Swift 3：无法在iOS中使用完成处理程序

Question

I have created one function using completion handler in NSObject class for consumption of web services. However I am not getting a way to call that function with handler return.

func getUser(url:String, completionHandler: @escaping (NSDictionary?, NSError?) -> ()) {

    let config = URLSessionConfiguration.default // Session Configuration
    let session = URLSession(configuration: config) // Load configuration into Session
    let url = URL(string: url)!

    let task = session.dataTask(with: url, completionHandler: {
        (data, response, error) in

        if error != nil {
           completionHandler(nil, error as NSError?)
        } else {
            do {
                if let json = try JSONSerialization.jsonObject(with: data!, options: .allowFragments) as? [NSDictionary: Any] {
                    completionHandler(json as NSDictionary?,nil)
                }
            } catch {
                print("error in JSONSerialization")
            }
        }
    })
    task.resume()
}

Answer 1

You should make sure that your completionHandler is called in every cases: for example, when the JSONSerialization throws, you catch and print the error, but you're not calling your completionHandler . The same if the JSON result is nil

ADDING

You can handle it in this way:

do {
    let json = try JSONSerialization.jsonObject(with: data!, options: .allowFragments) as? [NSDictionary: Any]
    completionHandler(json,nil)
} catch(let error) {
    print(error)
    completionHandler(nil, error)
}