Unable to access variable outside of query function even though variable is declared after class

Question

I have a few queries running to load variables with data from a Parse server into variables that are declared right below class. When I print the variable to the console within the query function, it prints correctly, but when I call it outside of the function or print it, it is empty. Any idea where I'm going wrong?

Variable declaration:

class AddTaskViewController: UIViewController, UITextFieldDelegate, UIPickerViewDataSource, UIPickerViewDelegate {
var varCost = ""

Query loading variable:

let varCostQuery = PFQuery(className: "Class1")
varCostQuery.findObjectsInBackground(block: { (objects, error) in
    if error != nil {
        print(error!)
    } else {
        varCostQuery.whereKey("Header", equalTo: self.HeaderSelection.text)
        varCostQuery.findObjectsInBackground(block: { (objects, error) in
            for object in objects! {
                self.varCost = object["pricePerUnit"] as! String
                print(self.varCost) //Option1
            }
        })
    }
})
print(varCost) //Option2

When I print Option1, I get the data exactly like I'm looking for, but when I print Option2 or try to do anything with the varCost variable at this level, I get "" like the variable has never been updated.

Answer 1

This occurs because the code being passed to the findObjectsInBackground method is code that is run only once all the objects have been found.

Because this code will only be called when the objects have been found, this may take a bit of time, so this code is send to a background queue to wait for the objects to be found.

We don't want the rest of our code to pause and wait for this though! That would slow our program down a lot. So the program continues past this block until it completes. That's why Option 2 is empty, because findObjectsInBackground hasn't had time to get the objects yet so the code has jumped straight to where Option 2 is.

When the objects have finally been found, the block of code is called and Option 1 is printed.

This means that you can only be sure self.varCost will have the right value from within this block (or closure, as it is called in Swift).