define ThreadFlag {
  @value [ConditionWait & ConditionResume] cond
  @value Bool enabled
  @value Bool canceled

  new () {
    return ThreadFlag{ ThreadCondition.new(), false, false }
  }

  start () {
    scoped {
      MutexLock lock <- MutexLock.lock(cond)
    } cleanup {
      \ lock.freeResource()
    } in {
      enabled <- true
      \ cond.resumeAll()
    }
    return self
  }

  stop () {
    scoped {
      MutexLock lock <- MutexLock.lock(cond)
    } cleanup {
      \ lock.freeResource()
    } in enabled <- false
    return self
  }

  getEnabled () {
    scoped {
      MutexLock lock <- MutexLock.lock(cond)
    } cleanup {
      \ lock.freeResource()
    } in return enabled && !canceled
  }

  cancel () {
    scoped {
      MutexLock lock <- MutexLock.lock(cond)
    } cleanup {
      \ lock.freeResource()
    } in {
      canceled <- true
      \ cond.resumeAll()
    }
    return self
  }

  shouldContinue () {
    scoped {
      MutexLock lock <- MutexLock.lock(cond)
    } cleanup {
      \ lock.freeResource()
    } in while (true) {
      if (canceled) {
        return false
      } elif (enabled) {
        break
      } else {
        // The Mutex associated with cond is unlocked during wait().
        \ cond.wait()
      }
    }
    return true
  }
}