Pause execution in a playground until a condition is met (like an async network load completing)

waitUntilTrue.swift

// in a playground, pause execution wait until the condition pred is true
func waitUntilTrue(@autoclosure pred:()->Bool, secondsUntilTimeout duration:NSTimeInterval = 25)
{
  let previousPlayGroundRunStatus = XCPExecutionShouldContinueIndefinitely()
  XCPSetExecutionShouldContinueIndefinitely(continueIndefinitely: true)
  
  let start = NSDate()
  while true {
    if pred() {
      NSLog("condition met.")
      break
    }
    else if fabs(start.timeIntervalSinceNow) > duration {
      NSLog("timeout")
      break
    }
    else {
      sleep(1)
    }
  }
  XCPSetExecutionShouldContinueIndefinitely(continueIndefinitely: previousPlayGroundRunStatus)
}