Asynchronous Call and For Loop - loops

Question

I recently had an issue with Alamofire (More generally with asynchronous calls)
I have two models, Listings and Users. Listings contains a user's email, and I would also like to get the user's first and last name (I understand I could solve this in the backend as well, however, I would like to see if there is a frontend solution as this problem comes up for something more complicated as well)
Currently I'm making a GET request to get all listings, and I'm looping through them, and making another GET request to get firstname, lastname.
I need to wait to get the result of this get request, or at the minimum append it to my listings dictionary. Likewise, before I do anything else (Move on to the next screen of my app), I'd like to have all the listings be linked to a firstname, lastname. Because theres a loop, this specifically seems to cause some issues (ie if it was just two nested GET requests, it could be in a callback). Is there an easy way to get around this. I've attached psuedocode below:
GET Request to grab listings:
for each listing:
GET request to grab first_name, last_name
Once all listings have gotten first_name, last_name -> Load next page

Answer 1

The answer for your question is called a dispatch group
Dispatch groups can be entered and left only to execute some code when no code is currently inside the dispatch group.
GET Request to grab listings{
var downloadGroup = dispatch_group_create()
//Create a dispatch group
for each listing{
dispatch_group_enter(downloadGroup)
//Enter the dispatch group
GET request to grab first_name, last_name (Async){
dispatch_group_leave(downloadGroup)
//Leave the dispatch group
}
}
dispatch_group_notify(downloadGroup, dispatch_get_main_queue()) {
//Run this code when all GET requests are finished
}
}
As demonstrated by this code.
Source and interesting reading material about dispatching: Grand Central Dispatch Tutorial for Swift by Ray Wenderlich

Answer 2

With Scala-style futures and promises you can do something like this:
let future: Future<[Listing]> = fetchListings().flatMap { listings in
listings.traverse { listing in
fetchUser(listing.userId).map { user in
listing.userName = "\(user.firstName) \(user.lastName)"
return listing
}
}
}
The result of the above expression is a future whose value is an array of listings.
Print the listing's user name, once the above expression is finished:
future.onSuccess { listings in
listings.forEach {
print($0.userName)
}
}
Scala-style future and promise libraries: BrightFutures or FutureLib
Below a ready-to-use code example which you can paste into a playgrounds file to experiment with any of the above libraries (works in FutureLib, BrightFutures might require slight modifications).
import FutureLib
import Foundation
import XCPlayground
XCPlaygroundPage.currentPage.needsIndefiniteExecution = true
class Listing {
let userId: Int
init(userId: Int) {
self.userId = userId
userName = ""
}
var userName: String
}
struct User {
let id: Int
let firstName: String
let lastName: String
init (_ id: Int, firstName: String, lastName: String) {
self.id = id
self.firstName = firstName
self.lastName = lastName
}
}
func fetchListings() -> Future<[Listing]> {
NSLog("start fetching listings...")
return Promise.resolveAfter(1.0) {
NSLog("finished fetching listings.")
return (1...10).map { Listing(userId: $0) }
}.future!
}
// Given a user ID, fetch a user:
func fetchUser(id: Int) -> Future<User> {
NSLog("start fetching user[\(id)]...")
return Promise.resolveAfter(1.0) {
NSLog("finished fetching user[\(id)].")
return User(id, firstName: "first\(id)", lastName: "last\(id)")
}.future!
}
let future: Future<[Listing]> = fetchListings().flatMap { listings in
listings.traverse { listing in
fetchUser(listing.userId).map { user in
listing.userName = "\(user.firstName) \(user.lastName)"
return listing
}
}
}
future.onSuccess { listings in
listings.forEach {
print($0.userName)
}
}

Answer 3

Here's a possible solution:
GET Request to grab listings:
var n = the number of listings
var i = 0 //the number of retrieved
for each listing:
GET request to grab first_name, last_name, callback: function(response){
assign first/last name using response.
i+=1
if(i==n)
load_next_page()
}
So what this does is keep a counter of how many firstname/lastname records you've fetched. Make sure that you handle cases where a call to get the name fails.
Or, as suggested in a comment on the question, you could use promises. They make async code like this much nicer.