How to wait the function execution until json loaded in Swift? - arrays

Question

I am getting json file from url, then, all json output will be added in a array. When I print the array, it says that array is empty, but actually array is not empty I know. If I use like: DispatchQueue.main.asyncAfter(deadline: .now() + 0.5) { }, then, I can print the array.
My code to get json and adding its values to the array:
var allTags:Array<String> = []
func getJSON(getUrl:URL){
let url:URL = getUrl
let session = URLSession.shared
var request = URLRequest(url: url)
request.httpMethod = "GET"
request.cachePolicy = NSURLRequest.CachePolicy.reloadIgnoringCacheData
let task = session.dataTask(with: request as URLRequest) {
(data, response, error) in
let dataString = String(data: data, encoding: String.Encoding.utf8)!
do {
let data = Data(dataString.utf8)
let dictionaries = (try? JSONSerialization.jsonObject(with: data)) as? [[String:Any]] ?? []
for dict in dictionaries {
let tags = dict["Tags"] as? [String] ?? []
for tag in tags {
allTags.append(tag)
}
}
}
}task.resume()
}
getJSON(getUrl: self.jsonUrl())
print(self.allTags.count) //it prints 0, but there are some data I know
DispatchQueue.main.asyncAfter(deadline: .now() + 0.5) {
print(self.allTags.count) //it prints 168
}

Answer 1

Use a callback:
var allTags:Array<String> = []
func getJSON(getUrl:URL, callback: #escaping () -> Void) {
let url:URL = getUrl
let session = URLSession.shared
var request = URLRequest(url: url)
request.httpMethod = "GET"
request.cachePolicy = NSURLRequest.CachePolicy.reloadIgnoringCacheData
let task = session.dataTask(with: request as URLRequest) { (data, response, error) in
let dataString = String(data: data, encoding: String.Encoding.utf8)!
do {
let data = Data(dataString.utf8)
let dictionaries = (try? JSONSerialization.jsonObject(with: data)) as? [[String:Any]] ?? []
for dict in dictionaries {
let tags = dict["Tags"] as? [String] ?? []
for tag in tags {
allTags.append(tag)
}
}
callback()
}
}
task.resume()
}
getJSON(getUrl: self.jsonUrl()) {
print(self.allTags.count) // Should work now
}

Answer 2

var allTags:Array<String> = []
func getJSON(getUrl:URL, completion: #escaping () -> ()) {
let url:URL = getUrl
let session = URLSession.shared
var request = URLRequest(url: url)
request.httpMethod = "GET"
request.cachePolicy = NSURLRequest.CachePolicy.reloadIgnoringCacheData
let task = session.dataTask(with: request as URLRequest) {
(data, response, error) in
let dataString = String(data: data, encoding: String.Encoding.utf8)!
do {
let data = Data(dataString.utf8)
let dictionaries = (try? JSONSerialization.jsonObject(with: data)) as? [[String:Any]] ?? []
for dict in dictionaries {
let tags = dict["Tags"] as? [String] ?? []
for tag in tags {
allTags.append(tag)
}
}
completion()
}
}task.resume()
}
getJSON(getUrl: self.jsonUrl()) { [weak self] in
guard let strongSelf = self else { return }
print(strongSelf.allTags.count)
}