DispatchgGroup.swift

import Foundation

class DispatchGroup {
    private var blocks: [dispatch_block_t] = []
    private let dispatch_queue = dispatch_queue_create("com.dispatch_group.foo", DISPATCH_QUEUE_CONCURRENT)
    private let dispatch_group = dispatch_group_create()

    func andThen(block: dispatch_block_t) -> Group {
        blocks += [block]
        return self
    }

    func noAndThen(completion: dispatch_block_t) -> Group {
        for block in blocks {
           dispatch_group_async(dispatch_group, dispatch_queue, block)
        }

        dispatch_group_notify(dispatch_group, dispatch_queue, completion)
        return self
    }
}

let foo = Group()

for i in 1...10 {
    foo.andThen { println(i) }
}

foo.andThen {
    println("foo")
   }.andThen {
       println("bar")
   }.andThen {
       println("baz")
   }.noAndThen {
       println("completion")
}

// ❯ ./group.swift
// 51426
//
// 3
// 7
//
// 891
//
// fbb
//
// 0oaa
// orz
//
//
// completion