I am writing a Network class for my app. One function carries out the actual request to the API I am using
as part of this function I am using a URLSession.shard.dataTask().
var decodedResponse = SongLinkAPIResponse()
let task = session.dataTask(with: url) { data, response, error in
let result = data.map(Result.success) ?? Result.failure(DataLoaderError.network(error!))
let handlerResult = handler(result)
// This handler just decodes the downloaded JSON into a SongLinkAPIResponse struct.
// This works fine and I can see this is working
DispatchQueue.main.async {
switch handlerResult {
case .success(let response):
// When I run this code in a playground I can see that this line shows as working in the sidebar
decodedResponse = response
case .failure(let error):
print(error)
}
}
}
task.resume()
//However when returning this variable it just has the empty default configuration of the struct
return decodedResponse
Any ideas on why the variable is not updating?
Thanks in advance
session.dataTask is asynchronous. It will execute some time in the future.
Your code will execute in the following order.
// 1
var decodedResponse = SongLinkAPIResponse()
// 2
let task = session.dataTask(with: url) { data, response, error in
// 4
...
}
task.resume()
// 3
return decodedResponse
Here are some possible explanations/solutions:
Function that returns asynchronously retrieved value
Returning data from async call in Swift function
Swift write an async/await method with return value
Related
I'm beginner in SwiftUI and I'm not familiar with variable management.
I'd like to send a very simple post request like this one with SwiftUI:
let full_path : String = "https://www.example.com/get_answer?my_message=current temperature"
I've tried with this piece of code but it didn't work.
if (URL(string: full_path) != nil) {
let url = URL(string: full_path)!
var request = URLRequest(url: url)
request.httpMethod = "POST"
var decodedAnswer = String("")
do {
let task = URLSession.shared.dataTask(with: url) { data, response, error in
print(response)
decodedAnswer = String(decoding: response, as: UTF8.self)
}
}
I have the following error:
Value of optional type 'URLResponse?' must be unwrapped to a value of
type 'URLResponse'
I don't know how to get the response.
How can I get the response from a simple Post request in SwiftUI?
Multiple issues here.
You are trying to decode the URLResponse object, but what you want is the data object in the decoder.
You seem to not know about optionals. I would refer you to the basic Apple tutorials about this topic. You can find it with your favorite search engine.
You are in an async context here. Everything inside the url datasession closure will be execute after your network request returns. The code in your function will be completed by that moment and your var decodedAnswer will be out of scope. So move it out of the function in to the class/struct.
You probably want something like this:
This should be defined in class scope or you won´t be able to use it:
var decodedAnswer: String = ""
This should be in a function:
let full_path: String = "https://www.example.com/get_answer?my_message=current temperature"
if let url = URL(string: full_path) {
var request = URLRequest(url: url)
request.httpMethod = "POST"
do {
let task = URLSession.shared.dataTask(with: url) { data, response, error in
//This converts the optionals in to non optionals that could be used further on
//Be aware this will just return when something goes wrong
guard let data = data, let response = response, error == nil else{
print("Something went wrong: error: \(error?.localizedDescription ?? "unkown error")")
return
}
print(response)
decodedAnswer = String(decoding: data, as: UTF8.self)
}
task.resume()
}
}
In a swift 2 command line tool (main.swift), I have the following:
import Foundation
print("yay")
var request = HTTPTask()
request.GET("http://www.stackoverflow.com", parameters: nil, completionHandler: {(response: HTTPResponse) in
if let err = response.error {
print("error: \(err.localizedDescription)")
return //also notify app of failure as needed
}
if let data = response.responseObject as? NSData {
let str = NSString(data: data, encoding: NSUTF8StringEncoding)
print("response: \(str)") //prints the HTML of the page
}
})
The console shows 'yay' and then exits (Program ended with exit code: 0), seemingly without ever waiting for the request to complete. How would I prevent this from happening?
The code is using swiftHTTP
I think I might need an NSRunLoop but there is no swift example
Adding RunLoop.main.run() to the end of the file is one option. More info on another approach using a semaphore here
I realize this is an old question, but here is the solution I ended on. Using DispatchGroup.
let dispatchGroup = DispatchGroup()
for someItem in items {
dispatchGroup.enter()
doSomeAsyncWork(item: someItem) {
dispatchGroup.leave()
}
}
dispatchGroup.notify(queue: DispatchQueue.main) {
exit(EXIT_SUCCESS)
}
dispatchMain()
You can call dispatchMain() at the end of main. That runs the GCD main queue dispatcher and never returns so it will prevent the main thread from exiting. Then you just need to explicitly call exit() to exit the application when you are ready (otherwise the command line app will hang).
import Foundation
let url = URL(string:"http://www.stackoverflow.com")!
let dataTask = URLSession.shared.dataTask(with:url) { (data, response, error) in
// handle the network response
print("data=\(data)")
print("response=\(response)")
print("error=\(error)")
// explicitly exit the program after response is handled
exit(EXIT_SUCCESS)
}
dataTask.resume()
// Run GCD main dispatcher, this function never returns, call exit() elsewhere to quit the program or it will hang
dispatchMain()
Don't depend on timing.. You should try this
let sema = DispatchSemaphore(value: 0)
let url = URL(string: "https://upload.wikimedia.org/wikipedia/commons/4/4d/Cat_November_2010-1a.jpg")!
let task = URLSession.shared.dataTask(with: url) { data, response, error in
print("after image is downloaded")
// signals the process to continue
sema.signal()
}
task.resume()
// sets the process to wait
sema.wait()
If your need isn't something that requires "production level" code but some quick experiment or a tryout of a piece of code, you can do it like this :
SWIFT 3
//put at the end of your main file
RunLoop.main.run(until: Date(timeIntervalSinceNow: 15)) //will run your app for 15 seconds only
More info : https://stackoverflow.com/a/40870157/469614
Please note that you shouldn't rely on fixed execution time in your architecture.
Swift 4: RunLoop.main.run()
At the end of your file
// Step 1: Add isDone global flag
var isDone = false
// Step 2: Set isDone to true in callback
request.GET(...) {
...
isDone = true
}
// Step 3: Add waiting block at the end of code
while(!isDone) {
// run your code for 0.1 second
RunLoop.main.run(until: Date(timeIntervalSinceNow: 0.1))
}
I want to update my code in the past when I used swift 2 or 3. I am stuck where I want to use Alamofire but the way to use it changed and I don't know how to use it anymore. Can anybody update this part of the code and explain a little bit? Thank you.
This is the original code.
Alamofire.request(.POST, url)
.response{ (request, response, data, error) in
let xml = SWXMLHash.parse(data!)
let sunsetTime = xml["result"]["rise_and_set"]["sunset_hm"].element?.text
self.sunsetTimeLabel.text = sunsetTime
self.getDateFromString(sunsetTime,year: comp.year,month: comp.month,day: comp.day)
if (error != nil) {
print(error)
}
}
this is the code I was writing.
AF.request(url, method: .post).responseJSON { (responseData) in
let xml = SWXMLHash.parse(responseData as Data)
let sunsetTime = xml["result"]["rise_and_set"]["sunset_hm"].element?.text
self.sunsetTimeLabel.text = sunsetTime
There is an error saying "Cannot convert value of type 'AFDataResponse' (aka 'DataResponse') to type 'Data' in coercion"
Your first code snippet is Alamofire 3 syntax. I infer from the second code snippet that you are now using Alamofire 5.
There are a few issues:
You are calling responseJSON (which you’d only use if your response was JSON, not XML). Use response or, better, responseData.
The response object passed to this closure is not a Data, itself. In the case of responseData method, it is a AFDataResponse object, which has a data property (which is a Data?). You have to extract the Data object from this AFDataResponse, either by unwrapping the contents of the data property, or from the result (see next point).
You should probably check for success or failure and extract the Data from the response.result object.
So, pulling this together, you end up with something like:
AF.request(url, method: .post).responseData { response in
switch response.result {
case .failure(let error):
print(error)
case .success(let data):
let xml = SWXMLHash.parse(data)
...
}
}
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'm using a lot of async network request (btw any network request in iOS need to by async) and I'm finding way to better handle errors from Apple's dataTaskWithRequest which not supports throws.
I have code like that:
func sendRequest(someData: MyCustomClass?, completion: (response: NSData?) -> ()) {
let request = NSURLRequest(URL: NSURL(string: "http://google.com")!)
if someData == nil {
// throw my custom error
}
let task = NSURLSession.sharedSession().dataTaskWithRequest(request) {
data, response, error in
// here I want to handle Apple's error
}
task.resume()
}
I need to parse my possible custom errors and handle possible connection errors from dataTaskWithRequest. Swift 2 introduced throws, but you can't throw from Apple's closure because they have no throw support and running async.
I see only way to add to my completion block NSError returning, but as I know using NSError is old-style Objective-C way. ErrorType can be used only with throws (afaik).
What's the best and most modern method to handle error when using Apple network closures? There is no way no use throws in any async network functions as I understand?
there are many ways you can solve this, but i would recommend using a completion block which expects a Result Enum. this would probably be the most 'Swift' way.
the result enum has exactly two states, success and error, which a big advantage to the usual two optional return values (data and error) which lead to 4 possible states.
enum Result<T> {
case Success(T)
case Error(String, Int)
}
Using the result enum in a completion block finishes the puzzle.
let InvalidURLCode = 999
let NoDataCode = 998
func getFrom(urlString: String, completion:Result<NSData> -> Void) {
// make sure the URL is valid, if not return custom error
guard let url = NSURL(string: urlString) else { return completion(.Error("Invalid URL", InvalidURLCode)) }
let request = NSURLRequest(URL: url)
NSURLSession.sharedSession().dataTaskWithRequest(request) { data, response, error in
// if error returned, extract message and code then pass as Result enum
guard error == nil else { return completion(.Error(error!.localizedDescription, error!.code)) }
// if no data is returned, return custom error
guard let data = data else { return completion(.Error("No data returned", NoDataCode)) }
// return success
completion(.Success(data))
}.resume()
}
because the return value is a enum, you should switch off of it.
getFrom("http://www.google.com") { result in
switch result {
case .Success(let data):
// handle successful data response here
let responseString = String(data:data, encoding: NSASCIIStringEncoding)
print("got data: \(responseString)");
case .Error(let msg, let code):
// handle error here
print("Error [\(code)]: \(msg)")
}
}
another solution would be to pass two completion blocks, one for success and one for error. something along the lines of:
func getFrom(urlString: String, successHandler:NSData -> Void, errorHandler:(String, Int) -> Void)
It's very similar to Casey's answer,
but with Swift 5, now we have Result (generic enumeration) implementation in standard library,
//Don't add this code to your project, this has already been implemented
//in standard library.
public enum Result<Success, Failure: Error> {
case success(Success), failure(Failure)
}
It's very easy to use,
URLSession.shared.dataTask(with: url) { (result: Result<(response: URLResponse, data: Data), Error>) in
switch result {
case let .success(success):
handleResponse(success.response, data: success.data)
case let .error(error):
handleError(error)
}
}
https://developer.apple.com/documentation/swift/result
https://github.com/apple/swift-evolution/blob/master/proposals/0235-add-result.md
There's an elegant approach utilising a JavaScript-like Promise library or a Scala-like "Future and Promise" library.
Using Scala-style futures and promises, it may look as follows:
Your original function
func sendRequest(someData: MyCustomClass?, completion: (response: NSData?) -> ())
may be implemented as shown below. It also shows, how to create a promise, return early with a failed future and how to fulfill/reject a promise:
func sendRequest(someData: MyCustomClass) -> Future<NSData> {
guard let url = ... else {
return Future.failure(MySessionError.InvalidURL) // bail out early with a completed future
}
let request = ... // setup request
let promise = Promise<NSData>()
NSURLSession.sharedSession().dataTaskWithRequest(request) { data, response, error in
guard let error = error else {
promise.reject(error) // Client error
}
// The following assertions should be true, unless error != nil
assert(data != nil)
assert(response != nil)
// We expect HTTP protocol:
guard let response = response! as NSHTTPURLResponse else {
promise.reject(MySessionError.ProtocolError) // signal that we expected HTTP.
}
// Check status code:
guard myValidStatusCodeArray.contains(response.statusCode) else {
let message: String? = ... // convert the response data to a string, if any and if possible
promise.reject(MySessionError.InvalidStatusCode(statusCode: response.statusCode, message: message ?? ""))
}
// Check MIME type if given:
if let mimeType = response.MIMEType {
guard myValidMIMETypesArray.contains(mimeType) else {
promise.reject(MySessionError.MIMETypeNotAccepted(mimeType: mimeType))
}
} else {
// If we require a MIMEType - reject the promise.
}
// transform data to some other object if desired, can be done in a later, too.
promise.fulfill(data!)
}.resume()
return promise.future!
}
You might expect a JSON as response - if the request succeeds.
Now, you could use it as follows:
sendRequest(myObject).map { data in
return try NSJSONSerialization.dataWithJSONObject(data, options: [])
}
.map { object in
// the object returned from the step above, unless it failed.
// Now, "process" the object:
...
// You may throw an error if something goes wrong:
if failed {
throw MyError.Failed
}
}
.onFailure { error in
// We reach here IFF an error occurred in any of the
// previous tasks.
// error is of type ErrorType.
print("Error: \(error)")
}