URLSession Cancellation in swift while auto search - ios

Question

My moto is Auto search, I have been trying with URLSession, When i am trying to search slowly the requests are handled as expected(when there is no text the response is empty i mean the placesarray) but when i am trying to clear the text or hit any searchtext speedily then the previous request response are being appended in the placesarray. I tried with cancelling the previous request yet i am not getting the result(i.e previous response not be appended)
func autoSearch(text:String){
let urlRequest = URLRequest(url: self.getQueryFormedBingURL()!)
let session = URLSession.shared
session.getTasksWithCompletionHandler
{
(dataTasks, uploadTasks, downloadTasks) -> Void in
// self.cancelTasksByUrl(tasks: dataTasks as [URLSessionTask])
self.cancelTasksByUrl(tasks: uploadTasks as [URLSessionTask])
self.cancelTasksByUrl(tasks: downloadTasks as [URLSessionTask])
}
let task = session.dataTask(with: urlRequest, completionHandler: { (data, response, error) -> Void in
print("response \(response)")
if let data = data {
let json = try? JSONSerialization.jsonObject(with: data, options: [])
if let response = response as? HTTPURLResponse , 200...299 ~= response.statusCode {
if let jsonDic = json as? NSDictionary {
let status = jsonDic.returnsObjectOrNone(forKey: "statusCode") as! Int
if status == 200 {
if let resourceSetsArr = jsonDic.returnsObjectOrNone(forKey: "resourceSets") as? NSArray {
if let placesDict = resourceSetsArr.object(at: 0) as? NSDictionary {
if let resourceArr = placesDict.object(forKey: "resources") as? NSArray, resourceArr.count > 0 {
if let _ = self.placesArray {
self.placesArray!.removeAll()
}
for loopCounter in 0...resourceArr.count - 1 {
let modalClass:BingAutoCompletePlace = BingAutoCompletePlace(responseDict: resourceArr[loopCounter] as! NSDictionary)
self.placesArray?.append(modalClass)
}
completion(self.placesArray!)
}
else { //URL Success, where there no places with the given search string
completion([])
}
}
}
}
}
}
else if let response = response as? HTTPURLResponse , 400...499 ~= response.statusCode {// When url fails
if let _ = error {
print("error=\(error!.localizedDescription)")
}
completion([])
}
else {
if let _ = error {
print("error=\(error!.localizedDescription)")
}
completion([])
}
}
})
task.resume()
}
//Request cancellation
private func cancelTasksByUrl(tasks: [URLSessionTask]) {
for task in tasks
{
task.cancel()
}
}

Answer 1

Unfortunately, the framework does not guarantee any order in which tasks finish -- because this depends on the running time. It could also be that you're in a completion handler of a currently cancelled task.
To circumvent this, you could do the following:
Create a private instance variable to store the most-recent task
Cancel everything else as before
In the completion handler
check if the task is still the most recent task (like if (task !== self.currentTask) {return})
create a local Array to store the data
Update the view controllers array in the main thread (DispatchQueue.main.async(...))
I cleaned up you code a litte (using guard statments to minimize the nesting). Maybe you should also
Empty the array in all the error / empty cases (instead of simple return from the guard statement)
hand-in the task to the completion call and check there again if the task is still the currentTask. This would also be a good way to reset currentTask to nil.
Just adopt it to your needs :-)
var currentTask:URLSessionDataTask?
func autoSearch(text:String){
let completion:(_ x:[AnyObject])->() = {_ in }
let urlRequest = URLRequest(url: self.getQueryFormedBingURL()!)
let session = URLSession.shared
session.getTasksWithCompletionHandler
{
(dataTasks, uploadTasks, downloadTasks) -> Void in
// self.cancelTasksByUrl(tasks: dataTasks as [URLSessionTask])
self.cancelTasksByUrl(tasks: uploadTasks as [URLSessionTask])
self.cancelTasksByUrl(tasks: downloadTasks as [URLSessionTask])
}
var task:URLSessionDataTask!
task = session.dataTask(with: urlRequest, completionHandler: { (data, response, error) -> Void in
print("response \(response)")
if (task !== self.currentTask) {
print("Ignore this task")
return
}
if let error = error {
print("response error \(error)")
}
guard let data = data else { return }
let json = try? JSONSerialization.jsonObject(with: data, options: [])
var newPlacesArray = [AnyObject]() // Empty array of whichever type you want
if let response = response as? HTTPURLResponse , 200...299 ~= response.statusCode {
guard let jsonDic = json as? NSDictionary else { return }
let status = jsonDic.returnsObjectOrNone(forKey: "statusCode") as! Int
if status == 200 {
guard let resourceSetsArr = jsonDic.returnsObjectOrNone(forKey: "resourceSets") as? NSArray else { return }
guard let placesDict = resourceSetsArr.object(at: 0) as? NSDictionary else { return }
guard let resourceArr = placesDict.object(forKey: "resources") as? NSArray, resourceArr.count > 0 else {
//URL Success, where there no places with the given search string
DispatchQueue.main.async {completion(newPlacesArray)}
return
}
for loopCounter in 0...resourceArr.count - 1 {
let modalClass:BingAutoCompletePlace = BingAutoCompletePlace(responseDict: resourceArr[loopCounter] as! NSDictionary)
newPlacesArray.append(modalClass)
}
DispatchQueue.main.async {completion(newPlacesArray)}
}
}
else if let response = response as? HTTPURLResponse , 400...499 ~= response.statusCode {// When url fails
if let _ = error {
print("error=\(error!.localizedDescription)")
}
DispatchQueue.main.async {completion(newPlacesArray)}
}
else {
if let _ = error {
print("error=\(error!.localizedDescription)")
}
DispatchQueue.main.async {completion(newPlacesArray)}
}
})
self.currentTask = task
task.resume()
}