Limiting concurrent tasks.

concurrency.swift

import Foundation

func dispatch_async_batch(tasks: [() -> ()], limit: Int, completion: (() -> ())?) {
    if tasks.count > 0 || completion != nil {
        let q = dispatch_queue_create("dispatch_async_batch", DISPATCH_QUEUE_CONCURRENT);
        let sema = dispatch_semaphore_create(limit);

        dispatch_async(q, {
            for task in tasks {
                dispatch_semaphore_wait(sema, DISPATCH_TIME_FOREVER)
                dispatch_async(q, {
                    task() // The task must be synchronous!
                    dispatch_semaphore_signal(sema)
                })
            }

            if let completion = completion {
                dispatch_barrier_async(q, completion)
            }
        })
    }
}

func dummy(t: NSTimeInterval, _ dt: NSTimeInterval) {
    let _t = t + dt * (Double(arc4random()) / Double(UInt32.max) * 2 - 1)
    NSThread.sleepForTimeInterval(_t)
}

let tasks = (0 ..< 12).map { i in
    return {
        print("+ Task #\(i) started")
        dummy(4, 2)
        print("- Task #\(i) stopped")
    }
}
print("Scheduling \(tasks.count) task(s)...")
let sema = dispatch_semaphore_create(1)
dispatch_semaphore_wait(sema, DISPATCH_TIME_FOREVER)
dispatch_async_batch(tasks, limit: 4) {
    print("All tasks has been completed.")
    dispatch_semaphore_signal(sema)
}
dispatch_semaphore_wait(sema, DISPATCH_TIME_FOREVER) // Don't let the app terminate until the background work is finished.

/* Sample output

Scheduling 12 task(s)...
+ Task #0 started
+ Task #1 started
+ Task #2 started
+ Task #3 started
- Task #1 stopped
+ Task #4 started
- Task #0 stopped
+ Task #5 started
- Task #2 stopped
+ Task #6 started
- Task #3 stopped
+ Task #7 started
- Task #6 stopped
+ Task #8 started
- Task #4 stopped
+ Task #9 started
- Task #7 stopped
+ Task #10 started
- Task #5 stopped
+ Task #11 started
- Task #8 stopped
- Task #9 stopped
- Task #10 stopped
- Task #11 stopped
All tasks has been completed.

*/