Tracking changes when updating models

[Tom Götz <to...@decoded.de>]

Hi there,

I need to track all modifications made to FormComponents' target objects in order to generate some kind of changelog when saving my entity. Therefore I created a custom Model that wraps the components' default model and remembers the original value when setObject is called.

This works fine but breaks for FormComponents that use FormComponent#updateCollectionModel to update their model object (CheckGroup, ListMultipleChoice, MultiFileUploadField) because of the implementation of updateCollectionModel, which manipulates the collection in place and calls setObject with the already updated collection (see here: http://bit.ly/1nPoniW <http://bit.ly/1nPoniW>).

I tried to find another way to react upon model object changes and found Page#componentModelChanging (http://bit.ly/ZZdiRr), which is called from within Component#modelChanging. Unfortunately this method is final void, and thus can’t be overridden (does this make sense? Nothing can’t be done with the „component" parameter …).

Any pointer where I should look to get this done with the existing Wicket code (I use Wicket 6.17.0)?

I could think of several solutions for supporting that usecase:

* FormComponent#updateCollectionModel: don’t update the collection in place but hand the unmodified collection to setObject
* remove the „final“ from Page#componentModelChanging and add at least a „protected“
* my favorite: introduce a new listener for component model changes, so that it’s easy to hook into that process, analogue to IComponentOnBeforeRenderListener and friends (e.g. IModelUpdateListener, called before and after model updates ...)

What do you think?

Cheers,
   -Tom