Where to place a completionHandler when inside loops? - ios

Question

I'm using a completionHandler in this function, however it's nested within several for loops (below). The problem is the handler where it is now gets called every time the loop it's in runs, whereas I only want the handler to pass in the Set when the entire function has completed processing. If I place it outside of the loop, then it gets called too early and is empty. What should I do here?
Right now when I print to the console to test it prints:
Set item 1
Set item 1, 2
Set item 1, 2, 3 etc.
struct RekoRequest {
public func getRekos(rekoType: rekoCategory, handler: #escaping (Set<String>) -> Void) {
var urls = [NSURL]()
var IDs = Set<String>()
TwitterRequest().fetchTweets(searchType: "things") { result in
guard let tweets = result as? [TWTRTweet] else {print("Error in getRekos receiving tweet results from TwitterRequest.fetchTweets"); return}
for tweet in tweets {
let types: NSTextCheckingResult.CheckingType = .link
let detector = try? NSDataDetector(types: types.rawValue)
guard let detect = detector else { print("NSDataDetector error"); return }
let matches = detect.matches(in: text, options: .reportCompletion, range: NSMakeRange(0, (text.characters.count)))
for match in matches {
if let url = match.url {
guard let unwrappedNSURL = NSURL(string: url.absoluteString) else {print("error converting url to NSURL");return}
//Show the original URL
unwrappedNSURL.resolveWithCompletionHandler {
guard let expandedURL = URL(string: "\($0)") else {print("couldn't covert to expandedURL"); return}
guard let urlDomain = expandedURL.host else { print("no host on expandedURL"); return }
switch urlDomain {
case "www.somesite.com":
let components = expandedURL.pathComponents
for component in components {
if component == "dp" {
guard let componentIndex = components.index(of: component) else {print("component index error"); return}
let IDIndex = componentIndex + 1
let ID = components[IDIndex]
//Filter out Dups and add to Set
IDs.insert(ID)
handler(IDs)
print(ID) //this prints multiple sets of IDs, I only want one when the function is finished completely
}
}
break;
default:
break;
}
}
} else { print("error with match.url") }
} //for match in matches loop
} //for tweet in tweets loop
}
}
}
// Create an extension to NSURL that will resolve a shortened URL
extension NSURL
{
func resolveWithCompletionHandler(completion: #escaping (NSURL) -> Void)
{
let originalURL = self
let req = NSMutableURLRequest(url: originalURL as URL)
req.httpMethod = "HEAD"
URLSession.shared.dataTask(with: req as URLRequest)
{
body, response, error in completion(response?.url as NSURL? ?? originalURL)
}
.resume()
}
}

Answer 1

Call your completion handler after the for loop.
for component in components {
if component == "dp" {
...
}
}
handler(IDs)
Important: The handler should be called outside of the for loop, but within the TwitterRequest().fetchTweets() trailing closure.
Approaches to handling an empty set
Your IDs are being initialized to an empty set. Only after meeting certain conditions within your for loop are values being inserted into this set. If these conditions aren't met, then your IDs set will be empty.
If this is undesirable, then you will have to either make changes to your completion handler or alter your conditional logic so that you always get a non-empty set.
One approach might be to have optional set in your callback. Something like:
(Set<String>?) -> Void
If IDs are empty, then callback with a nil and have your calling code handle the possibility of a nil set.
Another approach might be to create an enum to encapsulate your result and use this in your callback. Something like:
Enum
enum Result {
case success(Set<String>)
case failure
}
Callback
handler: (Result) -> Void
Usage
handler(.success(IDs))
// or
handler(.failure)
Calling Code
getReckos(rekoType: .someType) { result in
switch result {
case .success(let IDs):
// Use IDs
case .failure:
// Handle no IDs
}
}