In the thread "Where to process PageParameters" I was requested to
explain why I think you should not override isVisible, but rather should
call setVisible in onBeforeRender (slide 100 in my presentation
http://www.grons.nl/~erik/pub/20081112%20Effective%20Wicket.pdf).
There are 2 reasons, but only the second one is really important.
-1- isVisible is called a lot. It is easily called ten times within 1
request
So if you have anything processor intensive going on, it will be a
performance hit. Just doing a simple expression is of course no problem.
(For fun, just set a breakpoint in something like
FeedbackPanel#isVisible and request a page that contains one.)
-2- isVisible can make your model be reloaded multiple times within 1
request
Consider the following case (pseudo code):
MyPanel(id, personId) {
super(id, new CompoundPropertyModel(new
LoadableDetachablePersonModel(personId)));
add( new Label("address") {
@Override
isVisible() {
return getDefaultModel() != null;
}
});
}
The label uses the property 'address' of a person to see if the label
should be visible. The person is retrieved by a LoadableDetachableModel
subclass (LDM) and then wrapped by a CompoundPropertyModel (CPM).
During the render phase, isVisible will delegate getting the address
property to the CPM and this model delegates it to the LDM. LDM will
load the person from the database only once (well until it is detached).
At the end of the render phase everything will be detached. But now
something weird happens. The problem is that isVisible is called during
the detach phase, on the label /after/ the CPM (and therefore also the
LDM) are detached. As isVisible retrieves the model from the CPM, and
therefore from the LDM, it will trigger a reload of the person inside
the LDM.
Now, as visibility is often (if not almost always) determined by a
business object (e.g. very often a LDM) I think it makes sense to avoid
having to think about the situation described above, and just avoid it
all together.
Note: I observed this behavior in Wicket 1.3 (1.3.3 I think). If it
works differently now, I would be very glad to withdraw this recommendation.
Regards,
Erik.
--
Erik van Oosten
http://day-to-day-stuff.blogspot.com/
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