Use JSON data offline Swift 3 - ios

Question

I'm a beginner so please be gentle.
Ok so I successfully parse my JSON from the server and can access all of its contents but know I'm kind of stuck here:
//Functionn for fetching API data
func fetchRates(){
//Set EndPoint URL
let url = URL(string: endPoint)
URLSession.shared.dataTask(with: url!) { (data, response, error) in
if error != nil {
print(error as Any)
}
do {
let json = try? JSONSerialization.jsonObject(with: data!, options: [])
let dictonary = json as? [String: Any?]
let ratesJson = dictonary?["rates"] as? [String: Double]
print(ratesJson as Any)
} catch let jsonError {
print(jsonError)
}
}.resume()
}
Now I want to get my ratesJson Dictionary out of this function and actually use its contents to do some calculations, but after hours of trying and searching I still have no idea how.
I just want to:
Get JSON data stored presistently (for offline use)
Update the Data when the app launches and overwrite the old data
The possibilities I've come across so far are CoreData, which seems really confusing and complicated and Realm which I don't really know at all.
Help would be very much appreciated and sorry if this seems really stupid

Answer 1

Since you have the JSON parsing, I would create a object class to store the data using Core Data. There is a lot that goes into core data usage at first, but it is well worth the time investment to learn if you plan on continuing iOS development. Core Data Tuts
I also do not know what you plan on doing with your app, or the purpose, and Core Data might be a little overkill according to your purpose, but you could always save the updated JSON each time, overwriting a file in the device's document's dir, then read the data back in and parse it again.
You can write the JSON data to a outputstream using:
JSONSerialization.writeJSONObject(_ obj: Any, to stream: OutputStream, options opt: JSONSerialization.WritingOptions = [], error: NSErrorPointer)
//Write JSON data into a stream. The stream should be opened and configured.
//The return value is the number of bytes written to the stream, or 0 on error.
//All other behavior of this method is the same as the dataWithJSONObject:options:error: method.
UPDATE: Ok I think I see your question now. There are two ways that you can go about getting your data back to your view controller: 1. Delegate pattern 2.Closures. I personally like using delegates over closures.
With delegates you will need to create a protocol:
protocol SessionDataTaskComplete {
func dataDownloaded(parsedJson : [String : Double])
}
And inside of your URLSession class you will need a class variable to hold that protocol delegate:
var dataTaskCompleteDelegate : SessionDataTaskComplete?
Then you will need to have your view controller implement that protocol:
class MyViewController: UIViewController, SessionDataTaskComplete {
override func viewDidLoad() {
super.viewDidLoad()
//Also assign your viewController as your URLSession's SessionDataTaskComplete delegate
myURLSession.dataTaskCompleteDelegate = self
}
func dataDownloaded(parsedJson : [String : Double]) {
//Handle data as you wish here
}
}
Now in your fetchRates() you can use that delegate to pass the data back to your viewController:
func fetchRates(){
//Set EndPoint URL
let url = URL(string: endPoint)
URLSession.shared.dataTask(with: url!) { (data, response, error) in
if error != nil {
print(error as Any)
}
do {
let json = try? JSONSerialization.jsonObject(with: data!, options: [])
let dictonary = json as? [String: Any?]
let ratesJson = dictonary?["rates"] as? [String: Double]
//print(ratesJson as Any)
if (dataTaskCompleteDelegate != nil) {
dataTaskCompleteDelegate!.dataDownloaded(parsedJson: ratesJson)
}
} catch let jsonError {
print(jsonError)
}
}.resume()
}
If you are not comfortable with using the delegate pattern, that is something else I would suggest spending time learning, as it is used all over with the iOS SDK. Try to think of them as an object assigning a duty/task to another object.