Result of call is unused

问题

Right below the second comment, I receive an error of "Result of call to 'taskForDeleteMethod' is unused. Why is this when I use the results and error in the closure following the call?

func deleteSession(_ completionHandlerForDeleteSession: @escaping (_ success: Bool, _ error: NSError?) -> Void) {

    /* 1. Specify parameters, method (if has {key}), and HTTP body (if POST) */
    // There are none...

    /* 2. Make the request */
    taskForDELETEMethod { (results, error) in

        /* 3. Send the desired value(s) to completion handler */
        if let error = error {
            print("Post error: \(error)")
            completionHandlerForDeleteSession(false, error)
        } else {
            guard let session = results![JSONKeys.session] as? [String: AnyObject] else {
                print("No key '\(JSONKeys.session)' in \(results)")
                return
            }

            if let id = session[JSONKeys.id] as? String {
                print("logout id: \(id)")
                completionHandlerForDeleteSession(true, nil)
            }
        }
    }
}

回答1:

You are confusing the results variable, which is, indeed, used inside the closure, and the result of the taskForDELETEMethod call itself, which is NSURLSessionDataTask object.

From the examples of using taskForDELETEMethod that I was able to find online it looks like it is perfectly OK to ignore the return value, so you can avoid this warning by assigning the result to _ variable, i.e.

let _ = taskForDELETEMethod {
    ... // The rest of your code goes here
}

回答2:

In earlier swift versions, you need not bother about the return value of a method. You may store it in any variable snd use it later or you may ignore it completely. Neither it gave any error nor a warning.

But in swift 3.0 you need to specify whether you want to ignore the returned value or use it.

1. If you want to use the returned value, you can create a variable/constant and store the value in it, i.e

let value = taskForDELETEMethod {
    // Your code goes here
}

2. If you want to ignore the returned value, you can use _ ,i.e

let _ = taskForDELETEMethod {
    // Your code goes here
}

来源：https://stackoverflow.com/questions/39692602/result-of-call-is-unused

标签

swift

function

closures