DispatchGroup [weak self] handling in nested notify

Question

I have two asynchronous functions. I need to run second function based on first's output and run third function after second is complete. I did it like this. Is it correct way to handle serialization? And how to handle weak self reference?

let dispatchGroup = DispatchGroup()

dispatchGroup.enter()
function1() // async completion contains leave()

dispatchGroup.notify(queue: DispatchQueue.main) { [weak self] in
    guard let strongSelf = self else {return}
    strongSelf.dispatchGroup.enter()    
    strongSelf.function2() // Also async, dependent on the result of function1. contains leave()

    strongSelf.dispatchGroup.notify(queue: DispatchQueue.main) {
        strongSelf????.function3()
        print("results of function1 and function2")
        // I must wait to finish first two tasks in order, in other words serial queue
    }
}

Answer 1

1) Correct way to: call the method after the previous one has completed

class SomeAsyncType {

    typealias SomeResultType = String

    func async(_ someDate: Any, completion: @escaping (SomeResultType) -> Void) {
        self.asyncfunctionA(someDate, completion: completion)
    }

    private func asyncfunctionA(_ someDate: Any, completion: @escaping (SomeResultType) -> Void) {
        DispatchQueue.main.async { [weak self] in
            sleep(1)
            print("ResultA")
            self?.asyncfunctionB("ResultA", completion: completion)
        }
    }

    private func asyncfunctionB(_ someDate: Any, completion: @escaping (SomeResultType) -> Void) {
        DispatchQueue.main.async { [weak self] in
            sleep(2)
            print("ResultB")
            self?.asyncfunctionC("ResultB", completion: completion)
        }
    }

    private func asyncfunctionC(_ someDate: Any, completion: @escaping (SomeResultType) -> Void) {
       DispatchQueue.main.async {
            sleep(3)
            print("ResultC")
            completion("🙈🙊🙉")
            print("All completed")
        }
    }
}

---

usage:

let test = SomeAsyncType() // hold reference 
test.async("noDate") { result in
    print(result)
}

console:

ResultA
ResultB
ResultC
🙈🙊🙉
All completed

2) how to handle weak self reference?

This is new scope then you can use [weak self] again

dispatchGroup.notify(queue: DispatchQueue.main) { [weak self] in
    guard let strongSelf = self else {return}
    strongSelf.dispatchGroup.enter()
    strongSelf.function2() // Also async, dependent on the result of function1. contains leave()

    strongSelf.dispatchGroup.notify(queue: DispatchQueue.main) { [weak self] in
        guard let strongSelf = self else {return}
        strongSelf.function3()
        print("results of function1 and function2")
    }
}