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

Question

我尝试了两种技术来获取数据并从完成处理程序中填充数组。 在这两种方法中，dataArray的计数都显示为0。而我能够放置断点，并在执行在闭包内时看到正在填充数组：

第一种方法尝试过：

在下面的代码中，即使在内部和外部completionHandlers执行期间都在填充dataArray，dataArray也会显示零计数。

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()
}

}

尝试的第二种方法：

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()
}}}

请帮助我了解如何从闭包中获取数据并将其存储在数组中。 建议的其他解决方案是使用委托，这就是我在方法2中尝试过的方法。谢谢。

Answer 1

您在调用以异步方法填充数组后立即在viewDidLoad方法中查询该数组。

在didFetchData()第二种方法中检查结果。

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


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

您将需要将协议更改为：

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

然后为委托添加变量：

var mDelegate = MyDelegate?

然后分配结果：

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

现在，在您的关闭代码中完成innerTask ，将调用更改为

self.didFetchDataCompeleted(dataArray:self.dataArray)

或致电：

self.mDelegate?.didFetchData(self.dataArray)

当innerTask完成时

Answer 2

我没有仔细查看，但是您似乎正确地附加到了数组。 您出问题的地方是过早要求计数。 URL请求是异步运行的，并且从CPU的角度来看需要很长时间：

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

尝试这个：

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)
    }
}