Is Bircat Shehechiyanu a bircat shevach (a blessing of praise)? If it is, why sometimes do we make the beracha before the event (or in some cases an action representing an event, such as putting up a mezuza), unlike all other berachot shevach, which we make afterwards?
Shehechiyuanu is a brikat hashevach.
Now I have to explain to you how every instance of shehechiyanu is really recited after the occasion.
Edit: I found this article which you might find useful.