When you say looping until I assume you really mean wait until.
First of if this waiting is to be done on the main thread just forget about it, never block the main thread.
Instead of a loop you probably want to use a lock, and wait for the condition. This requires a shared lock between the code where you wait for the array to populate, and the code where you populate the array.
First create a shared condition lock like this:
typedef enum {
MYConditionStateNoObjects,
MYConditionStateHaveObjects
} MYConditionState;
...
sharedArray = [[NSMutableArray alloc] init];
sharedLock = [[NSConditionLock alloc] initWithCondition:MYConditionStateNoObjects];
Your method that populates the array should then do:
[sharedLock lockWhenCondition:MYConditionStateNoObjects];
// Your stuff to get the objects to add here.
[sharedArray addObjectsFromArray:theObjectsToAdd];
[sharedLock unlockWithCondition:MYConditionStateHaveObjects];
And the receiver that should wait until the array has objects do this:
[sharedLock lockWhenCondition:MYConditionStateHaveObjects];
// ... Do something with the objects you got here
[sharedArray removeAllObjects];
[sharedLock unlockWithCondition:MYConditionStateNoObjects];
