import Foundation

/// Execute a function on a background thread and then handle result
/// in main thread.
func doBackgroundTask<Params, Result>(
    params: Params,
    backgroundTask: (Params) -> Result,
    onComplete: (Result) -> ())
{
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0)) {
        let result = backgroundTask(params)
        dispatch_async(dispatch_get_main_queue()) {
            onComplete(result)
        }
    }
}

/// Execute a procedure on a background thread and then handle result
/// in main thread.
func doAsyncTask<Result>(
    backgroundTask: () -> Result,
    onComplete: (Result) -> ())
{
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0)) {
        let result = backgroundTask()
        dispatch_async(dispatch_get_main_queue()) {
            onComplete(result)
        }
    }
}


// Example:

func test(lo: Int, hi: Int) {
    doBackgroundTask((lo, hi), { (lo: Int, hi: Int) -> Int in
        println("(Background task running in thread \(pthread_self()))")
        var result = 0
        for i in lo...hi {
            println("(Step \(i) in thread \(pthread_self()))")
            result = result + i
            NSThread.sleepForTimeInterval(0.5)
        }
        return result
    }, { (result: Int) in
        println("(Completion on main thread \(pthread_self()))")
        println("Sum of \(lo)...\(hi) is \(result)")
    });
}

import XCPlayground

XCPSetExecutionShouldContinueIndefinitely(continueIndefinitely: true)

// These tests will run concurrently
test(1, 5)
test(10, 15)
test(20, 25)