Can't update Array within a function - ios

Question

Hey I'm working with Swift 2 and I'm trying to make a method that returns an array of strings of IDs downloaded from a database through a query. My problem is that within the function I cannot update my Array, meaning that I can access the downloaded information from the server but I cannot append it to my array for some reason. Or better, I can, but it doesn't really do anything. My array seems to stay empty.
func ATMsAroundMe(myLocation : PFGeoPoint) -> [String]{
var results = [String]()
let query = PFQuery(className: "ATMs")
query.whereKey("location", nearGeoPoint: myLocation, withinMiles: 5)
query.limit = 10
query.findObjectsInBackgroundWithBlock { (atms: [PFObject]?, error: NSError?) -> Void in
if (error == nil) {
for atm in atms! {
print(atm.objectId) //Works!
results.append(atm.objectId!) //Doesn't work
}
} else {
// Log details of the failure
}
}
print(results) //Prints "[]"
return results
}
So yeah if you have any suggestion or any idea on what am I doing wrong it'd be really helpful and appreciated if you could let me know.
Thanks.

Answer 1

The reason why results is not updated is because it is updated in another block scope. So updated values persists only in that block scope. To get the updated result you would need to use closures or __block in variable declaration in Objective-c.
which is quite nicely explained here here in BLOCKS VS CLOSURES

Answer 2

The issue here is that the call-
query.findObjectsInBackgroundWithBlock
This is an asynchronous call, and thus, your method simply returns as it does not wait for the results that will be returned by this asynchronous call. Thus you will need to think of an asynchronous API ATMsAroundMe in the form-
func ATMsAroundMe(myLocation : PFGeoPoint, completionHandler:(Bool,[String]?) ->Void){
let query = PFQuery(className: "ATMs")
query.whereKey("location", nearGeoPoint: myLocation, withinMiles: 5)
query.limit = 10
query.findObjectsInBackgroundWithBlock { (atms: [PFObject]?, error: NSError?) -> Void in
if (error == nil) {
for atm in atms! {
print(atm.objectId) //Works!
var results = [String]()
results.append(atm.objectId!)
completionHandler(true, results)
}
} else {
// Report the failure
completionHandler(false, nil)
}
}
}
You can now call this API like-
ATMsAroundMe(myLocation){(success :Bool, results:[String]?) in
if(success){
if let results = results {
//Process results
}
}
}
Synchronous solution:
func ATMsAroundMe(myLocation : PFGeoPoint) -> [String]{
var results = [String]()
let query = PFQuery(className: "ATMs")
query.whereKey("location", nearGeoPoint: myLocation, withinMiles: 5)
query.limit = 10
//Declare a semaphore to help us wait until the background task is completed.
let sem = dispatch_semaphore_create(0);
query.findObjectsInBackgroundWithBlock { (atms: [PFObject]?, error: NSError?) -> Void in
if (error == nil) {
for atm in atms! {
print(atm.objectId) //Works!
results.append(atm.objectId!)
dispatch_semaphore_signal(sem);
}
} else {
// Log details of the failure
dispatch_semaphore_signal(sem);
}
}
dispatch_semaphore_wait(sem, DISPATCH_TIME_FOREVER);
print(results) //Should print your results
return results
}
Note Be care full in calling this synchronous API from your main thread, it can potentially stall the main thread until the call returns like any other synchronous calls.