1.swift

import Foundation

final class Worker {
  let timeout = 5
  let notify: Void -> Void

  init(notify: Void -> Void) {
    self.notify = notify
  }

  func run() {
    configureTimer()

    dispatch_main()
  }

  private func configureTimer() {
    let source = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, dispatch_get_main_queue())
    dispatch_source_set_timer(source, 0, UInt64(timeout) / 2 * NSEC_PER_SEC, 0)
    dispatch_source_set_event_handler(source) {
      self.notify()
    }
    dispatch_resume(source)
  }
}

let worker = Worker(notify: {
  print("notify!")
})

worker.run()

// original version, not working

2.swift

import Foundation

final class Worker {
  let timeout = 5
  let notify: Void -> Void

  init(notify: Void -> Void) {
    self.notify = notify
  }

  func run() {
    let source = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, dispatch_get_main_queue())
    dispatch_source_set_timer(source, 0, UInt64(timeout) / 2 * NSEC_PER_SEC, 0)
    dispatch_source_set_event_handler(source) {
      self.notify()
    }
    dispatch_resume(source)
    dispatch_main()
  }
}

let worker = Worker(notify: {
  print("notify!")
})

worker.run()

// inlined and working

3.swift

import Foundation

final class Worker {
  let timeout = 5
  let notify: Void -> Void

  init(notify: Void -> Void) {
    self.notify = notify
  }

  func run() {
    dispatch_resume(configureTimer())

    dispatch_main()
  }

  private func configureTimer() -> dispatch_source_t {
    let source = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, dispatch_get_main_queue())
    dispatch_source_set_timer(source, 0, UInt64(timeout) / 2 * NSEC_PER_SEC, 0)
    dispatch_source_set_event_handler(source) {
      self.notify()
    }
    return source
  }
}

let worker = Worker(notify: {
  print("notify!")
})

worker.run()

// modified version, not working

4.swift

import Foundation

final class Worker {
  let timeout = 5
  let notify: Void -> Void

  init(notify: Void -> Void) {
    self.notify = notify
  }

  func run() {
    let source = configureTimer()
    dispatch_resume(source)

    dispatch_main()
  }

  private func configureTimer() -> dispatch_source_t {
    let source = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, dispatch_get_main_queue())
    dispatch_source_set_timer(source, 0, UInt64(timeout) / 2 * NSEC_PER_SEC, 0)
    dispatch_source_set_event_handler(source) {
      self.notify()
    }
    return source
  }
}

let worker = Worker(notify: {
  print("notify!")
})

worker.run()

// another modified version, working

Tested using Apple Swift version 3.0-dev (LLVM a7663bb722, Clang 4ca3c7fa28, Swift 1c2f40e246) on OS X El Capitan 10.11.3 (15D21)