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Introduction to Revit 2011 API
Dynamic Model Update
Overview
§“Ability for a Revit API application to modify the Revit model as a reaction to changes happening in the model”.
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§Helps track element addition, modification and deletion
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Dynamic model update offers the ability for a Revit API application to modify the Revit model as a reaction to changes happening in the model. Application.DocumentChanged event is read-only event and thus it does not support modification to Revit database. For that, we can use the DMU mechanism.

With this mechanism, Revit will now let you know when elements are added, modified or deleted.

One important thing to note is that this facility allows the application to perform its changes to react to the notification within the same transaction as the one that triggered the changes. Thus this enables you to edit the model during the model regeneration cycle triggered by the element change. For example – if the user stretches a concrete beam and our application is based on rebars, we can modify the rebar within the beam right after the users are done with resizing the beam. And those changes to the model will be contained in that overall transaction, and so if the user does an undo, our rebar sizing and the beam resizing will all be done at once in one shot (instead of having a clunky two stage resizing). This has lot of implication since we are now able to react to events in model and we can update our analysis applications,  or make changes in the model in reaction to users interaction to the model elements.