Hello I have been trying alot and looking around on SO, but I cant find a good solution to my problem.
The problem is that i do 2 requests within one function and the first one finishes, then it does updateUI function on the main thread instead of waiting for the second download to finish.
I know that i am doing something wrong but in my mind the two requests work on different threads and that is why updateUI only will trigger after first download is complete.
Is it possible to use dispatchqueue? I dont know how completionhandlers work either sadly..
I couldnt see what i have to use from their github page either, im quite new at Swift.
Please do not set this as duplicate. would really appreciate it
func getMovieData(){
self.posterLoading.startAnimating()
//Set up URL
let testCall: String = "https://api.themoviedb.org/3/discover/movie?api_key=935f5ddeed3fb57e&language=en-US&sort_by=popularity.desc&include_adult=false&include_video=false&page=1&with_genres=12"
Alamofire.request(testCall).responseJSON { response in
//print(response.request) // original URL request
//print(response.response) // HTTP URL response
//print(response.data) // server data
//print(response.result) // result of response serialization
if let json = response.result.value as? Dictionary<String,AnyObject> {
if let movies = json["results"] as? [AnyObject]{
for movie in movies{
let movieObject: Movie = Movie()
let title = movie["title"] as! String
let releaseDate = movie["release_date"] as! String
let posterPath = movie["poster_path"] as! String
let overView = movie["overview"] as! String
let movieId = movie["id"] as! Int
let genre_ids = movie["genre_ids"] as! [AnyObject]
movieObject.title = title
movieObject.movieRelease = releaseDate
movieObject.posterPath = posterPath
movieObject.overView = overView
movieObject.movieId = movieId
for genre in genre_ids{//Genre ids, fix this
movieObject.movieGenre.append(genre as! Int)
}
Alamofire.request("http://image.tmdb.org/t/p/w1920" + posterPath).responseImage {
response in
debugPrint(response)
//print(response.request)
//print(response.response)
//debugPrint(response.result)
if var image = response.result.value {
image = UIImage(data: response.data!)!
movieObject.poster = image
}
}
self.movieArray.append(movieObject)
print("movie added")
}//End of for each movie
DispatchQueue.main.async(){
print("is ready for UI")
self.updateUI()
}
}
}
}//End of Json request
}//End of getmoviedata
func updateUI(){
uiMovieTitle.text = movieArray[movieIndex].title
uiMoviePoster.image = movieArray[movieIndex].poster
}
Just make your getMovieData func with a completion block.
func getMovieData(finished: () -> Void) {
Alamofire.request(testCall).responseJSON { response in
// call me on success or failure
finished()
}
}
and then you can call your update UI in the completion block of the func, where you calling it the second time getMovieData()
Your function should look like this.
func getMovieData(completionHandler: #escaping (_ returnedData: Dictionary<String,AnyObject>)-> Void ) {
Alamofire.request(testCall).response { response in
if let JSON = response.result.value {
completionHandler(JSON)
}
}
}
And your function call look like
getMovieData(completionHandler: {(returnedData)-> Void in
//Do whatever you want with your returnedData JSON data.
//when you finish working on data you can update UI
updateUI()
})
You don't have to do your data operations in your function call btw.You can do your stuff in your function and call the completionHandler() end of it. Then you can update your ui only at function call.
Related
This question already has an answer here:
Return a string from a web scraping function in swift
(1 answer)
Closed 4 years ago.
How can I return a value within an if let statement to be further returned within a function? Here is the code:
func loadUrl(url:String) -> String {
DispatchQueue.global().async {
do {
let appUrl = URL(string:url)!
let data = try Data(contentsOf:appUrl)
let json = try JSONSerialization.jsonObject(with: data) as! [String:Any]
print("Test from do")
if let results = json["results"] as? [[String:Any]] {
print("Test from if let 1")
if let first = results[0] as? [String:Any] {
print("Test from if let 2")
var cityStateLocation = first["formatted_address"]!
return cityStateLocation
//What needs to be returned
}
}
DispatchQueue.main.async {
print("No Error")
}
} catch {
DispatchQueue.main.async {
print("Cannot connect to the server.")
}
}
}
}
What I would like to be able to do is take cityStateLocation and return it in the func, but because it is a part of an if let statement within an .async method I don't know how to do that. Could someone please explain?
EDIT: I need the return value of cityStateLocation to equal a variable in a separate function. Here is the separate function:
#IBAction func continueButton(_ sender: Any) {
var cityState:String
if locationSwitch.isOn == true {
print(location.latitude)
print(location.longitude)
let url = "https://maps.googleapis.com/maps/api/geocode/json?latlng=\(location.latitude),\(location.longitude)&result_type=locality&key=AIzaSyDI-ZacHyPbLchRhkoaUTDokwj--z_a_jk"
loadUrl(url: url)
cityState = loadUrl(url: url)
} else {
cityState = ""
}
CoreDataHandler.saveObject(locationLocality: cityState)
}
Edit 2: The main reason why the "duplicate answer" is not a duplicate is that my code needs to call the return of this function within a separate function then save it to Core Data. Also, my code is not using an array.
You could modify your function to include a closure. For instance:
func loadUrl(url: String, completionHandler: #escaping (_ location: String?) -> (Void)) {
And then, where you want to return it, you'd pass it in as such.
completionHandler(cityStateLocation)
I made it an optional so that, in your fail paths, you could return nil.
Then, where you call the function would change. Using trailing closure syntax, it could look like this:
loadUrl(url: "someurl.com/filepath.txt") { optionalLocation in
guard let nonOptionalLocation = optionalLocation else {
// Location was nil; Handle error case here
return
}
// Do something with your location here, like setting UI or something
}
This is a fairly common pattern when dealing with asynchronous activity, such as working with network calls.
The simplest (perhaps no the prettiest), way of doing this would simply be to declare and instantiate a variable above the dispatch queue. Then you can set the variable equal to whatever you want, within the dispatch queue, and return it afterwards. You can change the type of ret, so that it suits your needs more directly.
func loadUrl(url:String) -> String {
var ret = NSObject()
DispatchQueue.global().async {
do {
let appUrl = URL(string:url)!
let data = try Data(contentsOf:appUrl)
let json = try JSONSerialization.jsonObject(with: data) as! [String:Any]
print("Test from do")
if let results = json["results"] as? [[String:Any]] {
print("Test from if let 1")
if let first = results[0] as? [String:Any] {
print("Test from if let 2")
var cityStateLocation = first["formatted_address"]!
ret = cityStateLocation
//What needs to be returned
}
}
DispatchQueue.main.async {
print("No Error")
}
} catch {
DispatchQueue.main.async {
print("Cannot connect to the server.")
}
}
}
return ret
}
DispatchQueue.global().async will cause the coded included in the closure to be executed at some point the future, meaning you loadUrl function will return (almost) immediately.
What you need is some kind of callback which can be called when you have a result (AKA closure)
This is just another way to approach the problem, the difference between this and Josh's example is simply, I provide an additional closure to handle the errors
func loadUrl(url:String, complition: #escaping (String?) -> Void, fail: #escaping (Error) -> Void) {
DispatchQueue.global().async {
do {
let appUrl = URL(string:url)!
let data = try Data(contentsOf:appUrl)
let json = try JSONSerialization.jsonObject(with: data) as! [String:Any]
print("Test from do")
if let results = json["results"] as? [[String:Any]], !results.isEmpty {
print("Test from if let 1")
let first = results[0]
print("Test from if let 2")
if let cityStateLocation = first["formatted_address"] as? String {
complition(cityStateLocation)
} else {
complition(nil)
}
} else {
complition(nil)
}
} catch let error {
fail(error)
}
}
}
Which you might call using something like...
loadUrl(url: "your awesome url", complition: { (value) in
guard let value = value else {
// No value
return
}
// process value
}) { (error) in
// Handle error
}
This question already has answers here:
Returning data from async call in Swift function
(13 answers)
Closed 4 years ago.
I am new to Swift and was wondering how can I get a value from an Async task. I have a function that gets Json data from an API on return I would like to get the value of a particular field outside of the async task... My code is below essentially I have a variable called status I want to get the value of status after the async called is returned then I want to check if the value is 1 . In the code below the value returned is 1 however it seems like the async called is executed before the line if Status == 1 {} . If the value is One then I want to navigate to a different ViewController . Any suggestions would be great ... I obviously can not put the code to go to a different ViewController inside the Async code since that is called many times .
func GetData() {
var status = 0
// Code that simply contains URL and parameters
URLSession.shared.dataTask(with:request, completionHandler: {(data, response, error) in
if error != nil {
print("Error")
} else {
do {
let parsedData = try JSONSerialization.jsonObject(with: data!, options: .allowFragments) as! [String:Any]
DispatchQueue.main.async {
if let Replies = parsedData["Result"] as? [AnyObject] {
for Stream in Replies {
if let myvalue = Stream["status"] as? Int {
status = myvalue
}
}
}
}
} catch let error as NSError {
print(error)
}
}
}).resume()
if status == 1 {
// This code is executed before the async so I don't get the value
let nextViewController = self.storyboard?.instantiateViewController(withIdentifier: "Passed") as! Passed
self.present(nextViewController, animated:false, completion:nil)
}
}
You can use a callback function for that like this:
func GetData(callback: (Int) -> Void) {
//Inside async task, Once you get the values you want to send in callback
callback(status)
}
You will get a callback from where you called the function.
For your situation, Anbu's answer will work too.
I am working on a simple Flickr app that gets some data from their API and displays it on a tableview instance. Here's a piece of the code for the TableViewController subclass.
var photos = [FlickrPhotoModel]()
override func viewDidLoad() {
super.viewDidLoad()
getFlickrPhotos()
}
override func didReceiveMemoryWarning() {
super.didReceiveMemoryWarning()
}
private func getFlickrPhotos() {
DataProvider.fetchFlickrPhotos { (error: NSError?, data: [FlickrPhotoModel]?) in
//data is received
dispatch_async(dispatch_get_main_queue(), {
if error == nil {
self.photos = data!
self.tableView.reloadData()
}
})
}
}
The application does not seem to load the data if the { tableView.reloadData() } line is removed. Does anyone know why this would happen since I call getFlickrPhotos() within viewDidLoad(). I believe I am also dispatching from the background thread in the appropriate place. Please let me know what I am doing incorrectly.
EDIT -- Data Provider code
class func fetchFlickrPhotos(onCompletion: FlickrResponse) {
let url: NSURL = NSURL(string: "https://api.flickr.com/services/rest/?method=flickr.photos.getRecent&api_key=\(Keys.apikey)&per_page=25&format=json&nojsoncallback=1")!
let task = NSURLSession.sharedSession().dataTaskWithURL(url) { (data, response, error) in
if error != nil {
print("Error occured trying to fetch photos")
onCompletion(error, nil)
return
}
do {
let jsonResults = try NSJSONSerialization.JSONObjectWithData(data!, options: NSJSONReadingOptions.MutableContainers) as? NSDictionary
let photosContainer = jsonResults!["photos"] as? NSDictionary
let photoArray = photosContainer!["photo"] as? [NSDictionary]
let flickrPhoto: [FlickrPhotoModel] = photoArray!.map{
photo in
let id = photo["id"] as? String ?? ""
let farm = photo["farm"] as? Int ?? 0
let secret = photo["secret"] as? String ?? ""
let server = photo["server"] as? String ?? ""
var title = photo["title"] as? String ?? "No title available"
if title == "" {
title = "No title available"
}
let model = FlickrPhotoModel(id: id, farm: farm, server: server, secret: secret, title: title)
return model
}
//the request was successful and flickrPhoto contains the data
onCompletion(nil, flickrPhoto)
} catch let conversionError as NSError {
print("Error parsing json results")
onCompletion(conversionError, nil)
}
}
task.resume()
}
I'm not familiar with that API, but it looks like the fetchFlickrPhotos method is called asynchronously on a background thread. That means that the rest of the application will not wait for it to finish before moving on. viewDidLoad will call the method, but then move on without waiting for it to finish.
The completion handler that you provide is called after the photos are done downloading which, depending on the number and size of the photos, could be seconds later. So reloadData is necessary to refresh the table view after the photos are actually done downloading.
Let me preface this by saying I'm VERY new to Swift 2 and am building my first app which calls an api (php) for data (JSON). The problem I'm running into is when I make the call to the api the other functions ran before the api can send back the data.
I've researched some type of a onComplete to call a functions after the api response is done. I'm sure for most of you this is easy, but I cant seem to figure it our.
Thanks in advance!
class ViewController: UIViewController {
var Selects = [Selectors]()
var list = [AnyObject]()
var options = [String]()
var index = 0
#IBOutlet var Buttons: [UIButton]!
override func viewDidLoad() {
super.viewDidLoad()
self.API()
self.Render()
}
func API() {
let url = NSURL(string: "http:api.php")
let request = NSMutableURLRequest(URL: url!)
let task = NSURLSession.sharedSession().dataTaskWithRequest(request) {
data, response, error in
if data == nil {
print("request failed \(error)")
return
}
do {
let json = try NSJSONSerialization.JSONObjectWithData(data!, options: .AllowFragments)
if let songs = json["songs"] as? [[String: AnyObject]] {
for song in songs {
self.list.append(song)
}
}
self.Selects = [Selectors(Name: self.list[self.index]["name"] as? String, Options: self.BuildOptions(), Correct: 2)]
}
catch let error as NSError {
print("json error: \(error.localizedDescription)")
}
}
task.resume()
}
func BuildOptions() {
// BuildOptions stuff happens here
}
func Render() {
// I do stuff here with the data
}
}
So I assume your Render() method is called before data gets back from the api? Keeping your api-calling code in the view controllers is bad design, but as you're new i won't expand on that. In your case it's as simple as not calling your Render() method in viewDidLoad() - call it after you're done with parsing the data from JSON (after the self.Selects = [Selectors... line). NSURLSession.sharedSession().dataTaskWithRequest(request) method is called asynchronously , and the callback block with data, response, error parameters is executed after this method is done with fetching your data, so it can happen after the viewDidLoad is long done and intially had no data to work on as the asynchronous method was still waiting for response from the API.
Edit - speaking of handling api calls, it's a wise thing to keep them separated from specific view controllers to maintain a clean reusable code base. You should call the API and wait for a callback from it, so i'd just do that to your API function, it would look like this:
static func callAPI(callback: [AnyObject]? -> Void ) {
let url = NSURL(string: "http:api.php")
let request = NSMutableURLRequest(URL: url!)
let task = NSURLSession.sharedSession().dataTaskWithRequest(request) {
data, response, error in
if data == nil {
completion(nil)
}
do {
var list = [AnyObject]()
let json = try NSJSONSerialization.JSONObjectWithData(data!, options: .AllowFragments)
if let songs = json["songs"] as? [[String: AnyObject]] {
for song in songs {
self.list.append(song)
}
}
completion(list)
}
catch let error as NSError {
print("json error: \(error.localizedDescription)")
completion(nil)
}
}
task.resume()
}
Generally speaking methods should do one specific thing - in your case call the api and return data or error. Initialize your selectors in the view controllers on callback. Your view controller's viewDidLoad would look like this using the code above:
override func viewDidLoad() {
super.viewDidLoad()
YourApiCallingClass.callApi() {
result in
if let list = result {
self.list = list
self.Selects = [Selectors(Name: self.list[self.index]["name"] as? String, Options: self.BuildOptions(), Correct: 2)]
self.Render()
} else {
//Handle situation where no data will be returned, you can add second parameter to the closue in callApi method that will hold your custom errors just as the dataTaskWithRequest does :D
}
}
}
Now you have a nice separation of concerns, API method is reusable and view controller just handles what happens when it gets the data. It'd be nice if you slapped an UIActivityIndicator in the middle of the screen while waiting, it'd look all neat and professional then :P
I'm having trouble retrieving data from my Alamofire request asynchronously.
class BookGetter {
static let instance = BookGetter()
func getBook(bookId: String) -> Book {
let rootUrl = "https://www.someusefulbookapi.com/bookid=?"
let url = rootUrl + bookId
var title = ""
Alamofire.request(.GET, url).response { response in
let jsonDict = JSON(data: response.2!)
title = String(jsonDict["items"][0]["volumeInfo"]["title"])
}
let book = Book(title: title)
print(book.title)
return book
}
}
The output of print(book.title) is "", and I understand this is because the print statement is running before the request returns.
How do I get the book instance to be returned only when it is instantiated with the data from the request?
The problem you have is that you are calling an asynchronous method and expecting to return the result synchronously. When your code is executed, the getBook function completes and returns before even the GET request has complete.
Basically, you have two options:
Update your getBook method to be asynchronous and return the result with a completion block/callback
Wait for the asynchronous call to complete, blocking the current thread (this is OK as long as it is not the main thread you are blocking), and return the result synchronously.
1. Update your method to be asynchronous
To do this, you must return the result on a block/callback function.
class BookGetter {
static let instance = BookGetter()
func getBook(bookId: String, complete: (book: Book?, error: NSError?) -> Void) {
let rootUrl = "https://www.someusefulbookapi.com/bookid=?"
let url = rootUrl + bookId
var title = ""
Alamofire.request(.GET, url).response { request, response, data, error in
// TODO: You should check for network errors here
// and notify the calling function and end-user properly.
if error != nil {
complete(book: nil, error: error as? NSError)
return
}
let jsonDict = JSON(data: response.2!)
title = String(jsonDict["items"][0]["volumeInfo"]["title"])
let book = Book(title: title)
print(book.title)
complete(book: book, error: nil)
}
}
}
As mentioned in the above code, ideally you should handle errors in the callback response (including exceptions while parsing the JSON). Once handled, you can update the callback parameters to (book: Book?, error: NSError?) -> Void or similar, and check for book or error to be set on the caller function.
To call the function, you need to pass a block to handle the response:
BookGetter.instance.getBook("bookID") { (book, error) in
if error != nil {
// Show UIAlertView with error message (localizedDescription)
return
}
// Update User Interface with the book details
}
2. Wait for the asynchronous call to complete
As mentioned above, this is a good idea only if you were running this code on a background thread. It is OK to block background threads, but it is never OK to block the main thread on a graphic application, as it will freeze the user interface. If you do not know what blocking means, please use the option #1.
class BookGetter {
static let instance = BookGetter()
func getBook(bookId: String) -> Book {
let rootUrl = "https://www.someusefulbookapi.com/bookid=?"
let url = rootUrl + bookId
var title = ""
let semaphore = dispatch_semaphore_create(0)
Alamofire.request(.GET, url).response { response in
let jsonDict = JSON(data: response.2!)
title = String(jsonDict["items"][0]["volumeInfo"]["title"])
dispatch_semaphore_signal(semaphore)
}
//Wait for the request to complete
while dispatch_semaphore_wait(semaphore, DISPATCH_TIME_NOW) != 0 {
NSRunLoop.currentRunLoop().runMode(NSDefaultRunLoopMode, beforeDate: NSDate(timeIntervalSinceNow: 10))
}
let book = Book(title: title)
print(book.title)
return book
}
}
You can use closures and return a completionHandler with your book like in the following way:
func getBook(bookId: String, completionHandler: (book: Book?) -> ()) {
let rootUrl = "https://www.someusefulbookapi.com/bookid=?"
let url = rootUrl + bookId
Alamofire.request(.GET, url).response { response in completionHandler(
book:
{
// In this block you create the Book object or returns nil in case of error
if response == nil {
return nil
}
let jsonDict = JSON(data: response.2!)
let title = String(jsonDict["items"][0]["volumeInfo"]["title"])
let book = Book(title: title)
return book
}())
}
}
And then you can call it like in the following way:
getBook("idOfYourBook") { book in
if let book = book {
println(book.title)
}
}
I hope this help you.