Returning data after async task - swift

Question

I am uploading an image with using a library. This library is working async.
My function:
func upload() -> String {
let imageData:NSData = UIImageJPEGRepresentation(pureImage!, 100)!
var picture=""
SRWebClient.POST("http://domain.com/upload.php")
.data(imageData, fieldName:"image_field", data: ["username":"test","key":"test"])
.send({(response:AnyObject!, status:Int) -> Void in
if status == 200 {
let responseJSON = response! as! Dictionary<String, AnyObject>
let s_status=responseJSON["status"] as! Int
if s_status == 1 {
picture=responseJSON["picture"] as! String
print(picture)
}
}
},failure:{(error:NSError!) -> Void in
picture=""
})
return picture
}
As you can see, I have to return picture name. But now it is always returning empty string because upload process is async. How can I return the picture name after upload process?

Answer 1

Obviously you cannot return the picture name as function result, not unless you want to wait till the async task is done and waiting would make it a synchronous task again.
There are three very common ways to make async tasks deliver results:
Pass the task a callback (either a callback function or a completion block if you need to capture state or references). Once the task is done, it calls the callback. In your case, the callback could get the image name as argument and the callback code then needs to decide what to do with it.
If the task is encapsulated in an object, allow the object to have a delegate. Once the task is done, a delegate method is called. Either the method gets the image name as argument or can query the image name from the object it is delegate of (usually you'd pass the object itself as an argument to the delegate, that is common practice and good coding style according to Apple).
Send a notification that an image was uploaded. The image name can be the object of the notification; or some object that encapsulates the image name and possibly other properties. Whoever is interested to know when an upload task completed can register for that notification.
Some notes regarding the options above:
I'd use notifications with care. While they are easy to use and very useful if a lot of components spread across a huge project need to be informed about events, they are hard to debug (you cannot follow the code flow easily in a debugger) and they create a very lose coupling (which may or may not be desirable), yet a strong coupling to the notification itself. Also notifications cannot return a value in case that is every required.
A delegate is always a great option, but it forces users to create a class that implements the delegate protocol. This usually only pays off if you need more than just a single callback method or when you plan to call the delegate methods very frequently. Delegates are great for unit testing.
A callback is like a tiny delegate with just a single callback method. If you commonly make "fire and forget" tasks on the go and there is only a single callback required that will be called in case of success and in case of failure; and it will only be called once and there is no need to ever recycle it, then a callback is often preferable to a delegate. It has all the advantages of a delegate but it is more lightweight.

Answer 2

This is the sort of problem that Promises were designed for. You could implement callbacks but it quickly becomes unmanageable if you have more than a few of them to deal with.
Do yourself a big favor and import PromiseKit into you code. Take the half-hour to learn how to use it.
You will end up with something like
func upload() -> Promise<String>

Answer 3

you can use blocks to get a call back
func upload(completionHandler : (pictureName : NSString?)-> Void){
let imageData:NSData = UIImageJPEGRepresentation(pureImage!, 100)!
var picture=""
SRWebClient.POST("http://domain.com/upload.php")
.data(imageData, fieldName:"image_field", data: ["username":"test","key":"test"])
.send({(response:AnyObject!, status:Int) -> Void in
if status == 200 {
let responseJSON = response! as! Dictionary<String, AnyObject>
let s_status=responseJSON["status"] as! Int
if s_status == 1 {
picture=responseJSON["picture"] as! String
print(picture)
completionHandler(pictureName: picture)
}
}
},failure:{(error:NSError!) -> Void in
picture=""
completionHandler(pictureName: nil)
})
}