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configuration is parsed, simply add the listener before calling C<-E<gt>parse_config()>.
Otherwise, add listeners later.

 # an example with an unnamed block
 $conf->on_change('myUnnamedBlock', 'myKey', sub {
     my ($event, $old, $new) = @_;
     ...
 });

 # an example with a name block.
 $conf->on_change(['myNamedBlockType', 'myBlockName'], 'someKey', sub {
     my ($event, $old, $new) = @_;
     ...
 });

 # an example with an unnamed block and ->register_event() options.
 $conf->on_change('myUnnamedBlock', 'myKey', sub {
     my ($event, $old, $new) = @_;
     ...
 }, priority => 100, name => 'myCallback');

B<Parameters>

=over 4

=item *

B<block>: for unnamed blocks, should be the string block type. for named blocks, should be
an array reference in the form of C<[block type, block name]>.

=item *

B<key>: the key of the configuration value being listened for.

=item *

B<code>: a code reference to be called when the value is changed.

=item *

B<opts>: I<optional>, a hash of any other options to be passed to Evented::Object's
C<-E<gt>register_event()>.

=back

=head1 EVENTS

Evented::Configuration fires events when configuration values are changed.

In any case, events are fired with arguments C<(old value, new value)>.

Say you have an unnamed block of type C<myBlock>. If you changed the key C<myKey> in
C<myBlock>, Evented::Configuration would fire the event
C<change:myBlock:myKey>.

Now assume you have a named block of type C<myBlock> with name C<myName>. If you changed
the key C<myKey> in C<myBlock:myName>, Evented::Configuration would fire event
C<change:myBlock/myName:myKey>.

However, it is recommended that you use the C<-E<gt>on_change()> method rather than
directly attaching event callbacks. This will insure compatibility for later versions that
could possibly change the way events are fired.

=head1 SEE ALSO

=over 4

=item *

L<Evented::Object> - the event class that powers Evented::Configuration.

=back

=head1 AUTHOR

L<Mitchell Cooper|https://github.com/cooper> <cooper@cpan.org>

Copyright E<copy> 2011-2020. Released under New BSD license.

Comments, complaints, and recommendations are accepted. Bugs may be reported on
L<GitHub|https://github.com/cooper/evented-object/issues>.