Quick and dumb main thread watchdog that fires *before* the main thread is done

Watchdog.swift

// WARNING: No thought involved while writing this, introduce some prior to production use. I warned you.

var timer: dispatch_source_t!
func startWatchdog() {
    let q = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0)
    guard let timer = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, q) else { return }
    
    self.timer = timer
    dispatch_source_set_timer(timer, dispatch_time(DISPATCH_TIME_NOW, 0), NSEC_PER_SEC / 10, (UInt64(1) * NSEC_PER_SEC) / 10)
    dispatch_source_set_event_handler(timer) {
        var flag = false
        dispatch_async(dispatch_get_main_queue()) {
            flag = true
        }
        dispatch_after(dispatch_time(DISPATCH_TIME_NOW, Int64(NSEC_PER_SEC / 10)), q) {
            if !flag {
                // Break here and look at your main thread call stack
                print("lag")
            }
        }
    }
    dispatch_resume(timer)
}