How to prevent Timing Problems When Using Closures? - ios

Question

I'm trying to find the sum of numbers, returned from different escaping closures. The sum to be returned in main thread.
import Foundation
var randomTime: Int {
return Int.random(in: 0...1000)
}
func first(completion: #escaping (Int) -> Void) {
DispatchQueue.main.asyncAfter(deadline: .now() + .milliseconds(randomTime)) {
completion(1)
}
}
func second(completion: #escaping (Int) -> Void) {
DispatchQueue.main.asyncAfter(deadline: .now() + .milliseconds(randomTime)) {
completion(2)
}
}
func third(completion: #escaping (Int) -> Void) {
DispatchQueue(label: "anotherThread").async {
completion(3)
}
}
func fourth(completion: #escaping (Int) -> Void) {
DispatchQueue.main.asyncAfter(deadline: .now() + .milliseconds(randomTime)) {
completion(4)
}
}

Answer 1

If I got your question clear, You want to sum numbers but their values come at different times depending on a server response or some kind of delays. If that is the case then you have to use DispatchGroup
Here is a helper function, It is calling your methods first(completion: #escaping (Int) -> Void)..... fourth(completion: #escaping (Int) -> Void) and notify main queue only when the last value is received. I put some comments on the code to help understand. Let me know if something is not clear.
func computeOutPutAfterReceivingAllValues(completion: #escaping(_ sum: Int) -> Void) {
// Make a dispatch group which will notify main queue after making sure that all requests have been proceed.
let computeGroup = DispatchGroup()
var allNumbers: [Int] = []
computeGroup.enter()
first { (firstNumber) in
allNumbers.append(firstNumber)
self.second(completion: { (secondNumber) in
allNumbers.append(secondNumber)
self.third(completion: { (thirdNumber) in
allNumbers.append(thirdNumber)
self.fourth(completion: { (fourthNumber) in
allNumbers.append(fourthNumber)
// IMPORTANT: Leave a group after the last call.
computeGroup.leave()
})
})
})
}
// Notify Main queue and sum all your numbers
computeGroup.notify(queue: .main) {
/// Sum all your numbers in main queue
let sum = allNumbers.reduce(0, +)
completion(sum)
}
}
Usage:
You can test this in view didLoad.
override func viewDidLoad() {
super.viewDidLoad()
computeOutPutAfterReceivingAllValues { (sum) in
print("Here is the sum of all numbers: \(sum)")
}
}
// Output on console
Here is the sum of all numbers: 10