Minor Description
The current implementation of wicket:child and wicket:extend only allows for a single extension per subpage. However, this restriction is neither mandated by java class hierarchy nor by any other reason. Therefore, it should be possible to extend the current implementation to support multiple 'abstract' sections, just like abstract methods in java classes. This could be done by replacing
<wicket:child>
<wicket:extend>
some content
</wicket:extend>
</wicket:child>
with
<wicket:abstract id="foo">
<wicket:implement id="foo">
some content
</wicket:extend>
</wicket:child>
(new names have been suggested in http://www.nabble.com/Multiple-%3Cwicket%3Achild--%3E-tags-on-a-single-base-page--tf4738673.html)
A possible application is a layout with two columns, e.g. a header with navigation, a left column with sub-navigation and a right column with content (where the sub-navigation may change depending on the section. In deed, this is already possible using panels or similar means. However, it would allow to take advantage of markup inheritance only:
BasePage extens WebPage:
<div wicket:id="links>[some nav links here]</div>
<div><wicket:abstract id="subNavigation">[left navigation goes here]</wicket:abstract></div>
<div><wicket:abstract id="content">[content goes here]</wicket:abstract</div>
SectionPage extends BasePage:
<wicket:implement id="subNavigation">[sub navigation links here]</wicket:implement>
FooPage extends SectionPage:
<wicket:implement id="content">[content goes here]</wicket:implement>