Swift - Parsing JSON and retrieve data - json

Question

I am working with Swift on Xcode and I try to parse a JSON file to retrieve some data about nearby stores.
My source code is the following:
import GooglePlaces
import SwiftyJSON
class Place {
let name: String
let coordinates: CLLocationCoordinate2D
init(diction:[String : Any])
{
let json = JSON(diction)
name = json["name"].stringValue //as! String
let lat = json["geometry"]["location"]["lat"].doubleValue as CLLocationDegrees
let long = json["geometry"]["location"]["lng"].doubleValue as CLLocationDegrees
coordinates = CLLocationCoordinate2DMake(lat, long)
}
}
class ViewController: UIViewController, MKMapViewDelegate, SceneLocationViewDelegate {
var urlString = "https://maps.googleapis.com/maps/api/place/nearbysearch/json?"
urlString += "&location=51.507514,-0.073603"
urlString += "&radius=1500" //meters
urlString += "&name=Specsavers"
urlString += "&key=**************************"
guard let url = URL(string: urlString) else {return}
var places = [Place]()
var request = URLRequest(url:url)
request.httpMethod = "GET"
let task = URLSession.shared.dataTask(with: request as URLRequest) {
data, response, error in
print("HEREurlSession")
if let content = data {
do {
let json = try JSONSerialization.jsonObject(with: content, options: JSONSerialization.ReadingOptions.mutableContainers) as AnyObject
print(json) // json results are printed fine here
if let results = json["results"] as? [[String : Any]] {
for place in results {
places.append(Place(diction: place))
}
}
else {
print("return")
}
}
catch{
}
}
}
task.resume()
let size = places.count
print("HERE: ", size)
}
The build is successful but the output is size = 0 which means that I do not retrieve the data and the variable places is empty.
I do not know if it is exactly relevant but I get the following warning: Cast from 'MDLMaterialProperty?!' to unrelated type '[[String : Any]]' always fails for the line if let results = json["results"] as? [[String : Any]] in my source code.
Why I do not parse the JSON file correctly and I do not retrieve the data the I want to?

Answer 1

URLSession.shared.dataTask(with:) is asynchronous. This means, it runs in the background. You are executing
let size = places.count
print("HERE: ", size)
while the dataTask is still working.
Instead, you should use your result in the completion handler:
let task = URLSession.shared.dataTask(with: request as URLRequest) {
data, response, error in
print("HEREurlSession")
if let content = data {
do {
let json = try JSONSerialization.jsonObject(with: content, options: JSONSerialization.ReadingOptions.mutableContainers) as AnyObject
print(json)
if let results = json["results"] as? [[String : Any]] {
for place in results {
places.append(Place(diction: place))
}
}
else {
print("return")
}
}
catch{
}
}
// Use your result here
let size = places.count
useResultSize(size)
}
task.resume()
func useResultSize(_ size: Int) {
// Use your result here
print("HERE: ", size)
}
UPDATE
It seems, that you are missing what asynchronous execution actually means. Let me try to explain.
Lets mark the execution order in the code:
First, the red parts of your code are executed. Program execution starts at the top, then moves to the bottom red box and only after that (once the network request is finished) the green part is executed.
That means, that you can only use the result of the network request in the green part of the code. Outside of the green part, the result is not guaranteed to be available.
If you follow my initial advice, than everything should work. Please see the successful execution in my playground: