How to define a fallback case if a remote GET request fails? - ios

Question

I recently started with iOS development, and I'm currently working on adding new functionality to an existing app. For this feature I need to obtain a JSON file from a web server. However, if the server is unreachable (no internet/server unavailable/etc), a local JSON needs to be used instead.
In my current implementation I tried using a do catch block, but if there's no internet connection, the app just hangs instead of going to the catch block. JSON parsing and local data reading seem to work fine, the problem is likely in the GET method, as I tried to define a callback to return the JSON data as a separate variable, but I'm not sure if that's the correct way.
What is the best way to handle this scenario?
let url = URL(string: "https://jsontestlocation.com") // test JSON
do {
// make a get request, get the result as a callback
let _: () = getRemoteJson(requestUrl: url!, requestType: "GET") {
remoteJson in
performOnMainThread {
self.delegate.value?.didReceiveJson(.success(self.parseJson(jsonData: remoteJson!)!))
}
}
}
catch {
let localFile = readLocalFile(forName: "local_json_file")
let localJson = parseJson(jsonData: localFile!)
if let localJson = localJson {
self.delegate.value?.didReceiveJson(.success(localJson))
}
}
getRemoteJson() implementation:
private func getRemoteJson(requestUrl: URL, requestType: String, completion: #escaping (Data?) -> Void) {
// Method which returns a JSON questionnaire from a remote API
var request = URLRequest(url: requestUrl) // create the request
request.httpMethod = requestType
// make the request
let task = URLSession.shared.dataTask(with: request) { (data, response, error) in
// check if there is any error
if let error = error {
print("GET request error: \(error)")
}
// print the HTTP response
if let response = response as? HTTPURLResponse {
print("GET request status code: \(response.statusCode)")
}
guard let data = data else {return} // return nil if no data
completion(data) // return
}
task.resume() // resumes the task, if suspended
}
parseJson() implementation:
private func parseJson(jsonData: Data) -> JsonType? {
// Method definition
do {
let decodedData = try JSONDecoder().decode(JsonType.self, from: jsonData)
return decodedData
} catch {
print(error)
}
return nil
}

Answer 1

If you don't have to use complex logic with reachability, error handling, request retry etc. just return nil in your completion in case of data task, HTTP and No data errors:
func getRemoteJson(requestUrl: URL, requestType: String, completion: #escaping (Data?) -> Void) {
var request = URLRequest(url: requestUrl)
request.httpMethod = requestType
let task = URLSession.shared.dataTask(with: request) { (data, response, error) in
// Task error
guard error == nil else {
print("GET request error: \(error!)")
completion(nil)
return
}
// HTTP error
guard let response = response as? HTTPURLResponse, response.statusCode == 200 else {
print("GET request failed: \(response!.description)")
completion(nil)
return
}
// No data
guard let data = data else {
completion(nil)
return
}
completion(data)
}
task.resume()
}
let url = URL(string: "https://jsontestlocation.com")!
getRemoteJson(requestUrl: url, requestType: "GET") { remoteJson in
if let json = remoteJson {
print(json)
...
}
else {
print("Request failed")
...
}
}

Answer 2

func NetworkCheck() -> Bool {
var isReachable = false
let reachability = Reachability()
print(reachability.status)
if reachability.isOnline {
isReachable = true
// True, when on wifi or on cellular network.
}
else
{
// "Sorry! Internet Connection appears to be offline
}
return isReachable
}
Call NetworkCheck() before your API request. If It returns false, read your local json file. if true do remote API call.
Incase after remote API call, any failure check with HTTP header response code.
if let httpStatus = response as? HTTPURLResponse, httpStatus.statusCode != 200 {
}

Answer 3

I think you need to stop the request from hanging when it’s waiting for a response. The app might be running on a poor connection and be able to get some but not all the data in which case you likely want to failover to the local JSON.
I think you can roughly use what you have but add a timeout configuration on the URLSession as described here: https://stackoverflow.com/a/23428960/312910