I am trying to downlod multiple JSON files with a URLSession and when I run the funtion one time it works. But the moment I call the getSMAPrices function from a loop it does not work and I can not find out why.
Here is the working download function that works if i call it.
func getSMAPrices(symbol: String) {
let urlString = "https://www.alphavantage.co/query?function=SMA&symbol=\(symbol)&interval=daily&time_period=9&series_type=close&apikey=KPLI12AW8JDXM77Y"
guard let url = URL(string: urlString) else {
return
}
dataTask = defaultSession.dataTask(with: url, completionHandler: { (data, response, error) in
if error != nil {
print(error!.localizedDescription)
}
guard let data = data else {
return
}
//Implement JSON decoding and parsing
do {
//Decode retrived data with JSONDecoder and assing type of Article object
let decoder = JSONDecoder()
decoder.dateDecodingStrategy = .iso8601
let stockData = try decoder.decode(SimpelMovingAvarage.self, from: data)
//Get back to the main queue
DispatchQueue.main.async {
print(stockData)
}
} catch let jsonError {
print(jsonError)
}
})
dataTask?.resume()
}
And here is my very simple loop that replaces a part in the URL every run cycle. But nothing happens.
public func scanSymbols() {
for symbol in self.symbols {
progress += 1
progresBar.maxValue = Double(symbols.count)
progresBar.doubleValue = progress
//This does not work
getSMAPrices(symbol: symbol.key)
}
}
It's because your dataTask variable appears to be an instance property and is getting overwritten every time you call this method.
Related
I have a three year old barcode scanning ios app that has been making a synchronous call to an api with no problems until I was forced to use a slighly slower api. The latest version of XCode offer this advice: Synchronous URL loading of https://get-thumb.herokuapp.com/getThumb.php?objectid=20019 should not occur on this application's main thread as it may lead to UI unresponsiveness. Please switch to an asynchronous networking API such as URLSession.
The code below asychronusly loads an ArtObject with string data including a path to another api. How can I get the data from the second, getThumb api asynchronously and load it into the UI on the main thread?
func downLoadJson (forObject: String) {
let myURLStr = urlObj + forObject
print (myURLStr)
guard let downLoadURL = URL(string: myURLStr) else {return}
URLSession.shared.dataTask(with: downLoadURL) { (data, urlResponse, error) in
guard let data = data, error == nil, urlResponse != nil else {
print ("something is wrong in URLSessions call")
return
}
do {
let decoder = JSONDecoder()
let anObject = try decoder.decode(ArtObject.self, from: data)
/*
print("object Name is \(anObject.ObjectName)")
print("Creators is \(anObject.Creators)")
print("Medium is \(anObject.Medium)")
print("Titles is \(anObject.Titles)")
print("LabelUUID is \(anObject.LabelUUID)")
*/
gArtObject = anObject // this populates the global gArtObject with the local anObject so that the vals are avial in other funcs
var displayText = "ObjName: " + anObject.ObjectName
displayText += "\nCreator(s): " + anObject.Creators
displayText += "\nMedium: " + anObject.Medium
displayText += "\nTitle(s): " + anObject.Titles
displayText += "\nObjectNumber: " + String(anObject.ObjectNumber)
displayText += "\nComponentNumber: " + anObject.compNumber
//--- UI update must happen on main queue THIS IS THE PROBLEM
DispatchQueue.main.async {
self.objectDetails.text = displayText
self.imageVW.setImageFromURl(stringImageUrl: anObject.imageSmall)
}
} catch {
print("something wrong after download step")
print(error.localizedDescription)
}
}.resume()
extension UIImageView{
func setImageFromURl(stringImageUrl url: String){
if let url = NSURL(string: url) {
if let data = NSData(contentsOf: url as URL) {
self.image = UIImage(data: data as Data)
}
}
}
}
You can achieve this the same way you are loading your JSON.
In the setImageFromURl function remove the synchronous call to Data and use:
if let url = URL(string: url) {
URLSession.shared.dataTask(with: url) { (data, urlResponse, error) in
if let data = data {
DispatchQueue.main.async{
self.image = UIImage(data: data)
}
}
}.resume()
}
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?
This question already has answers here:
Returning data from async call in Swift function
(13 answers)
Closed 1 year ago.
I'm trying to learn a bit about making API calls in SwiftUI.
I have a function called loadData which runs via the onAppear modifier.
The goal of that function is to see if I currently have data in CoreData.
If there is no data in CoreData, then I'd like to call another function that makes the API call to get the data, but only return the fetched data.
With the example I have below, the getCurrentSol function returns before the async portion is finished. Resulting in no data being returned. What is the appropriate way for me to return the data?
As you can see, I did try a while(true) "trick". But for whatever reason, my results variable never even updates with the fetched data, even though the decodedData variable does contain the proper results.
}.onAppear(perform: loadData)
}
func loadData() {
print("data: \(storedData) ")
print("data.count: \(storedData.count)")
if(storedData.count == 0){
let fetchedData = getCurrentSol()
let currentSol = fetchedData.sol
print("fetchedData: \(fetchedData)")
print("currentSol: \(currentSol)")
}
}
func getCurrentSol() -> CuriosityRoverModel {
var results = CuriosityRoverModel(sol: 0, low: 0, high: 0, opacity: "Sunny", sunrise: "00:00", sunset: "00:00", month: "Month 0")
let urlString = "https://api.maas2.apollorion.com"
let url = URL(string: urlString)
URLSession.shared.dataTask(with: url!) {data, response, error in
DispatchQueue.main.async {
if let data = data {
do {
let decoder = JSONDecoder()
let decodedData = try decoder.decode(CuriosityRoverModel.self, from: data)
//This recieves the proper data, but it doesn't get written to the results var
print("decodedData: \(decodedData)")
results = decodedData
} catch {
print("Error: ", error)
}
}
}
}.resume()
// I thought this would be a way to wait for the data
// but results never gets updated so it ends up running endlessly
while(true){
if(results.sol > 0){
return results
}
}
//return results // This would just return the "empty" results var from above before the data is actually retrieved
}
}
There are many ways to achieve what you want. This is one approach, using a closure:
....
.onAppear(perform: loadData)
}
func loadData() {
print("data: \(storedData) ")
print("data.count: \(storedData.count)")
if (storedData.count == 0) {
getCurrentSol() { results in // <--- here
if let fetchedData = results {
let currentSol = fetchedData.sol
print("fetchedData: \(fetchedData)")
print("currentSol: \(currentSol)")
}
}
}
}
// use a completion closure to "return" your results when done, not before
func getCurrentSol(completion: #escaping (CuriosityRoverModel?) -> Void) {
let urlString = "https://api.maas2.apollorion.com"
let url = URL(string: urlString)
URLSession.shared.dataTask(with: url!) {data, response, error in
DispatchQueue.main.async {
if let data = data {
do {
let decoder = JSONDecoder()
let decodedData = try decoder.decode(CuriosityRoverModel.self, from: data)
print("decodedData: \(decodedData)")
completion(decodedData) // <--- here, return the results
} catch {
print("Error: ", error) // need to deal with errors
completion(nil) // <--- here, should return the error
}
}
}
}.resume()
}
}
I am trying to recover a data set from a URL (after parsing a JSON through the parseJSON function which works correctly - I'm not attaching it in the snippet below).
The outcome returns nil - I believe it's because the closure in retrieveData function is processed asynchronously. I can't manage to have the outcome saved into targetData.
Thanks in advance for your help.
class MyClass {
var targetData:Download?
func triggerEvaluation() {
retrieveData(url: "myurl.com") { downloadedData in
self.targetData = downloadedData
}
print(targetData) // <---- Here is where I get "nil"!
}
func retrieveData(url: String, completion: #escaping (Download) -> ()) {
let myURL = URL(url)!
let mySession = URLSession(configuration: .default)
let task = mySession.dataTask(with: myURL) { [self] (data, response, error) in
if error == nil {
if let fetchedData = data {
let safeData = parseJSON(data: fetchedData)
completion(safeData)
}
} else {
//
}
}
task.resume()
}
}
Yes, it’s nil because retrieveData runs asynchronously, i.e. the data hasn’t been retrieved by the time you hit the print statement. Move the print statement (and, presumably, all of the updating of your UI) inside the closure, right where you set self.targetData).
E.g.
func retrieveData(from urlString: String, completion: #escaping (Result<Download, Error>) -> Void) {
let url = URL(urlString)!
let mySession = URLSession.shared
let task = mySession.dataTask(with: url) { [self] data, response, error in
guard
let responseData = data,
error == nil,
let httpResponse = response as? HTTPURLResponse,
200 ..< 300 ~= httpResponse.statusCode
else {
DispatchQueue.main.async {
completion(.failure(error ?? NetworkError.unknown(response, data))
}
return
}
let safeData = parseJSON(data: responseData)
DispatchQueue.main.async {
completion(.success(safeData))
}
}
task.resume()
}
Where
enum NetworkError: Error {
case unknown(URLResponse?, Data?)
}
Then the caller would:
func triggerEvaluation() {
retrieveData(from: "https://myurl.com") { result in
switch result {
case .failure(let error):
print(error)
// handle error here
case .success(let download):
self.targetData = download
// update the UI here
print(download)
}
}
// but not here
}
A few unrelated observations:
You don't want to create a new URLSession for every request. Create only one and use it for all requests, or just use shared like I did above.
Make sure every path of execution in retrieveData calls the closure. It might not be critical yet, but when we write asynchronous code, we always want to make sure that we call the closure.
To detect errors, I'd suggest the Result pattern, shown above, where it is .success or .failure, but either way you know the closure will be called.
Make sure that model updates and UI updates happen on the main queue. Often, we would have retrieveData dispatch the calling of the closure to the main queue, that way the caller is not encumbered with that. (E.g. this is what libraries like Alamofire do.)
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
}