Does this trick have to be announced at the point the GM says, "The troll is attacking you."?
Or does the GM roll, and then you can use the shield trick to beat his roll?
Shield Focus
Source Pathfinder Unchained pg. 131
Related Feat Shield Focus
When an attack is made against you while you are using a shield, you can spend up to 2 stamina points. If you do, the shield’s bonus to AC against that attack increases by an amount equal to the number of stamina points you spent.
"It says- When an attack "is made against you."
"Not an attack "is going to be made against you."
I can see it either way, but looking to clarify, thanks.
The decision is made before you know the results of the roll. so it can be either before or after the roll, just before the GM lets you know if the Troll hits or not.
It says an when attack is made, not after the attack. In game terms the roll is the attack. Determining if an attack hits is not the attack. Logically you cannot determine if an attack hit if that attack has not occurred.
Players often know what the roll needed to hit them is. In some cases the players figure it out before the GM. Even if the GM does not give you the targets AC it is often fairly easy to figure it out. Player often know the stat blocks of creatures as well if not better than the GM. Even if they don’t it is not that difficult to figure out. If I previously hit on a 12 I know that anything higher than a 12 is going to hit. If the creature does something to change its AC the players can often still figure it out.
If you're playing on a VTT you will probably need to be looser with this interpretation. It is very possible for an entire attack to be rolled and resolved before the player could have time to respond to what they were going to do about it.