在 URLSession.shared.dataTask 期间存储来自异步闭包的数据

Question

我尝试了两种技术来从完成处理程序中获取数据并填充数组。在这两种方法中，dataArray 计数都显示为 0。而我可以放置断点并看到在闭包内执行时正在填充数组：

尝试的第一种方法：

在下面的代码中，dataArray 显示计数为零，即使它在内部和外部完成处理程序执行期间填充 dataArray。

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()
}

}

尝试了第二种方法：

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它们存储在数组中。建议的其他解决方案是使用委托，这就是我在方法 2 中尝试的方法。谢谢。

Answer 1

您在调用 viewDidLoad 方法后立即在异步方法中填充数组。

检查didFetchData()第二种方法中的结果。

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?

然后分配你的结果：

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

现在将关闭代码中innerTask 完成时的调用更改为

self.didFetchDataCompeleted(dataArray:self.dataArray)

或者直接打电话：

self.mDelegate?.didFetchData(self.dataArray)

当innerTask 完成时

Answer 2

我没有仔细看，但您似乎正确地附加到数组中。你出错的地方是过早地要求计数。 URL 请求是异步运行的，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

试试这个：

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)
    }
}