I'm writing a basic API call in Swift using URLRequests, and for whatever reason my call is never executed. I have multiple calls to an external server API using the same method and the functionality is just as expected, however, for my server running locally I get no response or even behavior within the dataTask closure.
I have tried any relevant solutions I could find online such as: Swift URL Session and URL Request not working and Swift 3, URLSession dataTask completionHandler not called. But none of these solutions seem to fix my issue. I know that the local API is working as any calls through Postman go through without fail, yet even after using the Swift snippet provided by Postman, I get no functionality.
func doFoo(id: String, completion: #escaping ([[Float]]) -> ()) {
let semaphore = DispatchSemaphore(value: 0)
var request = URLRequest(url: URL(string: "127.0.0.1:8080/doFoo/\(id)")!, timeoutInterval: Double.infinity)
request.httpMethod = "GET"
print("THIS IS REACHED")
let task = URLSession.shared.dataTask(with: request) { data, response, error in
print("THIS IS NEVER REACHED")
guard let data = data else {
self.semaphore.signal()
return
}
do {
// Decode json using JSONDecoder
// Call completion with JSON data
} catch {
print(error)
}
self.semaphore.signal()
}
task.resume()
self.semaphore.wait()
}
Other posts suggest that this could be an issue with the thread or execution completing before the closure is executed; while I am not super familiar with how the request executes and the behavior of semaphores, my understanding is that they are a way to request threads and prevent the above from happening.
If anyone more familiar with these topics could help me identify and understand why this issue is occurring, it would be greatly appreciated!
Related
I'm trying to make a fairly simple API call in Swift but, for some reason, my dataTask code is not running. I've made sure that the .resume() is there. This code has worked in the past but, something has changed recently and I don't know what it is. The only thing I can think of is the url. I've changed the ingredients but, when putting the url into a browser, it returns JSON data normally. When running this function, I get two "Outside URLSession.shared.dataTask....." messages in a row with nothing in between, indicating that the URLSession block of code isn't running. I'm a little new to APIs so, any help would be greatly appreciated. Please let me know if there's any more information I can provide. Also, I'm on an older MacBook and am using Swift5 if that makes a difference. Thanks!
let url: URL! = URL(string: "https://api.spoonacular.com/recipes/findByIngredients?ingredients=" + ingredientString + "&apiKey=aaabbbccc111222333")
print("URL: " + url.absoluteString)
let request = URLRequest(url: url)
// Make the API call
print("Outide URLSession.shared.dataTask.....")
let session = URLSession.shared.dataTask(with: request) { data, response, error in
print("Inside URLSession.shared.dataTask.....")
DispatchQueue.main.async {
print("Inside DispatchQueue.main.async....")
if data == nil {
print("No data recieved.")
}
print("data != nil.... Moving on to JSONDecoder....")
self.model = try! JSONDecoder().decode([RecipeSearchElement].self, from: data!)
}
}
session.resume()
print("Outside URLSession.shared.dataTask.....")
Unrelated to your immediate question at hand (which I answered elsewhere), I would advise a few changes to the routine:
One should not build a URL through string interpolation. Use URLComponents. If, for example, the query parameter included a space or other character not permitted in a URL, URLComponents will percent-encode it for you. If do not percent-encode it properly, the building of the URL will fail.
I would avoid try!, which will crash the app if the server response was not what you expected. One should use try within a do-catch block, so it handles errors gracefully and will tell you what is wrong if it failed.
I would recommend renaming the URLSessionDataTask to be task, or something like that, to avoid conflating “sessions” with the “tasks” running on that session.
I would not advise updating the model from the background queue of the URLSession. Fetch and parse the response in the background queue and update the model on the main queue.
Thus:
var components = URLComponents(string: "https://api.spoonacular.com/recipes/findByIngredients")
components?.queryItems = [
URLQueryItem(name: "ingredients", value: ingredientString),
URLQueryItem(name: "apiKey", value: "aaabbbccc111222333")
]
guard let url = components?.url else {
print("Unable to build URL")
return
}
// Make the API call
let task = URLSession.shared.dataTask(with: url) { data, _, error in
DispatchQueue.main.async {
guard error == nil, let data = data else {
print("No data received:", error ?? URLError(.badServerResponse))
return
}
do {
let model = try JSONDecoder().decode([RecipeSearchElement].self, from: data)
DispatchQueue.main.async {
self.model = model
}
} catch let parseError {
print("Parsing error:", parseError, String(describing: String(data: data, encoding: .utf8)))
}
}
}
task.resume()
In a more advanced observation, I would never have a network call update the model directly. I would leave that to the caller. For example, you could use a completion handler pattern:
#discardableResult
func fetchIngredients(
_ ingredientString: String,
completion: #escaping (Result<[RecipeSearchElement], Error>) -> Void
) -> URLSessionTask? {
var components = URLComponents(string: "https://api.spoonacular.com/recipes/findByIngredients")
components?.queryItems = [
URLQueryItem(name: "ingredients", value: ingredientString),
URLQueryItem(name: "apiKey", value: "aaabbbccc111222333")
]
guard let url = components?.url else {
completion(.failure(URLError(.badURL)))
return nil
}
// Make the API call
let task = URLSession.shared.dataTask(with: url) { data, _, error in
print("Inside URLSession.shared.dataTask.....")
DispatchQueue.main.async {
guard error == nil, let data = data else {
DispatchQueue.main.async {
completion(.failure(error ?? URLError(.badServerResponse)))
}
return
}
do {
let model = try JSONDecoder().decode([RecipeSearchElement].self, from: data)
DispatchQueue.main.async {
completion(.success(model))
}
} catch let parseError {
DispatchQueue.main.async {
completion(.failure(parseError))
}
}
}
}
task.resume()
return task
}
And then the caller could do:
fetchIngredients(ingredientString) { [weak self] result in
switch result {
case .failure(let error): print(error)
case .success(let elements): self?.model = elements
}
}
This has two benefits:
The caller now knows when the model is updated, so you can update your UI at the appropriate point in time (if you want).
It maintains a better separation of responsibilities, architecturally avoiding the tight coupling of the network layer with that of the view or view model (or presenter or controller) layers.
Note, I am also returning the URLSessionTask object in case the caller would like to cancel it at a later time, but I made it an #discardableResult so that you do not have to worry about that if you are not tackling cancelation at this point.
If you (a) are reaching the “outside” message, but not seeing the “inside” message; and (b) are absolutely positive that you are reaching the resume statement, it is one of a few possibilities:
The app may be terminating before the asynchronous request has time to finish. This can happen, for example, if this is a command-line app and you are allowing the app to quit before the asynchronous request has a chance to finish. If you want a command-line app to wait for a network request to finish, you might run a RunLoop that does not exit until the network request is done.
It can also happen if you use a playground and neglect to set needsIndefiniteExecution:
import PlaygroundSupport
PlaygroundPage.current.needsIndefiniteExecution = true
For the sake of completeness, there are a few other, less common, possibilities:
You have some other network request whose completion handler is blocked/deadlocked, thereby preventing anything else from running on the URLSession dedicated, serial, queue.
You have thread explosion somewhere else in your code, exhausting the limited pool of worker threads, preventing other tasks/operations from being able to get an available worker thread.
I'm trying to remove my dependency on RxAlamofire.
I currently have this function:
func requestData(_ urlRequest: URLRequestConvertible) -> Observable<(HTTPURLResponse, Data)> {
RxAlamofire.request(urlRequest).responseData()
}
How can I refactor this and use Alamofire directly to build and return an RxSwift Observable?
I suggest you look at the way the library wraps URLRequest to get an idea on how to do it...
Below is an abbreviated example from the library. In essence, you need to use Observable.create, make the network call passing in a closure that knows how to use the observer that create gives you.
Make sure you send a completed when done and make sure the disposable knows how to cancel the request.
Your Base will be something in Alamofire (I don't use Alamofire so I'm not sure what that might be.)
extension Reactive where Base: URLSession {
/**
Observable sequence of responses for URL request.
Performing of request starts after observer is subscribed and not after invoking this method.
**URL requests will be performed per subscribed observer.**
Any error during fetching of the response will cause observed sequence to terminate with error.
- parameter request: URL request.
- returns: Observable sequence of URL responses.
*/
public func response(request: URLRequest) -> Observable<(response: HTTPURLResponse, data: Data)> {
return Observable.create { observer in
let task = self.base.dataTask(with: request) { data, response, error in
guard let response = response, let data = data else {
observer.on(.error(error ?? RxCocoaURLError.unknown))
return
}
guard let httpResponse = response as? HTTPURLResponse else {
observer.on(.error(RxCocoaURLError.nonHTTPResponse(response: response)))
return
}
observer.on(.next((httpResponse, data)))
observer.on(.completed)
}
task.resume()
return Disposables.create(with: task.cancel)
}
}
}
I am in the process of implementing a REST API with Swift. Of course, part of this API is using HTTP requests to retrieve and send data.
Full disclosure, I am inexperienced with Swift and am using this as a learning project to get my feet wet, so to speak. But it's turned into much more of a difficult project than I anticipated.
In implementing the first get method, I have (finally) gotten rid of all the compilation errors. However, when I call the function which utilizes the URLRequest, URLSession, dataTask, etc, it is never entered.
Upon debugging the program, I can watch the program execution reach the CompletionHandler, and skip over it right to "task.resume()."
A similar construction works in a Swift Playground, but does not work in the actual project proper.
So far I have tried a few things, namely making the function access a class instance variable, in hopes that that would force it to execute. But it does not.
I think the issue may be dealing with synchronicity, and perhaps I need to use a Semaphore, but I want to make sure I'm not missing anything obvious first.
import Foundation
/**
A class to wrap all GET and POST requests, to avoid the necessity of repeatedly writing request code in each API method.
*/
class BasicRequest {
private var url: URL
private var header: [String: String]
private var responseType: String
private var jsonResponse: Any?
init(url: URL, header: [String: String], responseType: String) {
self.url = url
self.header = header
self.responseType = responseType
} //END INIT
public func requestJSON() -> Any {
// Create the URLRequest object, and fill the header with the header fields as provided.
var urlRequest = URLRequest(url: self.url)
for (value, key) in self.header {
urlRequest.addValue(value, forHTTPHeaderField: key)
}
let task = URLSession.shared.dataTask(with: urlRequest) { (data, response, error) in
print("Entered the completion handler")
if error != nil {
return
}
guard let httpResponse = response as? HTTPURLResponse, 200 == httpResponse.statusCode else {
print("HTTP Request unsuccessful")
return
}
guard let mime = response?.mimeType, mime == "application/json" else {
print("Not a JSON response")
return
}
do {
let json = try JSONSerialization.jsonObject(with: data!, options: [])
print(json)
self.jsonResponse = json
} catch {
print("Could not transform to JSON")
return
}
}
task.resume()
return "Function has returned"
} //END REQUESTJSON
}
The expected result would be returning a JSON object, however that does not seem to be the case.
With respect to error messages, I get none. The only log I get in the debugger is the boilerplate "process exited with code 0."
To be truthful, I'm at a loss with what is causing this not to work.
It appears you're writing this in a command-line app. In that case the program is terminating before the URLRequest completes.
I think the issue may be dealing with synchronicity, and perhaps I need to use a Semaphore, but I want to make sure I'm not missing anything obvious first.
Exactly.
The typical tool in Swift is DispatchGroup, which is just a higher-level kind of semaphore. Call dispatchGroup.enter() before starting the request, and all dispatchGroup.leave() at the end of the completion handler. In your calling code, include dispatchGroup.wait() to wait for it. (If that's not clear, I can add code for it, but there are also a lot of SO answers you can find that will demonstrate it.)
I'm following this tutorial for making a simple REST API call in swift: https://grokswift.com/simple-rest-with-swift/
The problem I'm running into is that the data task completion handler next gets executed. When I'm debugging it step by step, it just jumps over the completion handler block. Nothing is printed in the console, either.
I've searched for other methods of making REST API calls, but they are all very similar to this one and not working, either.
Here is my code:
let endpoint: String = "https://jsonplaceholder.typicode.com/todos/1"
guard let url = URL(string: endpoint) else {
return
}
let urlRequest = URLRequest(url: url)
let session = URLSession.shared
let task = session.dataTask(with: urlRequest) { (data, response, error) -> Void in
guard error == nil else {
print("Error calling GET")
return
}
guard let responseData = data else {
print("Error receiving data")
return
}
do {
print ("Parsing response...")
}
}
task.resume()
Your code looks right to me. I tested it in a Playground and I'm getting the Parsing response... message printed to the console which makes me think the issue is elsewhere in your code or environment. I'd be happy to take a look at the whole project if you can post a Github link or something similar.
Here are the steps I would take to debug an issue like this:
1) Confirm my execution environment has an active internet connection. The Safari app can be used to confirm on iOS devices or the Simulator. Playgrounds can be tested by pasting the following lines.
let url = URL(string: "https://jsonplaceholder.typicode.com/todos/1")!
print (try? String(contentsOf: url))
Look for a line in the console output similar to:
Optional("{\n \"userId\": 1,\n \"id\": 1,\n \"title\": \"delectus aut autem\",\n \"completed\": false\n}")
2) Confirm the url is valid and returns data by pasting it into a web browser url bar and hitting enter. You will either see JSON printed in the browser or not.
3) Confirm my code is actually getting called when the application runs. You can do this with either breakpoints or print() statements. As OOPer2 pointed out asynchronous callback closures like that used in session.dataTask() execute in a different time than the rest of your code which is why "it just jumps over the completion handler block" while stepping through with the debugger. You'll need to put another breakpoint or print() statement inside the completion handler closure. I'd put the breakpoint on the guard error == nil else { line.
4) Make sure the application is still executing when the network request finishes and the completion handler closure executes. If your code is in a ViewController running in an iOS application it's probably fine, but if it's running in a Playground it may not be. Playgrounds by default stop execution once the last line of code has been evaluated which means the completion closure will never execute. You can tell a Playground to continue executing indefinitely by importing the PlaygroundSupport framework and setting needsIndefiniteExecution = true on the current Playground page. Paste the entire code block below into a Playground to see it in action:
import Foundation
import PlaygroundSupport
// Keep executing the program after the last line has evaluated so the
// closure can execute when the asynchronous network request finishes.
PlaygroundPage.current.needsIndefiniteExecution = true
// Generic Result enum useful for returning values OR an error from
// asynchronous functions.
enum Result<T> {
case failure(Error)
case success(T)
}
// Custom Errors to be returned when something goes wrong.
enum NetworkError: Error {
case couldNotCreateURL(for: String)
case didNotReceiveData
}
// Perform network request asynchronous returning the result via a
// completion closure called on the main thread.
//
// In really life the result type will not be a String, it will
// probably be an array of custom structs or similar.
func performNetworkRequest(completion: #escaping (Result<String>)->Void ) {
let endpoint: String = "https://jsonplaceholder.typicode.com/todos/1"
guard let url = URL(string: endpoint) else {
let error = NetworkError.couldNotCreateURL(for: endpoint)
completion(Result.failure(error))
return
}
let urlRequest = URLRequest(url: url)
let session = URLSession.shared
let task = session.dataTask(with: urlRequest) { (data, response, error) -> Void in
// This closure is still executing on a background thread so
// don't touch anything related to the UI.
//
// Remember to dispatch back to the main thread when calling
// the completion closure.
guard error == nil else {
// Call the completion handler on the main thread.
DispatchQueue.main.async {
completion(Result.failure(error!))
}
return
}
guard let responseData = data else {
// Call the completion handler on the main thread.
DispatchQueue.main.async {
completion(Result.failure(NetworkError.didNotReceiveData))
}
return
}
// Parse response here...
// Call the completion handler on the main thread.
DispatchQueue.main.async {
completion(Result.success("Sucessfully parsed results"))
}
}
task.resume()
}
performNetworkRequest(completion: { result in
// The generic Result type makes handling the success and error
// cases really nice by just using a switch statement.
switch result {
case .failure(let error):
print(error)
case .success(let parsedResponse):
print(parsedResponse)
}
})
Why you dont use this Library Alamofire is an HTTP networking library written in Swift.
Add this line to your Podfile
pod 'Alamofire', '~> 4.4'
Then, run the following command:
pod install
Then in your ViewController file:
import Alamofire
Alamofire.request("https://jsonplaceholder.typicode.com/todos/1").responseJSON { response in
print("Request: \(String(describing: response.request))") // original url request
print("Response: \(String(describing: response.response))") // http url response
print("Result: \(response.result)") // response serialization result
if let json = response.result.value {
print("JSON: \(json)") // serialized json response
}
If let data = response.data, let utf8Text = String(data: data, encoding: .utf8) {
print("Data: \(utf8Text)") // original server data as UTF8 string
}
}
And in here are an example of how to parse the result.
https://github.com/CristianCardosoA/JSONParser
For more info about Alamofire:
https://github.com/Alamofire/Alamofire
I hope this help.
I know how to make a regular API call using swift. What I am not able to understand is how to make the API call to be repeated until required.
I want to call the API every one second
API Call Code Snippet:
let url = URL(string: "https://api.darksky.net/forecast/34eaef38915078ea03c22bb9063bd7ea/37.8267,-122.4233")
let request = URLRequest(url: url!, cachePolicy: URLRequest.CachePolicy.reloadIgnoringCacheData, timeoutInterval: 10)
let session = URLSession(configuration: URLSessionConfiguration.default, delegate: nil, delegateQueue: OperationQueue.main)
let task: URLSessionDataTask = session.dataTask(with: request, completionHandler: { (data: Data?, response: URLResponse?, error: Error?) in
if let error = error {
print(error)
} else if let data = data,
let dataDictionary = try! JSONSerialization.jsonObject(with: data, options: []) as? NSDictionary {
print("API Data:")
print(dataDictionary)
}
})
task.resume()
Note: This is not the actual API I will be calling
Ideally, for software solutions like financials you mentioned, the server must have support for some sort of Long Polling / websockets mechanism where once connection is established server feeds the client with new values whenever there are updates (refer : https://stackoverflow.com/a/12855533/1436617)
If server does not support : (Not the ideal solution) :
You can actually use recursion in this. On response (both success & failure) of the request again call the same function. That way you can continuously keep polling.
Remember to keep request timer short (5 or 10 seconds instead of 60 seconds) so that if there happens to be an network issue you can quickly make the next call.