Example with Promises

gistfile1.swift

private func fetchData()  -> Promise<[Int]> {
        return Promise { seal in
            let someData = (1...100_000).map { _ in Int.random(in: (1...100)) }
            seal.fulfill(someData)
        }
    }
    
    private func calculateMinMax(forData array: [Int]) -> Promise<(Int, Int)> {
        return Promise { seal in
            guard let first = array.first else {
                seal.reject(DataError.emptyArray)
                return
            }
            
            var min = first
            var max = first
            
            for item in array.dropFirst() {
                if min > item {
                    min = item
                }
                if max < item {
                    max = item
                }
            }
            seal.fulfill((min, max))
        }
    }
    
    private func updateUI(with data: (Int, Int)) -> Promise<Void> {
        return Promise { seal in
            self.textLabel.text = "min is \(data.0), max is \(data.1)"
            seal.fulfill(())
        }
    }

    private func doJob() {
        fetchData()
            .then { array in self.calculateMinMax(forData: array) }
            .then (on: .main) { (min, max) in
                self.updateUI(with: (min, max))
            }.catch { error in
                print(error.localizedDescription)
            }
    }