Use callback to get JSON value from call to database - json

Question

I am trying to populate a UITable with a json result. I call a function that gets the json from the server and stores the result in NSDictionary. I want to be able to use this collection, and then populate a table. I run into a problem however because for the func numberOfRowsInSection I need the count of the collection, and since my json result is within another function inside a try/catch I cant seem to return the value.
This is what I have for the function which I call in ViewDidLoad():
func getSubjects() -> NSDictionary{
let myUrl = NSURL(string: "www.mydomain.com/script.php");
let request = NSMutableURLRequest(url:myUrl as! URL)
let user_id = UserDetails[0]
request.httpMethod = "POST";
let postString = "user_id=\(user_id)";
request.httpBody = postString.data(using: String.Encoding.utf8);
let task = URLSession.shared.dataTask(with: request as URLRequest){
data, response, error in
if error != nil {
print("error=\(error)")
return
}
var err: NSError?
do {
let json = try JSONSerialization.jsonObject(with: data!, options: .mutableContainers) as? NSDictionary
if let parseJSON = json {
let resultValue: NSDictionary = parseJSON["subjects"] as! NSDictionary
}
} catch let error as NSError {
err = error
print(err!);
}
}
task.resume();
}
If I print resultValue I get what I need, in this example being:
{
1 = (
Maths,
Lecture
);
2 = (
Science,
Lecture
);
3 = (
English,
Seminar
);
}
But the confusion is, how do I go about returning this value? and where? and how would I implement it in the table? If I try to return resultValue when I parse the JSON I get the error that it is unexpected non-void return in void function and if I try to return the value at the end of the function, I get an unresolved identifier error
I feel I am implementing this incorrectly. I have checked many tutorials on this, and no one seems to populate a table with a POST JSON so I don't know how to go about returning the value, or the proper method of implementation. Any help would be greatly appreciated!

Answer 1

The problem your having is that the dictionary hasn't been retrieved by the time you try to return it. You can use an asynchronous callback to get the dictionary after it has been retrieved from your database.
func getSubjects(callback: #escaping (NSDictionary)-> Void){
let myUrl = NSURL(string: "www.mydomain.com/script.php");
let request = NSMutableURLRequest(url:myUrl as! URL)
let user_id = UserDetails[0]
request.httpMethod = "POST";
let postString = "user_id=\(user_id)";
request.httpBody = postString.data(using: String.Encoding.utf8);
let task = URLSession.shared.dataTask(with: request as URLRequest){
data, response, error in
if error != nil {
print("error=\(error)")
return
}
var err: NSError?
do {
let json = try JSONSerialization.jsonObject(with: data!, options: .mutableContainers) as? NSDictionary
if let parseJSON = json {
let resultValue: NSDictionary = parseJSON["subjects"] as! NSDictionary
callback(resultValue)
}
} catch let error as NSError {
err = error
print(err!);
}
}
task.resume();
}
and then you would call this function like...
getSubjects(callback: {(resultValue)-> Void in
print(resultValue)
//here you could set your tableView data equal to the values in the dictionary that was just received and then call self.tableView.reloadData to update your table view
})
So perhaps in the viewDidLoad() function of you UITableViewController or in the viewDidAppear(_:), depending on the life cycle of your view, you would call getSubjects(...) as i have shown above and then when the callback is called you call self.tableView.reloadData() as I have explained in the function call. If you are unsure how to setup a tableview datasource and delegate then you should probably open another question for that
EDIT
In response to your comment asking how to use the retrieved value from your server as a variable available to your whole class, you could do something like this...
class: ExampleViewController {
var resultsDictionary: [Int: [String: String]]?
override func viewDidLoad(){
getSubjects(callback: {(resultValue)-> Void in
resultsDictionary = resultValue
})
}
//Use the actual getSubjects function I have already shown you above
func getSubjects(){...}
}