ios - NSNotification and control flow -

i want following:

1. display loader 2. download database update , reload data 3. hide loader 

my idea send notifications @ begining , @ end of database's update method main view controller, view controller displays , hides loader.

i worried control flow. have guarantee view controller receive notification , display loader before database's update method proceeds?

the notifications queued in system, there's no guarantee it's going instantly. anyway, quite fast, sure faster work on database.

if want 100% sure, set flag somewhere , when receive notification, check databases reloaded.

anyway, i'm thinking if notification received after reload of database, because notification queued, hide loader notification come right after first one, there nothing worry about.