Dependency for functions in Swift - ios

Question

How can I create dependency for functions in swift to run one after another? In other words, I want one function to wait until another one completes its task successfully.
var sales : NSMutableArray! = NSMutableArray()
var items : NSMutableArray! = NSMutableArray()
override func viewDidLoad() {
super.viewDidLoad()
This is where I need help, the below dispatch method is not working properly, I get the “index 0 beyond bounds for empty array” error since the array is not populated yet I guess. Also I am not sure that if this is the right method to handle this…
/*
dispatch_async(queue, {
dispatch_sync(queue, {
self.downloadItems()
})
dispatch_sync(dispatch_get_main_queue(), {
self.displayItems()
})
})
*/
}
func downloadItems() {
let urlPath = "……………………………"
let url = NSURL(string: urlPath)
let session = NSURLSession.sharedSession()
let task = session.dataTaskWithURL(url!, completionHandler: {data, response, error -> Void in
if (error != nil) {
println(error)
} else {
let jsonResult: AnyObject = NSJSONSerialization.JSONObjectWithData(data, options: NSJSONReadingOptions.MutableContainers, error: nil)!
dispatch_async(dispatch_get_main_queue()) {
for var i = 0; i < jsonResult.count; i++ {
self.sales[i] = jsonResult[i]["daily_sales"] as NSString
}
}
}
})
task.resume()
}
func displayItems() {
self.items.addObject(["amount": self.sales[0]])
}

Answer 1

Use 'continuation passing style'. Your operation downloadItems() will take an argument with a function to perform once downloadItems() completes - the argument is similar to a continuation (which some languages expose, see Scheme and ML for details). So it looks like:
func downloadItems (continuation: (Void -> Void)!) {
let urlPath = // ...
// ...
if nil != continuation { continuation() }
}
and then you'd use it with
self.downloadItems { [self unowned] in self.displayItems() }