Is it possible to forbidden the viewpoint activation ?

If certain conditions are not fulfilled, i wish, for example, that the viewpoint activation open a popup displaying a error message and the viewpoint will be not activated.
Is it possible ? Is there an API to implement for this kind of behavior ?

Yes. You can extend org.polarsys.capella.common.ef.domain.AbstractEditingDomainR esourceSetListenerImpl, as an example look at org.polarsys.capella.core.sirius.ui.listener.FileModificatio nPreCommitListener. In transacttionAboutToCommit you must check if the transaction events include activation of your vp, and if so, check your condition and throw a rollback exception to abort activation. You must also register your listener:

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