activity2.swift

var activityUpdate: NSBackgroundActivityScheduler?

func backgroundActivitiesSetup() {
    activityUpdate = NSBackgroundActivityScheduler(identifier: "…")
    activityUpdate?.interval = 1 * 60 // every 1 minute
    activityUpdate?.repeats = true
    activityUpdate?.schedule { completion in
        updateNewItems(completion)
    }
}

func updateNewItems(_ completion: NSBackgroundActivityScheduler.CompletionHandler) {
    let numWorkers = 10
    let batchCount = 160
    var (items, totalItems) = …

    while totalItems > 0 {
        let lock = DispatchSemaphore(value: numWorkers)
        var startTime = Date()
        print("\(startTime.localDate()) Update [\(items.count) of \(totalItems) items] started.")

        DispatchQueue.concurrentPerform(iterations: numWorkers) { index in
            defer { lock.signal() }
            let start = index * items.count / numWorkers
            let end = (index + 1) * items.count / numWorkers

            for _ in start ..< end {
                let random = Int.random(in: 0 ..< items.count)
                // work here
                print("\(Date().localDate()) Update completed for \(items[random].name).")
            }
        }
        lock.wait()
        let totalTime = Date().offsetFrom(date: startTime)
        print("\(Date().localDate()) Update [\(items.count) of \(totalItems) items] completed in \(totalTime).")

        (items, totalItems) = // retrieve more items
    }
    completion(.finished)
}