This code works perfectly when downloading one file. However when trying to download multiple files that contain data and caching the incoming data causes a mess. Since the download occurs non-stop, thus one file is done and the next starts. I can't cache them since I don't know which data belongs to which file.
lazy var downloadQueue: OperationQueue = {
var queue = OperationQueue()
queue.maxConcurrentOperationCount = 1000000
queue.name = "Files"
return queue
}()
func fetch(url: String, completionHandler: #escaping ([String:String]) -> (), completionHandlerQueue: OperationQueue?) {
let task = session.dataTask(with: URL(string: url)!, completionHandler: {
(data, response, error) in
guard let data = data, let type = String(data: data, encoding: String.Encoding.utf8), error == nil else {
print("Error with the data: \(error.debugDescription)")
return
}
guard let statusCode = (response as? HTTPURLResponse)?.statusCode, statusCode >= 200 && statusCode <= 299 else {
return
}
guard completionHandlerQueue != nil else {
return
}
completionHandlerQueue!.addOperation(BlockOperation(block: {
completionHandler(type)
}))
})
task.resume()
}
// Here is the links
let urls = [.....]
func start() {
for url in urls {
print("Start")
fetch(url: url, completionHandler: { type in
// The incoming data I don't know which belongs to which url.
}, completionHandlerQueue: downloadQueue)
}
print("End")
}
Since its async I will get
Start
End
Then the data will come. How can I overcome this?
Related
I need to make 2 API calls simultaneously. I have 2 URLs for the calls, and if one of the calls will return any error I want to stop all the code execution.
How I tried to do it:
I have a function called performRequest() with a completion block. I call the function in my ViewController to update the UI - show an error/or a new data if all was successful. Inside it I create a URLSession tasks and then parse JSON:
I created an array with 2 urls:
func performRequest(_ completion: #escaping (Int?) -> Void) {
var urlArray = [URL]()
guard let urlOne = URL(string: "https://api.exchangerate.host/latest?base=EUR&places=9&v=1") else { return }
guard let urlTwo = URL(string: "https://api.exchangerate.host/2022-05-21?base=EUR&places=9") else { return }
urlArray.append(urlOne)
urlArray.append(urlTwo)
}
Then for each of the url inside the array I create a session and a task:
urlArray.forEach { url in
let session = URLSession(configuration: .ephemeral)
let task = session.dataTask(with: url) { data, _, error in
if error != nil {
guard let error = error as NSError? else { return }
completion(error.code)
return
}
if let data = data {
let printData = String(data: data, encoding: String.Encoding.utf8)
print(printData!)
DispatchQueue.main.async {
self.parseJSON(with: data)
}
}
}
task.resume()
}
print("all completed")
completion(nil)
}
For now I receive print("all completed") printed once in any situation: if both tasks were ok, if one of them was ok or none of them.
What I want is to show the print statement only if all tasks were completed successfully and to stop executing the code if one of them returned with error (for example if we will just delete one of the symbols in url string which will take it impossible to receive a data).
How can I do it correctly?
So I am downloading a JSON file using a URLRequest().
I parse through it in order to get a specific string and I want to set the text of a label I have in my ViewController to that specific string.
I use a CompletionHandler in order to retrieve the function that gets the JSON file from another Swift file.
Here is the code of calling the function and setting the label:
class SecondViewController: UIViewController {
tr = TransportServices()
tr.getLyftData(origin: originstring, destination: destinationstring){ json in
//Parsing JSON in order to get specific data
self.lyftlabel.text = stringexample
}
}
and here is the code of getting the JSON
func getLyftData(origin: String, destination: String, completionHandler: #escaping ([String: Any]) -> ()){
let urlrequest = URLRequest(url: URL(string: urlstring)!)
let config = URLSessionConfiguration.default
let sessions = URLSession(configuration: config)
let task = sessions.dataTask(with: urlrequest) {(data, response, error) in
guard error == nil else {
print(error!)
return
}
guard let responseData = data else {
print("error, did not receive data")
return
}
do {
if let json = try JSONSerialization.jsonObject(with: responseData, options: []) as? [String: Any]{
completionHandler(json)
}
}
catch {
print("Error with URL Request")
}
}
task.resume()
}
This does the job, but in a very slow manner. I know that there is a runtime issue because UILabel.text must be set from main thread only, but I don't know any other way to fix it. Please help.
If you want to set label text in main thread use this:
DispatchQueue.main.async {
self.lyftlabel.text = stringexample
}
My application receives location updates, and when it does (if they are relevant), I call an API asynchronously using a completion handler. When the application opens, the completion handler responds only if there was no request that finished before (two requests come in at the same time usually). When I debug, after the first 2-3 requests (which come in at the same time) where everything works, when the location update passes as relevant, the whole completion handling part of code gets skipped.
This is how I call the completion handler:
if conditions {
let lat = Float(loc.lat)
let long = Float(loc.long)
// calls function using completion handler in order to add new location
BusStations.allBusStations(lat: lat, long: long) { (busStations, error) in
if let error = error {
// got an error in getting the data
print(error)
return
}
guard let busStations = busStations else {
print("error getting all: result is nil")
return
}
if !busStations.stops.isEmpty || self.locations.isEmpty {
// do stuff
}
}
}
This is how I make the API call:
static func allBusStations (lat: Float, long: Float, completionHandler: #escaping (BusStations?, Error?) -> Void) {
let endpoint = BusStations.endpointForBusStations(lat: lat, long: long)
guard let url = URL(string: endpoint) else {
print("Error: cannot create URL")
let error = BackendError.urlError(reason: "Could not construct URL")
completionHandler(nil, error)
return
}
let urlRequest = URLRequest(url: url)
let session = URLSession.shared
let task = session.dataTask(with: urlRequest) {
(data, response, error) in
guard let responseData = data else {
print("Error: did not receive data")
completionHandler(nil, error)
return
}
guard error == nil else {
completionHandler(nil, error)
return
}
let decoder = JSONDecoder()
do {
let stations = try decoder.decode(BusStations.self, from: responseData)
completionHandler(stations, nil)
} catch {
print("error trying to convert data to JSON")
print(error)
completionHandler(nil, error)
}
}
task.resume()
}
What am I doing wrong? Any help would be appreciated.
I would try to dispatch the completion handler to global or main queue to see if it is deferred by system to execute on a queue of lower levels.
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()
}
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)
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])
}
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
I'm currently using Alamofire for requesting data and writing to disk with Realm. Specifically, I am fetching 24 source URLS from a Facebook Graph GET request and then making 24 separate requests to retrieve the data for each image. Once the data is retrieved, I am writing to disk with Realm.
here is how I am fetching the 24 sources:
FBAPI
Alamofire.request(.GET, FBPath.photos, parameters: params).responseJSON { response in
guard response.result.error == nil else {
print("error calling GET on \(FBPath.photos)")
print(response.result.error!)
completion(latestDate: nil, photosCount: 0, error: response.result.error)
return
}
if let value = response.result.value {
let json = JSON(value)
if let photos = json[FBResult.data].array {
for result in photos {
let manager = PTWPhotoManager()
manager.downloadAndSaveJsonData(result)
}
As you can see, I have a for loop iterating through each JSON containing the source url for the photo's image in which I then make another network request for each url, like so:
Manager
func downloadAndSaveJsonData(photoJSON : JSON) {
let source = photoJSON[FBResult.source].string
let id = photoJSON[FBResult.id].string
let created_time = photoJSON[FBResult.date.createdTime].string
let imageURL = NSURL(string: source!)
print("image requested")
Alamofire.request(.GET, imageURL!).response() {
(request, response, data, error) in
if (error != nil) {
print(error?.localizedDescription)
}
else {
print("image response")
let photo = PTWPhoto()
photo.id = id
photo.sourceURL = source
photo.imageData = data
photo.createdTime = photo.createdTimeAsDate(created_time!)
let realm = try! Realm()
try! realm.write {
realm.add(photo)
}
print("photo saved")
}
}
}
There seems to be a very long delay between when each image's data is requested and when I receive a response, and it also does not appear to be asynchronous. Is this a threading issue or is there a more efficient way to request an array of data like this? It should also be noted that I am making this network request from the Apple Watch itself.
These requests will happen mostly asynchronous as you wish. But there is some synchronization happening, you might been not aware of:
The response closures for Alamofire are dispatched to the main thread. So your network responses competes against any UI updates you do.
Realm write transactions are synchronous and exclusive, which is enforced via locks which will block the thread where they are executed on.
In combination this both means that you will block the main thread as long as the network requests succeed and keep coming, which would also render your app unresponsive.
I'd recommend a different attempt. You can use GCD's dispatch groups to synchronize different asynchronous tasks.
In the example below, the objects are all kept in memory until they are all downloaded.
A further improvement could it be to write the downloaded data onto disk instead and store just the path to the file in the Realm object. (There are plenty of image caching libraries, which can easily assist you with that.)
If you choose a path, which depends only on the fields of PWTPhoto (or properties of the data, you can get through a quick HEAD request), then you can check first whether this path exists already locally before downloading the file again. By doing that you save traffic when updating the photos or when not all photos could been successfully downloaded on the first attempt. (e.g. app is force-closed by the user, crashed, device is shutdown)
class PTWPhotoManager {
static func downloadAllPhotos(params: [String : AnyObject], completion: (latestDate: NSDate?, photosCount: NSUInteger, error: NSError?)) {
Alamofire.request(.GET, FBPath.photos, parameters: params).responseJSON { response in
guard response.result.error == nil else {
print("error calling GET on \(FBPath.photos)")
print(response.result.error!)
completion(latestDate: nil, photosCount: 0, error: response.result.error)
return
}
if let value = response.result.value {
let json = JSON(value)
if let photos = json[FBResult.data].array {
let group = dispatch_group_create()
var persistablePhotos = [PTWPhoto](capacity: photos.count)
let manager = PTWPhotoManager()
for result in photos {
dispatch_group_enter(group)
let request = manager.downloadAndSaveJsonData(result) { photo, error in
if let photo = photo {
persistablePhotos.add(photo)
dispatch_group_leave(group)
} else {
completion(latestDate: nil, photosCount: 0, error: error!)
}
}
}
dispatch_group_notify(group, dispatch_get_main_queue()) {
let realm = try! Realm()
try! realm.write {
realm.add(persistablePhotos)
}
let latestDate = …
completion(latestDate: latestDate, photosCount: persistablePhotos.count, error: nil)
}
}
}
}
}
func downloadAndSaveJsonData(photoJSON: JSON, completion: (PTWPhoto?, NSError?) -> ()) -> Alamofire.Request {
let source = photoJSON[FBResult.source].string
let id = photoJSON[FBResult.id].string
let created_time = photoJSON[FBResult.date.createdTime].string
let imageURL = NSURL(string: source!)
print("image requested")
Alamofire.request(.GET, imageURL!).response() { (request, response, data, error) in
if let error = error {
print(error.localizedDescription)
completion(nil, error)
} else {
print("image response")
let photo = PTWPhoto()
photo.id = id
photo.sourceURL = source
photo.imageData = data
photo.createdTime = photo.createdTimeAsDate(created_time!)
completion(photo, nil)
}
}
}
}