for writable properties

    min,max     on int, float

The type of the ParamSpec, such as Glib::Param::Int etc, is used to know what sort of value comparison and validation should be done in the ConnectProperties propagation. For example Glib::Param::Int properties are compared using == whereas strings are compared with eq.

@signal_names = $elem->read_signal()

Return the name of a signal on $self->{'object'} to use to listen for changes to the property value.

The signal name may vary with the property name $self->{'pname'}, as for example the plain object properties using notify,

    sub read_signal {
      my ($self) = @_;
      return 'notify::'.$self->{'pname'};

For some element subclasses there might be a single signal for the changes in a set of related properties,

    use constant read_signal => 'foo-changed';

read_signal() is only used for readable properties. The read_signal option to ConnectProperties can override it.

The return from read_signal() is actually a list of signal names. If a class doesn't need any signal at all on $self->{'object'} then return an empty list.

    use constant read_signal => ();
$value = $elem->get_value()

Get or set the value of the $elem->{'pname'} property on $elem->{'object'}.

This is usually a method call on the object, some attribute it doesn't offer as a real property or which is more convenient when transformed in some way.

An element subclass will generally look up or derive a method name from $elem->{'pname'}.

    my %get_method = (width  => 'get_width',
                      height => 'get_height');
    sub get_value {
      my ($self) = @_;
      my $method = $method{$self->{'pname'}};
      return $self->{'object'}->$method;

Optional Methods


Check that $elem->{'pname'} is a valid property for the target $elem->{'object'}. Croak if it's not.

This is called during element initialization. The base implementation does $elem->find_property() and if that's undef then croaks with "no such property". An element subclass might make additional checks, or might give a better explanation if perhaps a property is sometimes available and sometimes not.

It's suggested that an element class should not check the class of the target $elem->{'object'}, but only perhaps for availability of required methods, signals, etc. That allows a similar object with compatible features to be used with the element class.



Glib::Object, Glib::ParamSpec



Copyright 2010, 2011, 2012 Kevin Ryde




syntax highlighting: