How to sync 2 asynchronous fetch in Parse?

So I have a PFobject A that contains 2 other PFobjects BC as a value. When I create my local object a, I need both B C objects. So first I retrieve a by doing the query:

let query = PFQuery(className: "A")
query.getObjectInBackgroundWithId("1", block: { (a, error) -> Void in

      

        
        
        
      

    

Then I get b and c from a

var b = a!["B"] as! PFObject
var c = a!["C"] as! PFObject

      

        
        
        
      

    

Then I will need to get objects b and c individually

b.fetchInBackgroundWithBlock({ (fetchedB, error) -> Void in

      

        
        
        
      

    

The problem is that the fetch methods are asynchronous, and if I put them on the same thread, I cannot guarantee that both of them will be fetched at the end.

One solution is for the selection to be nested within the callbacks so that as soon as one selection is done, it will trigger another.

b.fetchInBackgroundWithBlock({ (fetchedB, error) -> Void in
    c.fetchInBackgroundWithBlock({ (fetchedC, error) -> Void in
        println(fetchedB)
        println(fetchedC) // Now they have values
        var myA = A(validB: fetchedB, validC: fetchedC) // Construction can continue

      

        
        
        
      

    

But if more objects are required, it will be nested in the nest. How:

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 and e are independent of each other - it must be perfect to retrieve them on separate threads!

So, is there a way to make the callbacks wait for each other at some point on the main thread to make sure all objects are fetched?

Thank!