Swift: feedback when function ends - ios

Question

Let's say I have a ViewController A and a class B.
when I press some button inside A, it calls an IBAction that calls a function B.foo() which returns an Int
B.foo() takes 8~10 seconds to finish and while it runs I'd like to put an Loading... animation on A, and when B.foo() finishes, the animation would stop.
How can I do this? this is an pseudo-code example of what I wish:
#IBAction func buttonPressed(_ sender: UIButton){
UIView.animate(blablabla......)
DO({
self.answer = B.foo()
}, andWhenItFinishesDo: {
self.someone.layer.removeAllAnimation()
})
}

Answer 1

This is a very common problem. One way to solve it would be to use different queues (You can think of them as lines of work that can happen in parallel).
The the basic idea is that once your button is pressed, you show your loading indicator and "dispatch" the long work to a secondary queue, that will operate in the background and do the work. This ensures that your main queue does not block while the work happens and the user interface stays responsive.
The trick is now that you want to get notified when the long work is finished so that you can stop showing the loading indicator (and possibly do even more).
While you actually could use some kind of notification system, there are other, sometimes more appropriate ways. It would actually be even more convenient, if you could just tell the long running function to call you back specifically with code that you provide.
That would be the basic concept of a "completion handler" or "callback".
The whole thing would look something like that:
// Some made up class here
class B {
// This is only static because I do not have an instance of B around.
static func foo(completion: #escaping (Int) -> Void ) {
// The method now does all of its work on a background queue and returns immediately
DispatchQueue.global(qos: .background).async {
// In the background this may take as long as it wants
let result = longCalculation()
// VERY important. The caller of this function might have a certain
// expectation about on which queue the completion handler runs.
// Here I just use the main queue because this is relatively safe.
// You could let the caller provide a queue in the function
// parameters and use it here
DispatchQueue.main.async {
// The completion handler is a function that takes an Int.
// That is exactly what you are providing here
completion(result)
}
}
}
}
#IBAction func buttonPressed(_ sender: UIButton){
self.showLoadingIndicator()
// The foo function now takes a completion handler that gets the result in.
// You have to provide this function here and do something with the result
//
// The completion handler will only be run when the foo function calls it
// (which is after the computation as you can see in the method above.
//
// I am also telling the completion handler here that self should not be
// held on to as the view controller might already have gone away when the
// long calculation finished. The `[weak self]` thingy makes that inside
// your completion handler self is an optional and might be nil (and it
// doesn't hold a strong reference to self, but that's a whole other topic)
B.foo(completion: { [weak self] result in
// Do something with the result
// Since we are called back on the main queue we can also do UI stuff safely
self?.hideLoadingIndicator()
})
}
I hope this helps a bit.
Asynchronous programming can be quite difficult to learn but there are tons of tutorials and examples you can find on this topic.

Answer 2

Hey Hamish you can do this in two simple ways,
First one is using the defer statements provided for functions.
Defer statement block is executed after the functions goes out of scope.
here is a simple example to describe the same.
func print1000000() {
//start displaying the loading indicator
defer {
// hide the loading indicator and move to the next ViewController
let seVC = storyboard?.instantiateViewController(withIdentifier: "SecondVC") as! SecondVC
self.navigationController?.pushViewController(seVC, animated: true)
}
// here goes the task you want to execute such as downloading a file or the one i did here
for index in 0...1000000 {
print(index)
}
}
The above function prints numbers upto 1000000 and then pushes the control to another ViewController
=========================================================================
Second way of doing it is by using closures, as described by Thomas in his answer.