-- swift: async + JSON + completion + DispatchGroup - json

Question

The code in my viewcontroller-class is executed before the JSON-download-process is ready even though there is a completion handler in the func for downloading JSON an a DispatchGroup(). I store the JSON-data in an array called "fetchedModules" and this is filled with 11 items in this case. Why does this happen?
result in console:
---> in Class PostCell - func numberOfSections: 0
JSON call finished
// ViewController
override func viewDidLoad() {
super.viewDidLoad()
let group = DispatchGroup()
group.enter()
self.fetchJSON()
// here calling downloadJSONasync
group.leave()
group.notify(queue: .main) {
print("JSON call finished")
}
...
// networkService with completion
func downloadJSONasync(searchItem: String, completion: #escaping ([NSDictionary]) -> Void) {
//request.cachePolicy = URLRequest.CachePolicy.reloadIgnoringLocalCacheData
request.httpMethod = "GET"
let configuration = URLSessionConfiguration.default
//let session = URLSession(configuration: configuration, delegate: nil)
let session = URLSession(configuration: configuration)
let task = session.dataTask(with: request, completionHandler: {(data, response, error) in
guard let data = data, error == nil else { return }
if (error != nil) {
print("error!")
}
else{
do {
let fetchedData = try JSONSerialization.jsonObject(with: data, options: JSONSerialization.ReadingOptions.allowFragments) as! [NSDictionary]
completion(fetchedData)
}
catch {
print("error")
}
}
})
task.resume()
}
// call in viewController
override func numberOfSections(in tableView: UITableView) -> Int {
print("---> in Class PostCell - func numberOfSections: \(String(describing: fetchedModules.count))")
return fetchedModules.count
// code of fetchJSON
func fetchJSON()
{
let baseurl = AppConstants.Domains.baseurl // https://m.myapp2go.de
let compositURL = baseurl + "getmodules_noItems.php?id=\(AppConstants.appString.startString)"
let encodedUrl : String! = compositURL.addingPercentEncoding(withAllowedCharacters: CharacterSet.urlQueryAllowed) // remove the spaces in the url string for safty reason
let JSONurl = URL(string: encodedUrl)! // convert the string into url
var JSONrequest = URLRequest(url: JSONurl) // make request
JSONrequest.httpMethod = "GET"
//JSONrequest.cachePolicy = .reloadIgnoringCacheData
let networkService = NetworkService(request: JSONrequest)
networkService.downloadJSONasync(searchItem: AppConstants.appString.startString, completion: { (fetchedData) in
fetchedModules.removeAll()
DispatchQueue.main.async {
for eachFetchedModul in fetchedData {
let eachModul = eachFetchedModul
if
let custid = eachModul["custid"] as? String,
let modulcat = eachModul["modulcat"] as? String,
let modulname = eachModul["modulname"] as? String,
let comment = eachModul["comment"] as? String
{
fetchedModules.append(CModules(custid: custid, modulcat: modulcat, modulname: modulname, comment: comment))
print(custid)
print(modulcat)
print(modulname)
print(comment)
print("---------------------")
}
}// for end
// ... somehow set data source array for your table view
self.tableView.reloadData()
}// dispatch
}
)} // func end

Answer 1

Because fetchJSON returns immediately, before the JSON is downloaded. The effect is that the DispatchGroup is entereed and left right away, without waiting for the JSON:
group.enter()
self.fetchJSON() // returns immediately
group.leave() // the JSON has yet to be downloaded
To wait until the JSON has arrived, add a completion handler to fetchJSON:
override func viewDidLoad() {
group.enter()
self.fetchJSON {
group.notify(queue: .main) {
print("JSON call finished")
}
group.leave()
}
}
// Change the data type of the completion handler accordingly
func fetchJSON(completionHandler: #escaping (Data?) -> Void) {
// ...
networkService.downloadJSONasync(searchItem: AppConstants.appString.startString) { fetchedData in
defer { completionHandler(fetchedData) }
// ...
}
)
Using defer ensures that the completion handler will always be called, no matter how the outer closure returns. I'm not clear why you use a DispatchGroup here as there is no waiting, but I kept it in place to answer your question.

Answer 2

Your table view doesn't have any data from the beginning because any data hasn't been fetched yet. So it's ok, that table view has no cells. You just need to reloadData of your table view since now you appended elements to table view's data source array and now you should show this data.
Please, don't use DispatchGroup for this, just use your completion parameter of your method and inside of the completion closure after data are received set data source array for table view and then ... reload data of table view
downloadJSONasync(searchItem: "someString") { dictionary in
DispatchQueue.main.async { // don't forget that your code doesn't run on the main thread
// ... somehow set data source array for your table view
self.tableView.reloadData()
}
}
Note that you should avoid using NSDictonary and you should rather use Dictionary. Also from Swift 4+ you can use Codable instead of JSONSerialization.