ios - How to sync 2 async fetches in Parse? -

so have pfobject contains 2 other pfobjects b c value. when construct local object a, need both of objects b c. first fetch doing query:

let query = pfquery(classname: "a") query.getobjectinbackgroundwithid("1", block: { (a, error) -> void in 

then b , c a

var b = a!["b"] as! pfobject var c = a!["c"] as! pfobject 

then need fetch b , c objects individually

b.fetchinbackgroundwithblock({ (fetchedb, error) -> void in 

the problem is, fetching methods async , if put them in same thread won't guaranteed have both of them fetched in end.

one solution have fetching nested in call-backs once 1 fetch done start other.

b.fetchinbackgroundwithblock({ (fetchedb, error) -> void in     c.fetchinbackgroundwithblock({ (fetchedc, error) -> void in         println(fetchedb)         println(fetchedc) // have values         var mya = a(validb: fetchedb, validc: fetchedc) // construction can continue 

but if more objects needed, become nesting in nesting. like:

b.fetchinbackgroundwithblock({ (fetchedb, error) -> void in     c.fetchinbackgroundwithblock({ (fetchedc, error) -> void in         d.fetchinbackgroundwithblock({ (fetchedd, error) -> void in             e.fetchinbackgroundwithblock({ (fetchede, error) -> void in 

however b,c,d , e not dependent on each other - should perfect fetch them on separate threads!

so there way make call-backs wait each other @ point in main thread make sure objects fetched?

thanks!

use includekey() method on first pfquery tell parse fetch both class b , c when performing query. relevant documentation includekey