Multiple Request with Dispatch Group

multipleRequest.swift

let firstRequestGroup = DispatchGroup()
let secondRequestGroup = DispatchGroup()

for () {
    requestGroup.enter()
    firstRequest() {
        print("DEBUG: FIRST Request")
        if success {
            requestGroup.enter()
            secondRequest() {
                print("DEBUG: SECOND Request")
                requestGroup.leave()
            }
        }
        requestGroup.leave()
    }
}

// This only gets executed once all the requests in the authRequestGroup are done (i.e. FIRST, SECOND requests)

requestGroup.notify(queue: DispatchQueue.main, execute: {
    secondRequestGroup.enter()
    aRequest() {
        print("DEBUG: THIRD Request")
        secondRequestGroup.leave()
    }
}

// Note: Any code placed here will be executed before the THIRD request completes! To execute code after the THIRD request, we need the request secondRequestGroup.notify like below
print("This gets executed before the THIRD request completes")

// This only gets executed once all the requests in the secondRequestGroup are done (i.e. THIRD request)
secondRequestGroup.notify(queue: DispatchQueue.main, execute: {
    // Here, you can update the UI, HUD and turn off the network activity indicator
    print("DEBUG: all Done")
}

import Alamofire

struct SequentialRequest {

    static func fetchData() {

        let authRequestGroup =  DispatchGroup()
        let requestGroup = DispatchGroup()
        var results = [String: String]()

        //First request - this would be the authentication request
        authRequestGroup.enter()
        Alamofire.request("http://httpbin.org/get").responseData { response in
            print("DEBUG: FIRST Request")
            results["FIRST"] = response.result.description

            if response.result.isSuccess { //Authentication successful, you may use your own tests to confirm that authentication was successful

                authRequestGroup.enter() //request for data behind authentication
                Alamofire.request("http://httpbin.org/get").responseData { response in
                    print("DEBUG: SECOND Request")
                    results["SECOND"] = response.result.description

                    authRequestGroup.leave()
                }

                authRequestGroup.enter() //request for data behind authentication
                Alamofire.request("http://httpbin.org/get").responseData { response in
                    print("DEBUG: THIRD Request")
                    results["THIRD"] = response.result.description
                    authRequestGroup.leave()
                }
            }
            authRequestGroup.leave()
        }

        // This only gets executed once all the requests in the authRequestGroup are done (i.e. FIRST, SECOND AND THIRD requests)
        authRequestGroup.notify(queue: DispatchQueue.main, execute: {

            // Here you can perform additional request that depends on data fetched from the FIRST, SECOND or THIRD requests
            requestGroup.enter()
            Alamofire.request("http://httpbin.org/get").responseData { response in
                print("DEBUG: FOURTH Request")
                results["FOURTH"] = response.result.description
                requestGroup.leave()
            }
        
            //Note: Any code placed here will be executed before the FORTH request completes! To execute code after the FOURTH request, we need the request requestGroup.notify like below
            print("This gets executed before the FOURTH request completes")

            //This only gets executed once all the requests in the requestGroup are done (i.e. FORTH request)
            requestGroup.notify(queue: DispatchQueue.main, execute: {

                //Here, you can update the UI, HUD and turn off the network activity indicator
                for (request, result) in results {
                    print("\(request): \(result)")
                }
                print("DEBUG: all Done")
            })
        })
    }
}