In method A, get data from callback in method B?

Question

I have a function that is build to get the latest items from a API. There are several other ones, with different functionality, but they all work the same. It looks like this:

func getLatest(pageNumber: Int) -> Array<Any>{
    let urlRequest = URL(string: baseUrl + latestUrl + String(pageNumber))

    let requestedData = doRequest(url: urlRequest!, completion: { data -> Void in
        // We have the data from doRequest stored in data, but now what?!

    })

    return allData
}

I also have a async method that handles the requests. That one looks like this:

func doRequest(url: URL, completion: @escaping ([[ApiItem]]) -> ()){
    var allItems = [[ApiItem]]()

    URLSession.shared.dataTask(with: url) { (data, response, error) in
        guard let data = data else { return }

        do{
            let json = try JSONSerialization.jsonObject(with: data, options: .mutableContainers) as! [String: AnyObject]

            let results = json["items"] as? [AnyObject]

            for r in results!{
                let item = ApiItem(json: r as! [String: Any])

                allItems.append([item])
            }

            completion(allItems)
        } catch let jsonError{
            print("JSON error: \(jsonError)")
        }
    }.resume()

The doRequest function works absolutely fine. It gets the data, parses the JSON and send it back to getLatest --> requestedData. The problem right now is, is that getLatest() is a function that needs to return the data that is stored in the data variable of requestedData.

How can I make it so, that the getLatest() function returns the data that is stored in the data in requestedData()?

Give to getLatest the same treatment you gave to doRequest following our comments in your [previous question](https://stackoverflow.com/q/46002165/2227743). *A callback in a callback*. Inside the call to doRequest (where you have your "now what" comment), call the completion handler you will have added to the signature of getLatest. // It works great and I often use it. Beware though to not nest too many callbacks, you would go straight to Inception hell. Two is ok. — Eric Aya, Sep 01 '17 at 15:44
Please post your code in an answer, it could be nice for future users to see the complete solution. — Eric Aya, Sep 02 '17 at 09:05

score 0 · Accepted Answer · answered Sep 02 '17 at 10:30

So I've fixed it by doing this:

In the first method, the one that actually needs the data from the API, I added this:

let trendingData = restApiManager.getLatest(pageNumber: 0, completion: { data -> Void in
   let item = data[indexPath.row]

   let url = NSURL(string: item.still)

   let data = NSData(contentsOf: url as! URL)
   if data != nil {
       cell.image.image = UIImage(data:data! as Data)
   }             
})

The getLatest() method looks like this:

func getLatest(pageNumber: Int, completion: @escaping ([ApiItem]) -> ()) {
    let urlRequest = URL(string: baseUrl + trendingUrl + String(pageNumber))

    let requestedData = doRequest(url: urlRequest!, completion: { data -> Void in
        // We have the data from doRequest stored in data
        var requestedData = data
        completion(requestedData)
    })
}

And finally, the doRequest() method looks like this:

func doRequest(url: URL, completion: @escaping ([ApiItem]) -> ()){
    var allItems = [ApiItem]()

    URLSession.shared.dataTask(with: url) { (data, response, error) in
        guard let data = data else { return }

        do{
            let json = try JSONSerialization.jsonObject(with: data, options: .mutableContainers) as! [String: AnyObject]

            let results = json["items"] as? [AnyObject]

            for r in results!{
                let item = ApiItem(json: r as! [String: Any])

                allItems.append(item)
            }

            completion(allItems)
        } catch let jsonError{
            print("JSON error: \(jsonError)")
        }
    }.resume()
}

score -1 · Answer 2 · answered Sep 01 '17 at 21:34

What I would do is use a Singleton in which I can store the Data

class DataManager:NSObject
{
  static let instance  = DataManager()
  override private init(){}


  var items:[ApiItem] = []
}

Then in your first method I would do this:

func getLatest(pageNumber: Int){
    let urlRequest = URL(string: baseUrl + latestUrl + String(pageNumber))

    let requestedData = doRequest(url: urlRequest!, completion: { data -> items in
        // We have the data from doRequest stored in data, but now what?!


      DataManager.instance.items = items
    })

}

This is how I usually go about this kind of situations. There may be better options though...

In method A, get data from callback in method B?

2 Answers2