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iOS Build a Weather App with Swift Managing Complexity Methods with Closures

Posted by Sheep McSheep
Sheep McSheep
846 Points

Completion handler syntax

let dataTask = session.dataTaskWithRequest(request) {
     (let data, let response, let error) in
}

In the dataTaskWithRequest completion handler closure, what is the purpose of "let" in front of data, response, and error parameters? Removing it doesn't seem to cause any errors.

2 Answers

Dominic Bryan
Dominic Bryan
14,452 Points
Dominic Bryan
Dominic Bryan
14,452 Points

Without typing a HUGE answer, you are correct it makes no difference to the interpreter. However ironically enough the "let" is purely readability in this case.

Because the completionHandler creates url, response and error, they could be variables or constants. Pasan simply specified them to be constants, its honestly more for readability I believe and not overwriting the previous url constant declared in the global scope (They will be the same value but different in their respective scopes).

Sheep McSheep
Sheep McSheep
846 Points

Thanks! Just trying to get a handle on the fifty million different ways swift closures can be written.

Rafael Nicoleti
Rafael Nicoleti
87 Points
Rafael Nicoleti
Rafael Nicoleti
87 Points

This solution it's work

func downloadJSONFromURL(completion: JSONDictionaryCompletion) {

    let request = NSURLRequest(URL: queryURL)


    let task = session.dataTaskWithRequest(request) {
        (data:NSData?, response:NSURLResponse?, error:NSError?) -> Void in

        if error != nil {
            completion(nil)
        }

        if let data = data {
            do
            {
                let object = try NSJSONSerialization.JSONObjectWithData(data, options: .AllowFragments) as! [String: AnyObject]
                completion(object)
            } catch {
                completion(nil)
            }
        } else {
            completion(nil)
        }
    }

    task?.resume()

}
Dominic Bryan
Dominic Bryan
14,452 Points
Dominic Bryan
Dominic Bryan
14,452 Points

This is a working piece of code, but I think there is slight confusion around your answer and the question asked. Sheep McSheep wanted to understand why the parameters in the completion had "let" (constant) called in front of them.

But keep it up man, its good to try answer peoples questions on the forum.