在URLSession.shared.dataTask期间从异步关闭存储数据

Question

I have tried two techniques to get data and fill an array from completion handlers. In both methods, the dataArray count is showing as 0. Whereas I'm able to put breakpoints and see that the array is being populated when the execution is within the closure:

First Method Tried:

In the below code, dataArray shows a count of zero even though it is populating the dataArray during execution of both inner and outer completionHandlers.

class ViewController: UIViewController { 
var dataArray = []
var urlOuter = URL(string: "outer.url.com/json")

override func viewDidLoad() {
    super.viewDidLoad()
    self.downloadTask()
    print(dataArray.count)
}

func downloadTask() {    

    let outerTask = URLSession.shared.dataTask(with: urlOuter!, completionHandler: {
    (data, response, error) in
    let parsedData = try JSONSerialization.jsonObject(with: content, options: .mutableContainers) as! [[String: Any]]

    for arr in parsedData! {
        var urlInner = URL(string: "http://inner.url/" + arr["url"] + ".com/json")
        let innerTask = URLSession.shared.dataTask(with: urlInner!, completionHandler: {
        (data, response, error) in
        let innerParsedData = try JSONSerialization.jsonObject(with: content, options: .mutableContainers) as! [[String: Any]]
        self.dataArray.append(innerParsedData)

        })
        innerTask.resume()   
    }// end of for loop

    })
    outerTask.resume()
}

}

Second Method Tried:

protocol MyDelegate{ func didFetchData(data:String)}
class ViewController: UIViewController { 
var dataArray = []
var urlOuter = URL(string: "outer.url.com/json")

override func viewDidLoad() {
    super.viewDidLoad()
    self.downloadTask()
    print(dataArray.count)
}

func didFetchData(data:String) {
    self.dataArray.append(data)
}

func downloadTask() {    

    let outerTask = URLSession.shared.dataTask(with: urlOuter!, completionHandler: {
    (data, response, error) in
    let parsedData = try JSONSerialization.jsonObject(with: content, options: .mutableContainers) as! [[String: Any]]

    for arr in parsedData! {
        var urlInner = URL(string: "http://inner.url/" + arr["url"] + ".com/json")
        let innerTask = URLSession.shared.dataTask(with: urlInner!, completionHandler: {
        (data, response, error) in
        let innerParsedData = try JSONSerialization.jsonObject(with: content, options: .mutableContainers) as! String
        self. didFetchData(data:innerParsedData)

        })
        innerTask.resume()   
    }// end of for loop

    })
    outerTask.resume()
}}}

Please help me understand how to get data out of the closures and store them in the array. Other solutions suggested are to use delegates and that is what I tried in method 2. Thank you.

Answer 1

You are querying the array in the viewDidLoad method right after you call to populate it in a async method.

check the results in the didFetchData() second method.

override func viewDidLoad() {
    super.viewDidLoad()
    self.downloadTask()
}


func didFetchData(data:String) {
    self.dataArray.append(data)
    // Check the count here!!
    print(dataArray.count)
}

You will need to change your protocol to:

protocol MyDelegate{ func didFetchData(dataArray: [])}

Then add the variable for the delegate:

var mDelegate = MyDelegate?

Then assign your result:

func didFetchDataCompeleted(dataArray: []) {
    // hand over the data to the delegate
    mDelegate?.didFetchData(self.dataArray)
}

now change the the call when the innerTask is completed within your closure code to

self.didFetchDataCompeleted(dataArray:self.dataArray)

or just call:

self.mDelegate?.didFetchData(self.dataArray)

when the innerTask is finished

Answer 2

I haven't look to closely but you seemed to be appending to the array correctly. Where you went wrong is asking for the count too soon. URL requests are run asynchronously and takes ages from the CPU's perspective:

self.downloadTask()     // this function run async
print(dataArray.count)  // nothing has been downloaded yet

try this:

func downloadTask(completionHandler: () -> Void) {      
    let outerTask = URLSession.shared.dataTask(with: urlOuter!) { data, response, error in
        let parsedData = try JSONSerialization.jsonObject(with: content, options: .mutableContainers) as! [[String: Any]]
        let group = DispatchGroup()

        for arr in parsedData! {
            var urlInner = URL(string: "http://inner.url/" + arr["url"] + ".com/json")
            group.enter()

            let innerTask = URLSession.shared.dataTask(with: urlInner!) { data, response, error in
                let innerParsedData = try JSONSerialization.jsonObject(with: content, options: .mutableContainers) as! [[String: Any]]

                // Appending to an array concurrently from multiple queues can lead to
                // weird error. The main queue is serial, which make sure that the
                // array is appended to once at a time
                DispatchQueue.main.async {
                    self.dataArray.append(innerParsedData)
                }
                group.leave()
            }
            innerTask.resume()
        }// end of for loop

        // Make sure all your inner tasks have completed
        group.wait(timeout: .forever)
        completionHandler()         
    }
    outerTask.resume()
}

override func viewDidLoad() {
    super.viewDidLoad()
    self.downloadTask() {
        print(dataArray.count)
    }
}