How to know when asynchronous request has completed using a closure? - ios

Question

I am using Alamofire to download data and parse it with JSON. I know that in order to announce that data is available we should use closures and no the NotificationCenter. I do not understand closures though. How would I use a closure to reload a table view once the request has completed?
Here is the code.
func downloadEvents() {
let coreDataObject = CoreDataMethods()
Alamofire.request(URL(string:URL)!)
.responseJSON { response in
switch response.result {
case .success:
// we create the model object
let downloadedEvent = EventModel()
/* we have new data remove old data from core data*/
coreDataObject.deleteData(entityArgument: "Event")
events.removeAll() // remove the data from array this is no longer needed FIX
for JSONData in response.result.value as! [Dictionary<String, Any>] {
/* get the data from JSON and store in the event model*/
downloadedEvent.eTitle = JSONData[downloadedEvent.titleString] as? String
downloadedEvent.eDate = JSONData[downloadedEvent.dateString] as? String
downloadedEvent.eDescription = JSONData[downloadedEvent.descriptionString] as? String
downloadedEvent.eLocation = JSONData[downloadedEvent.locationline1String] as? String
downloadedEvent.eLocation2 = JSONData[downloadedEvent.locationline2String] as? String
/* if the event has an image save the url*/
if let image = JSONData[downloadedEvent.imageString] as? String {
downloadedEvent.eImageURL = image
} else {
/* else we save the default image url */
downloadedEvent.eImageURL = downloadedEvent.defaultImageURL
}
coreDataObject.save(eventParam: downloadedEvent)
}
/* post notification to reload table view FIX */
NotificationCenter.default.post(name: RELOAD_NOTIFICATION, object: nil)
case .failure(let error):
print("ALAMO REQUEST FIALED: \(error)")
}
}
}

Answer 1

Here is the downloadEvents function with the ability to notify the caller it was successful:
func downloadEvents(completion: #escaping (Bool, String?)-> Void) {
let coreDataObject = CoreDataMethods()
Alamofire.request(URL(string:URL)!)
.responseJSON { response in
switch response.result {
case .success:
// we create the model object
let downloadedEvent = EventModel()
/* we have new data remove old data from core data*/
coreDataObject.deleteData(entityArgument: "Event")
events.removeAll() // remove the data from array this is no longer needed FIX
for JSONData in response.result.value as! [Dictionary<String, Any>] {
/* get the data from JSON and store in the event model*/
downloadedEvent.eTitle = JSONData[downloadedEvent.titleString] as? String
downloadedEvent.eDate = JSONData[downloadedEvent.dateString] as? String
downloadedEvent.eDescription = JSONData[downloadedEvent.descriptionString] as? String
downloadedEvent.eLocation = JSONData[downloadedEvent.locationline1String] as? String
downloadedEvent.eLocation2 = JSONData[downloadedEvent.locationline2String] as? String
/* if the event has an image save the url*/
if let image = JSONData[downloadedEvent.imageString] as? String {
downloadedEvent.eImageURL = image
} else {
/* else we save the default image url */
downloadedEvent.eImageURL = downloadedEvent.defaultImageURL
}
coreDataObject.save(eventParam: downloadedEvent)
}
completion(true, nil)
/* post notification to reload table view FIX */
//NotificationCenter.default.post(name: RELOAD_NOTIFICATION, object: nil)
case .failure(let error):
print("ALAMO REQUEST FIALED: \(error)")
completion(false, "ALAMO REQUEST FIALED: \(error)")
}
}
}
You would then call the function like this:
func reloadTable(){
downloadEvents { (success, errMsg) in
if success{
DispatchQueue.main.async {
self.tableView.reloadData()
}
}
else{
let alertMessage: String
if let err = errMsg{
alertMessage = err
}
else{
alertMessage = "An unknown error occurred."
}
let alert = UIAlertController.init(title: "Request Failed", message: alertMessage, preferredStyle: .alert)
alert.addAction(UIAlertAction(title: "Ok", style: UIAlertActionStyle.cancel, handler: nil))
DispatchQueue.main.async {
self.present(alert, animated: true, completion: nil)
}
}
}
}