In function call, why parse statement get executed atlast in SWIFT? - function

Question

I am begining to SWIFT. I have tried following code to check how function call is working. Function call is working fine, as our wish. But, in PARSE, the order is not working in the parse statement. Parse statement get executed atlas when all function gets over. How to solve this.
if I run this code, I am getting output like,
MY OUTPUT:
START
FIRST CLASS TOP
FIRST CLASS BOTTOM
SECOND CLASS
THIRD CLASS
END
WELCOME TO PARSE // WHY THIS LINE IS PRINTING LAST??
But, I need output like,
REQUIRED OUTPUT:
START
FIRST CLASS TOP
WELCOME TO PARSE //I NEED THIS LINE TO BE EXECUTE IN ORDER.
FIRST CLASS BOTTOM
SECOND CLASS
THIRD CLASS
END
MY CODING IS BELOW. Kindly check and guide me.
class ViewController: UIViewController {
let one_1 = class_1()
let second_2 = class_2()
let third_3 = class_3()
override func viewDidLoad() {
super.viewDidLoad()
println("START")
one_1.one()
second_2.second()
third_3.third()
println("END")
// Do any additional setup after loading the view, typically from a nib.
}
override func didReceiveMemoryWarning() {
super.didReceiveMemoryWarning()
// Dispose of any resources that can be recreated.
}
}
//CLASS_1
class class_1 {
var par_query = PFQuery(className: "story")
func one() {
println("FIRST CLASS TOP")
par_query.findObjectsInBackgroundWithBlock({(NSArray objects, NSError error) in
if (error != nil) {
NSLog("error " + error.localizedDescription)
}
else {
println("WELCOME TO PARSE")
}//ELSE ENDING
}) //PARSE ENDING
println("FIRST CLASS BOTTOM")
}
}
//CLASS_2
class class_2 {
func second() {
println("SECOND CLASS")
}
}
//CLASS_3
class class_3 {
func third() {
println("THIRD CLASS")
}
}

Answer 1

It has nothing to do with parse.com in particular, it behaves that way because findObjectsInBackgroundWithBlock is executed asynchronously.
You can read about that here.
UPDATED:
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0)) {
//Do some time comsuming calculation
//And when it's done
dispatch_async(dispatch_get_main_queue()) {
//Update the UI with your results
}
}
UPDATED 2
Let me put it that way: if you print anything, like "WELCOME TO PARSE" in an asynch closure, you cannot determine when it will be executed. I will mark with an * that where can be the "WELCOME TO PARSE" message with your current code:
START
FIRST CLASS TOP
FIRST CLASS BOTTOM
*
SECOND CLASS
*
THIRD CLASS
*
END
*
If you want to print the exact lines as you want, you could do two things:
Do not put a println in the asynch block
Put "FIRST CLASS BOTTOM" in your asynch block, and put also
class_2().second()
class_3().third()
in the asych block, so that will be called after the block is executed. However, I do not recommend this, it's just for the example.