Swift 4: How to asynchronously use URLSessionDataTask but have the requests be in a timed queue? - swift

Question

Basically I have some JSON data that I wish to retrieve from a bunch of URL's (all from the same host), however I can only request this data roughly every 2 seconds at minimum and only one at a time or I'll be "time banned" from the server. As you'll see below; while URLSession is very quick it also gets me time banned almost instantly when I have around 700 urls to get through.
How would I go about creating a queue in URLSession (if its functionality supports it) and while having it work asynchronously to my main thread; have it work serially on its own thread and only attempt each item in the queue after 2 seconds have past since it finished the previous request?
for url in urls {
get(url: url)
}
func get(url: URL) {
let session = URLSession.shared
let task = session.dataTask(with: url, completionHandler: { (data, response, error) in
if let error = error {
DispatchQueue.main.async {
print(error.localizedDescription)
}
return
}
let data = data!
guard let response = response as? HTTPURLResponse, response.statusCode == 200 else {
DispatchQueue.main.async {
print("Server Error")
}
return
}
if response.mimeType == "application/json" {
do {
let json = try JSONSerialization.jsonObject(with: data) as! [String: Any]
if json["success"] as! Bool == true {
if let count = json["total_count"] as? Int {
DispatchQueue.main.async {
self.itemsCount.append(count)
}
}
}
} catch {
print(error.localizedDescription)
}
}
})
task.resume()
}

Answer 1

Recursion solves this best
import Foundation
import PlaygroundSupport
// Let asynchronous code run
PlaygroundPage.current.needsIndefiniteExecution = true
func fetch(urls: [URL]) {
guard urls.count > 0 else {
print("Queue finished")
return
}
var pendingURLs = urls
let currentUrl = pendingURLs.removeFirst()
print("\(pendingURLs.count)")
let session = URLSession.shared
let task = session.dataTask(with: currentUrl, completionHandler: { (data, response, error) in
print("task completed")
if let _ = error {
print("error received")
DispatchQueue.main.async {
fetch(urls: pendingURLs)
}
return
}
guard let response = response as? HTTPURLResponse, response.statusCode == 200 else {
print("server error received")
DispatchQueue.main.async {
fetch(urls: pendingURLs)
}
return
}
if response.mimeType == "application/json" {
print("json data parsed")
DispatchQueue.main.async {
fetch(urls: pendingURLs)
}
}else {
print("unknown data")
DispatchQueue.main.async {
fetch(urls: pendingURLs)
}
}
})
//start execution after two seconds
Timer.scheduledTimer(withTimeInterval: 2, repeats: false) { (timer) in
print("resume called")
task.resume()
}
}
var urls = [URL]()
for _ in 0..<100 {
if let url = URL(string: "https://google.com") {
urls.append(url)
}
}
fetch(urls:urls)

Answer 2

The easiest way is to perform recursive call:
Imagine you have array with your urls.
In place where you initially perform for loop with, replace it with single call get(url:).
self.get(urls[0])
Then add this line at the and of response closure right after self.itemsCount.append(count):
self.urls.removeFirst()
Timer.scheduledTimer(withTimeInterval: 2, repeats: false) { (_) in
self.get(url: urls[0])
}

Answer 3

Make DispatchQueue to run your code on threads. You don't need to do this work on Main Thread. So,
// make serial queue
let queue = DispatchQueue(label: "getData")
// for delay
func wait(seconds: Double, completion: #escaping () -> Void) {
queue.asyncAfter(deadline: .now() + seconds) { completion() }
}
// usage
for url in urls {
wait(seconds: 2.0) {
self.get(url: url) { (itemCount) in
// update UI related to itemCount
}
}
}
By the way, Your get(url: url) function is not that great.
func get(url: URL, completionHandler: #escaping ([Int]) -> Void) {
let session = URLSession.shared
let task = session.dataTask(with: url, completionHandler: { (data, response, error) in
if let error = error {
print(error.localizedDescription)
/* Don't need to use main thread
DispatchQueue.main.async {
print(error.localizedDescription)
}
*/
return
}
let data = data!
guard let response = response as? HTTPURLResponse, response.statusCode == 200 else {
print("Server Error")
/* Don't need to use main thread
DispatchQueue.main.async {
print("Server Error")
}
*/
return
}
if response.mimeType == "application/json" {
do {
let json = try JSONSerialization.jsonObject(with: data) as! [String: Any]
if json["success"] as! Bool == true {
if let count = json["total_count"] as? Int {
self.itemsCount.append(count)
// append all data that you need and pass it to completion closure
DispatchQueue.main.async {
completionHandler(self.itemsCount)
}
}
}
} catch {
print(error.localizedDescription)
}
}
})
task.resume()
}
I would recommend you to learn concept of GCD(for thread) and escaping closure(for completion handler).
GCD: https://www.raywenderlich.com/148513/grand-central-dispatch-tutorial-swift-3-part-1
Escaping Closure: https://developer.apple.com/library/content/documentation/Swift/Conceptual/Swift_Programming_Language/Closures.html#//apple_ref/doc/uid/TP40014097-CH11-ID546