App that quickly activates/deactivates lock on certain apps?

Anything like that exists?

So when someone asks to see my phone, I can just quickly lock things they shouldn't be going through, like my messages. But when it's not locked constantly so that I don't have to enter a password everytime I open that app?

I have an app that does that (LockMe Widget), but it doesn't lock certain apps, it just locks the whole phone. =/