iOS Swift playground implementing a delayed function multiple times

delay.swift

import Foundation
import XCPlayground

// Allows execution to be indefinite, giving delays or callbacks time to play
XCPlaygroundPage.currentPage.needsIndefiniteExecution = true


// Delay function
func delayer(duration: Double, closure:()->()){
    
    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, Int64(duration * Double(NSEC_PER_SEC))), dispatch_get_main_queue(), closure)
}


func doSomething(counter: Int, max: Int){
    
    print("executing \(counter)")
    
    if(counter == max){
        
        // Probably don't need this, but why not stop execution when all is said and done?
        XCPlaygroundPage.currentPage.finishExecution()
    }
}

// Run it once just to see if things are working
doSomething(-1, max: 9999)

// Loop to run delay multiple times
for index in 0...4{

    // For fun, we'll make our delay exponential...as might be done in an exponential backoff call
    let duration = Double(pow(Double(index), 1.414))
    
    // Next two lines show how bat-shit crazy and incomplete Swift is...left in for posterior posterity
    let counterStr = String(duration)
    
    let subbed = counterStr.substringWithRange(Range<String.Index>(counterStr.startIndex..<counterStr.startIndex.advancedBy(3)))
    
    // This is much better....and also rounds for us!
    let substr = String(format: "%.1f", duration)
    
    // Prints part of a list showing us what delays to expect
    print("\(index), delay: \(substr)")
    
    // Delay calling the function
    delayer(duration){
        
        doSomething(index, max: 4)
    }
}