Dispatch group - cannot notify to main thread - swift

Question

After reading Swift 3 evolution on GCD, I am trying to create dispatch group. The problem is the group.notify(queue: do not notify when I pass DispatchQueue.main as a queue, although it does work for background queue.
Also I am not sure my syntax is all correct, as I am trying to convert code from Swift 2 to Swift 3.
typealias CallBack = (result: Bool) -> Void
func longCalculations (completion: CallBack) {
let backgroundQ = DispatchQueue.global(attributes: .qosBackground)
let group = DispatchGroup()
var fill:[Int] = []
for item in 0...200 {
group.enter()
if item > 50 {
fill.append(item)
}
group.leave()
}
//Below in the notify argument If I pass `backgroundQ`, it seems to work correctly but not when DispatchQueue.main is passed.
This code do not work
group.notify(queue: DispatchQueue.main, execute: {
completion(result: true)
})
}
This works correctly
group.notify(queue: backgroundQ, execute: {
completion(result: true)
})
}
_______________________________________________________
longCalculations() { print($0) }

Answer 1

After reading post suggested by Matt, I found that I was submitting task to main queue and when I asked to be notified on main thread itself, it got in the deadlock.
I have altered the code and now it is working as intended,
typealias CallBack = (result: [Int]) -> Void
func longCalculations (completion: CallBack) {
let backgroundQ = DispatchQueue.global(attributes: .qosDefault)
let group = DispatchGroup()
var fill:[Int] = []
for number in 0..<100 {
group.enter()
backgroundQ.async(group: group, execute: {
if number > 50 {
fill.append(number)
}
group.leave()
})
}
group.notify(queue: DispatchQueue.main, execute: {
print("All Done"); completion(result: fill)
})
}
longCalculations(){print($0)}