修改全局变量内部闭包（Swift 4）

Question

I am trying to modify the global variable currentWeather (of type CurrentWeather) using this function, which is meant to update said variable with the information retrieved from the URL and return a bool signifying its success. However, the function is returning false, as currentWeather is still nil. I recognize that the dataTask is asynchronous, and that the task is running in the background parallel to the application, but I don't understand what this means for what I'm trying to accomplish. I also am unable to update currentWeather after the do block, as weather is no longer recognized after exiting the block. I did try using "self.currentWeather", but was told it was an unresolved identifier (perhaps because the function is also global, and there is no "self"?).

The URL is not currently valid because I took out my API key, but it is working as expected otherwise, and my CurrentWeather struct is Decodable. Printing currentWeatherUnwrapped is also consistently successful.

I did look around Stack Overflow and through Apple's official documentation and was unable to find something that answered my question, but perhaps I wasn't thorough enough. I'm sorry if this is a duplicate question. Direction to any further relevant reading is also appreciated! I apologize for the lack of conformity to best coding practices - I'm not very experienced at this point. Thank you all so much!

func getCurrentWeather () -> Bool {
let jsonUrlString = "https://api.wunderground.com/api/KEY/conditions/q/\(state)/\(city).json"

guard let url = URL(string: jsonUrlString) else { return false }

URLSession.shared.dataTask(with: url) { (data, response, err) in
    // check error/response

    guard let data = data else { return }

    do {
        let weather = try JSONDecoder().decode(CurrentWeather.self, from: data)
        currentWeather = weather
        if let currentWeatherUnwrapped = currentWeather {
            print(currentWeatherUnwrapped)
        }
    } catch let jsonErr {
        print("Error serializing JSON: ", jsonErr)
    }

    // cannot update currentWeather here, as weather is local to do block

    }.resume()

return currentWeather != nil
}

Answer 1

When you do an asynchronous call like this, your function will return long before your dataTask will have any value to return. What you need to do is use a completion handler in your function. You can pass it in as a parameter like this:

func getCurrentWeather(completion: @escaping(CurrentWeather?, Error?) -> Void) {
    //Data task and such here
    let jsonUrlString = "https://api.wunderground.com/api/KEY/conditions/q/\(state)/\(city).json"

    guard let url = URL(string: jsonUrlString) else { return false }

    URLSession.shared.dataTask(with: url) { (data, response, err) in
    // check error/response

        guard let data = data else { 
            completion(nil, err)
            return
        }

        //You don't need a do try catch if you use try?
        let weather = try? JSONDecoder().decode(CurrentWeather.self, from: data)
        completion(weather, err)
    }.resume()

}

Then calling that function looks like this:

getCurrentWeather(completion: { (weather, error) in
    guard error == nil, let weather = weather else { 
        if weather == nil { print("No Weather") }
        if error != nil { print(error!.localizedDescription) }
        return
    }
    //Do something with your weather result
    print(weather)
})

Answer 2

You fundamentally misunderstand how async functions work. You function returns before the URLSession's dataTask has even begun to execute. A network request may take multiple seconds to complete. You ask it to fetch some data for you, give it a block of code to execute ONCE THE DATA HAS DOWNLOADED, and then go on with your business.

You can be certain that the line after the dataTask's resume() call will run before the new data has loaded.

You need to put code that you want to run when the data is available inside the data task's completion block. (Your statement print(currentWeatherUnwrapped) will run once the data has been read successfully.)

Answer 3

All you need is a closure.

You cant have synchronous return statement to return the response of web service call which in itself is asynchronous in nature. You need closures for that.

You can modify your answer as below. Because you have not answered to my question in comment I have taken liberty to return the wether object rather than returning bool which does not make much sense.

func getCurrentWeather (completion : @escaping((CurrentWeather?) -> ()) ){
        let jsonUrlString = "https://api.wunderground.com/api/KEY/conditions/q/"

        guard let url = URL(string: jsonUrlString) else { return false }

        URLSession.shared.dataTask(with: url) { (data, response, err) in
            // check error/response

            guard let data = data else { return }

            do {
                let weather = try JSONDecoder().decode(CurrentWeather.self, from: data)
                CurrentWeather.currentWeather = weather
                if let currentWeatherUnwrapped = currentWeather {
                    completion(CurrentWeather.currentWeather)
                }
            } catch let jsonErr {
                print("Error serializing JSON: ", jsonErr)
                completion(nil)
            }

            // cannot update currentWeather here, as weather is local to do block

            }.resume()
    }

Assuming currentWeather is a static variable in your CurrentWeather class you can update your global variable as well as return the actual data to caller as shown above

EDIT:

As pointed out by Duncan in comments below, the above code executes the completion block in background thread. All the UI operations must be done only on main thread. Hence its very much essential to switch the thread before updating the UI.

Two ways :

1- Make sure you execute the completion block on main thread.

DispatchQueue.main.async {
      completion(CurrentWeather.currentWeather)
}

This will make sure that whoever uses your getCurrentWeather in future need not worry about switching thread because your method takes care of it. Useful if your completion block contains only the code to update UI. Lengthier logic in completion block with this approach will burden the main thread.

2 - Else In completion block that you pass as a parameter to getCurrentWeather whenever you update UI elements make sure you wrap those statements in

DispatchQueue.main.async {
    //your code to update UI
}

EDIT 2:

As pointed out by Leo Dabus in comments below, I should have run completion block rather than guard let url = URL(string: jsonUrlString) else { return false } That was a copy paste error. I copied the OP's question and in a hurry din realize that there is a return statement.

Though having a error as a parameter is optional in this case and completely depends on how you designed your error handling model, I appreciate the idea suggested by Leo Dabus which is more general approach and hence updating my answer to have error as a parameter.

Now there are cases where we may need to send our custom error as well for example if guard let data = data else { return } returns false rather than simply calling return you may need to return a error of your own which says invalid input or something like that.

Hence I have taken a liberty to declare a custom errors of my own and you can as well use the model to deal with your error handling

enum CustomError : Error {
    case invalidServerResponse
    case invalidURL
}

func getCurrentWeather (completion : @escaping((CurrentWeather?,Error?) -> ()) ){
        let jsonUrlString = "https://api.wunderground.com/api/KEY/conditions/q/"

        guard let url = URL(string: jsonUrlString) else {
            DispatchQueue.main.async {
                completion(nil,CustomError.invalidURL)
            }
            return
        }

        URLSession.shared.dataTask(with: url) { (data, response, err) in
            // check error/response

            if err != nil {
                DispatchQueue.main.async {
                    completion(nil,err)
                }
                return
            }

            guard let data = data else {
                DispatchQueue.main.async {
                    completion(nil,CustomError.invalidServerResponse)
                }
                return
            }

            do {
                let weather = try JSONDecoder().decode(CurrentWeather.self, from: data)
                CurrentWeather.currentWeather = weather

                if let currentWeatherUnwrapped = currentWeather {
                    DispatchQueue.main.async {
                        completion(CurrentWeather.currentWeather,nil)
                    }
                }

            } catch let jsonErr {
                print("Error serializing JSON: ", jsonErr)
                DispatchQueue.main.async {
                    completion(nil,jsonErr)
                }
            }

            // cannot update currentWeather here, as weather is local to do block

            }.resume()
    }

Answer 4

As you pointed out, the data ask is async , meaning you do not know when it will be completed.

One option is to modify your wrapper function getCurrentWeather to be async as well by not providing a return value, but instead a callback/closure. Then you will have to deal with the async nature somewhere else though.

The other option which is what you probably want in your scenario is to make the data task synchronous like so:

func getCurrentWeather () -> Bool {
    let jsonUrlString = "https://api.wunderground.com/api/KEY/conditions/q/\(state)/\(city).json"

    guard let url = URL(string: jsonUrlString) else { return false }

    let dispatchGroup = DispatchGroup() // <===
    dispatchGroup.enter() // <===

    URLSession.shared.dataTask(with: url) { (data, response, err) in
        // check error/response

        guard let data = data else { 
            dispatchGroup.leave() // <===
            return 
        }

        do {
            let weather = try JSONDecoder().decode(CurrentWeather.self, from: data)
            currentWeather = weather
            if let currentWeatherUnwrapped = currentWeather {
                print(currentWeatherUnwrapped)
            }
            dispatchGroup.leave() // <===
       } catch let jsonErr {
           print("Error serializing JSON: ", jsonErr)
           dispatchGroup.leave() // <===
       }
       // cannot update currentWeather here, as weather is local to do block

    }.resume()

    dispatchGroup.wait() // <===

    return currentWeather != nil
}

The wait function can take parameters, which can define a timeout. https://developer.apple.com/documentation/dispatch/dispatchgroup Otherwise your app could be stuck waiting forever. You will then be able to define some action to present that to the user.

Btw I made a fully functional weather app just for learning, so check it out here on GitHub https://github.com/erikmartens/NearbyWeather . Hope the code there can help you for your project. It's also available on the app store.

EDIT: Please understand that this answer is meant to show how to make async calls synchronous. I am not saying this is good practice for handling network calls. This is a hacky solution for when you absolutely must have a return value from a function even though it uses async calls inside.