I am working with the GoogleMaps API, this returns 20 nearby locations and up to 60 locations can be returned. The locations are returned with a nextPageToken which allows you to GET the next page of results (20 per page). I am trying to loop through the API to enable myself to get all the locations available but am having difficulty:
func getAllNearbyLocations(url: URL) {
I am using Alamofire to return the API request (I have also tried using URLSessions)
First I put together a function which returns the json dictionary in the completion block
// This function returns the JSON from a specific URL
func getJsonFromURL(url: URL, completionHandler: #escaping (NSDictionary) -> ()) {
Alamofire.request(url).responseJSON { response in
let json = response.result.value as! NSDictionary
completionHandler(json)
}
}
Next we have a getNearByLocation function which we initialise with a url. As you can see we return the results, add them to an array, check if we have the max number of results (60) or don't have a nextPageToken any more. If either of these are false we create the new URL and fire the function we are currently in again. The loop finishes when we return all the new locations.
func getAllNearbyLocations(url: URL) {
self.getJsonFromURL(url: url) { (dictionary) in
let newLocations: NSArray = dictionary.value(forKey: "results") as! NSArray
self.locations.addObjects(from: newLocations as! [Any])
if self.locations.count >= 60 {
self.sendNewLocations(locations: self.locations)
}
else {
// We want to now update the URL we are using and search again
if let newPageToken = dictionary["next_page_token"] {
let newURL = self.rootSearchURL + "&pagetoken=" + (newPageToken as! String)
let url = URL(string: newURL)
// We want to get our current URL and remove the last characters from it
self.getAllNearbyLocations(url: url!)
}
else {
// If we have no more pages then we return what we have
self.sendNewLocations(locations: self.locations)
}
}
}
}
The strange thing is when I test the code with breakpoints it returns as expected. It loops through the function, adds all the new locations and returns. When I run it in realtime the returned dictionary doesn't return properly (doesn't contain the locations or next page token) and so my function only returns the first 20 locations.
I have used API requests before but never so close in succession. I feel that it is a catch 22 as I can't know the new pageToken until I have called the request and as soon as I have returned the request I want to call the request with that token immediately.
As per the Documentation,
There is a short delay between when a next_page_token is issued, and when it will become valid. Requesting the next page before it is available will return an INVALID_REQUEST response. Retrying the request with the same next_page_token will return the next page of results.
Try to check what you are getting when you are retrieving data with new_page_token
Try to call like this :
let newURL = self.rootSearchURL + "&pagetoken=" + (newPageToken as! String)
let url = URL(string: newURL)
DispatchQueue.main.asyncAfter(deadline: DispatchTime.now() + 3) {
// We want to get our current URL and remove the last characters from it
self.getAllNearbyLocations(url: url!)
}
Related
I'm trying to create a feed that should include posts made by people a user follows, and I'm using following function to get posts for user feed:
func observePosts(for userID: String, completion: #escaping ([UserPost], String) -> Void) {
let databaseKey = "Posts/\(userID)/"
databaseRef.child(databaseKey).queryOrderedByKey().queryLimited(toLast: 12).observe(.value) { snapshot in
var retrievedPosts = [UserPost]()
var lastSeenPostKey = ""
let dispatchGroup = DispatchGroup()
for snapshotChild in snapshot.children.reversed() {
guard let postSnapshot = snapshotChild as? DataSnapshot,
let postDictionary = postSnapshot.value as? [String: Any]
else {
print("Couldn't cast snapshot as DataSnapshot")
return
}
dispatchGroup.enter()
lastSeenPostKey = postSnapshot.key
do {
let jsonData = try JSONSerialization.data(withJSONObject: postDictionary as Any)
let decodedPost = try JSONDecoder().decode(UserPost.self, from: jsonData)
UserService.shared.observeUser(for: decodedPost.userID) { postAuthor in
decodedPost.author = postAuthor
retrievedPosts.append(decodedPost)
dispatchGroup.leave()
}
} catch {
print("Couldn't create UserPost from postDictionary")
}
}
dispatchGroup.notify(queue: .main) {
completion(retrievedPosts, lastSeenPostKey)
}
}
}
First step I'm getting user feed posts with .observe function and after that I'm iterating through every received post calling a UserService.shared.observeUser method that observes user data for every post to set post author's profile picture and username.
The problem with my method is that I'm using DispatchGroup's .enter and .leave methods to notify once every observe function got user data so that I could call completion block in which I'm reloading tableView.
Sure it works fine first time but since it's observe function it's called every time there's a change to user so once user updates his data (profile pic, usernama etc.) it crashes on dispatchGroup.leave() line since it's not balanced with dispatchGroup.enter()
What would be a correct approach to solve this issue?
I've been trying to solve this problem for 3 days searching all the internet, and only advice I found is to fetch it one time using observeSingleValue() instead of observing, but I really need to observe user profile data to keep profile pics and other necessary data up to date.
Hello new to Swift and Alamofire,
The issue i'm having is when I call this fetchAllUsers() the code will return the empty users array and after it's done executing it will go inside the AF.request closure and execute the rest.
I've done some research and I was wondering is this is caused by Alamofire being an Async function.
Any suggestions?
func fetchAllUsers() -> [User] {
var users = [User]()
let allUsersUrl = baseUrl + "users/"
if let url = URL(string: allUsersUrl) {
AF.request(url).response { response in
if let data = response.data {
users = self.parse(json: data)
}
}
}
return users
}
You need to handle the asynchrony in some way. This this means passing a completion handler for the types you need. Other times it means you wrap it in other async structures, like promises or a publisher (which Alamofire also provides).
In you case, I'd suggest making your User type Decodable and allow Alamofire to do the decoding for you.
func fetchAllUsers(completionHandler: #escaping ([User]) -> Void) {
let allUsersUrl = baseUrl + "users/"
if let url = URL(string: allUsersUrl) {
AF.request(url).responseDecodable(of: [User].self) { response in
if let users = response.value {
completionHandler(users)
}
}
}
}
However, I would suggest returning the full Result from the response rather than just the [User] value, otherwise you'll miss any errors that occur.
I've never really gotten the nuances of async operations so time and again, I get stymied. And I just can't figure it out.
I'm trying to do some very simple web scraping.
My local volleyball association has a page (verbose HTML, not responsive, not mobile-friendly, yaddah, yaddah, yaddah) which shows the refs assigned to each game of the season. I'm trying to write a silly little app which will scrape that page (no API, no direct access to db, etc.) and display the data in a grouped table. The first group will show today's matches (time, home team, away team). The second group will show tomorrow's matches. Third group shows the entire season's matches.
Using code I found elsewhere, my viewDidLoad loads the page, scrapes the data and parses it into an array. Once I've parsed the data, I have three arrays: today, tomorrow, and matches, all are [Match].
override func viewDidLoad() {
super.viewDidLoad()
let url = URL(string: urlString)!
let request = NSMutableURLRequest(url: url)
let task = URLSession.shared.dataTask(with: request as URLRequest) {
data, response, error in
if let error = error {
print (error)
} else {
if let unwrappedData = data {
// scrape, scrape, parse, parse
matchRow = ...
self.matches.append(matchRow)
if matchRow.date == todaysDate {
self.today.append(matchRow)
} else if matchRow.date == tomorrowsDate {
self.tomorrow.append(matchRow)
}
}
}
}
task.resume()
}
As I'm sure is no surprise to anyone who understands async operations, my table is empty. I've checked and I see the the data is there and properly parsed, etc. But I can't for the life of me figure out how get the data in my table. The way I have it now, the data is not ready when numberOfSections or numberOfRowsInSection is called.
I've found the Ray Wenderlich tutorial on URLSession and I also have a Udemy course (Rob Percival) that builds an app to get the weather using web scraping, but in both those instances, the app starts and waits for user input before going out to the web to get the data. I want my app to get the data immediately upon launch, without user interaction. But I just can't figure out what changes I need to make so that those examples work with my program.
Help, please.
You can simply reload the tableviews once the data arrays are getting populated from the URLSession completion block. Have you tried that. Sample snippet may be like the one follows.
let task = URLSession.shared.dataTask(with: request as URLRequest) {
data, response, error in
if let error = error {
print (error)
} else {
if let unwrappedData = data {
// scrape, scrape, parse, parse
matchRow = ...
self.matches.append(matchRow)
if matchRow.date == todaysDate {
self.today.append(matchRow)
} else if matchRow.date == tomorrowsDate {
self.tomorrow.append(matchRow)
}
}
DispatchQueue.main.async { [weak self] in
self?.todayTableView.reloadData()
self?.tomorrowTableView.reloadData()
}
}
}
For one week I have been trying to get a string returned from dataTask().
I already read a lot here on StackOverFlow and also from serval sites where they tackle this topic. For example, this one. So I already understand that it's that the dataTask doesn't directly return values, cause it happens on different threads and so on. I also read about closures and completion handlers. I really got the feeling that I actually already got a little clue what this is about. But I can't get it to work.
So this is my code. I just post the whole code so no-one needs to worry that the problem sticks in a part which I don't show. Everything is working fine until I try to return a value and save it for example in a variable:
func requestOGD(code gtin: String, completion: #escaping (_ result: String) -> String) {
// MARK: Properties
var answerList: [String.SubSequence] = []
var answerDic: [String:String] = [:]
var product_name = String()
var producer = String()
// Set up the URL request
let ogdAPI = String("http://opengtindb.org/?ean=\(gtin)&cmd=query&queryid=400000000")
guard let url = URL(string: ogdAPI) else {
print("Error: cannot create URL")
return
}
let urlRequest = URLRequest(url: url)
// set up the session
let config = URLSessionConfiguration.default
let session = URLSession(configuration: config)
// make the request
let task = session.dataTask(with: urlRequest) {
(data, response, error) in
// check for any errors
guard error == nil else {
print("error calling GET on /todos/1")
print(error!)
return
}
// make sure we got data
guard let responseData = data else {
print("Error: did not receive data")
return
}
// parse the result, which is String. It willbecome split and placed in a dictionary
do {
let answer = (String(decoding: responseData, as: UTF8.self))
answerList = answer.split(separator: "\n")
for entry in answerList {
let entry1 = entry.split(separator: "=")
if entry1.count > 1 {
let foo = String(entry1[0])
let bar = String(entry1[1])
answerDic[foo] = "\(bar)"
}
}
if answerDic["error"] == "0" {
product_name = answerDic["detailname"]!
producer = answerDic["vendor"]!
completion(product_name)
} else {
print("Error-Code der Seite lautet: \(String(describing: answerDic["error"]))")
return
}
}
}
task.resume()
Here I call my function, and no worries, I also tried to directly return it to the var foo, also doesn't work The value only exists within the closure:
// Configure the cell...
var foo:String = ""
requestOGD(code: listOfCodes[indexPath.row]) { (result: String) in
print(result)
foo = result
return result
}
print("Foo:", foo)
cell.textLabel?.text = self.listOfCodes[indexPath.row] + ""
return cell
}
So my problem is, I have the feeling, that I'm not able to get a value out of a http-request.
You used a completion handler in your call to requestOGD:
requestOGD(code: listOfCodes[indexPath.row]) {
(result: String) in
// result comes back here
}
But then you tried to capture and return that result:
foo = result
return result
So you're making the same mistake here that you tried to avoid making by having the completion handler in the first place. The call to that completion handler is itself asynchronous. So you face the same issue again. If you want to extract result at this point, you would need another completion handler.
To put it in simple terms, this is the order of operations:
requestOGD(code: listOfCodes[indexPath.row]) {
(result: String) in
foo = result // 2
}
print("Foo:", foo) // 1
You are printing foo before the asynchronous code runs and has a chance to set foo in the first place.
In the larger context: You cannot use any asynchronously gathered material in cellForRowAt. The cell is returned before the information is gathered. That's what asynchronous means. You can't work around that by piling on further levels of asynchronicity. You have to change your entire strategy.
How i can return value from function used Alamofire . i try to print outside .responseJSON the value in ArrData is not set but i try print inside it work
this code:
func getDept()->NSMutableArray
{
var ArrData:NSMutableArray = []
let url = "http://www.xxxxxxxxxxxxx.com"
Alamofire.request(.GET, url).responseJSON { response in
let json = JSON(response.result.value!)
let count = json.count
for var index = 0; index < count;index++
{
ArrData.addObject(json[index]["dept"].stringValue)
}
}
return ArrData
}
it i good idea to check at least README.md of the framework which you are going to use in your code
Networking in Alamofire is done asynchronously. Asynchronous
programming may be a source of frustration to programmers unfamiliar
with the concept, but there are very good reasons for doing it this
way.
Rather than blocking execution to wait for a response from the server,
a callback is specified to handle the response once it's received. The
result of a request is only available inside the scope of a response
handler. Any execution contingent on the response or data received
from the server must be done within a handler.
Try to use a handler like this and a callback:
func getopt(callback:(array: [String]) -> void ){
func completion(request: NSURLRequest?, response:NSHTTPURLResponse?,result:Result<AnyObject>){
if let rdata = result.value{
let data = JSON(rdata)
print(data)
let myArray = [String]
let objects = data.array
for object in objects{
myArray.append(object)
}
callback(myArray)
}
}
let url = "http://www.xxxxxxxxxxxxx.com"
Alamofire.request(.GET,url),
encoding: .JSON).responseJSON(completionHandler: completion)
}
You pass the array to your callback So when you call getopt where you call it you can print the array. Some like this:
func something (){
getopt(callback)
}
func callback(array:[String]){
print array[0]
}