ios - 在函数内将数据传递到 segue.destination 时出现问题

Question

我尝试在 Segue 调用上进行 2 个 API 调用，并最终将第二次调用中的数据数组传递给 CollectionView。通过第一次调用，我得到一个值 catID，我需要该值才能进行其他调用:

let searchEndpoint: String = MY_ENDPOINT

// Add auth key
let serviceCallWithParams = searchEndpoint + "?PARAMETER"
guard let url = URL(string: serviceCallWithParams) else {
    print("Error: cannot create URL")
    return
}

let urlRequest = URLRequest(url: url)

// setting up the session
let config = URLSessionConfiguration.default
let session = URLSession(configuration: config)

// making the request
let task = session.dataTask(with: urlRequest) {
    (data, response, error) in
    // error check
    guard error == nil else {
        print("error")
        print(error)
        return
    }
    // make sure we got data
    guard let responseData = data else {
        print("Error: did not receive data")
        return
    }
    // parse JSON
    do {
        guard let catData = try JSONSerialization.jsonObject(with: responseData, options: []) as? [String: AnyObject] else {
            print("error converting data to JSON")
            return
        }
        if let data = catData["data"] as? [String: Any] {
            if let array = data["categories"] as? [Any] {
                if let firstObject = array.first as? [String: Any] {
                    if let catId = firstObject["catId"] as? Int {
                        getTitles(catId: catId)
                    }
                }
            }
        }

    } catch  {
        print("error converting data to JSON")
        return
    }
}
task.resume()

然后 getTitles 函数如下所示:

func getTitles(catId: Int) {
    let catIdString = String(catId)
    let titlesEndPoint: String = MY_ENDPOINT + catIdString
    // Add auth key
    let titlesEndPointWithParams = titlesEndPoint + "?PARAMETER"
    guard let titlesUrl = URL(string: titlesEndPointWithParams) else {
        print("Error: cannot create URL")
        return
    }

    let titlesUrlRequest = URLRequest(url: titlesUrl)

    // set up the session
    let config = URLSessionConfiguration.default
    let session = URLSession(configuration: config)

    // make the request
    let task = session.dataTask(with: titlesUrlRequest) {
        (data, response, error) in
        // check for any errors
        guard error == nil else {
            print("error calling GET on listCategoryTitles")
            print(error)
            return
        }
        // make sure we got data
        guard let titlesData = data else {
            print("Error: did not receive data")
            return
        }
        // parse the JSON
        do {
            guard let allTitles = try JSONSerialization.jsonObject(with: titlesData, options: []) as? [String: AnyObject] else {
                print("error converting data to JSON")
                return
            }
            if let titlesJson = allTitles["data"] as? [String: Any] {
                if let titlesArray = titlesJson["titles"] as? Array<AnyObject> {
                    self.books = []
                    for (index, value) in titlesArray.enumerated() {
                        var book = Book()
                        book.bookTitle = value["title"] as? String
                        book.bookAuthor = value["author"] as? String
                        if let imageSource = value["_links"] as? Array<AnyObject> {
                            book.bookImageSource = imageSource[1]["href"] as? String
                        }
                        self.books?.append(book)
                    }
                }
            }

        } catch  {
            print("error converting data to JSON")
            return
        }
    }
    task.resume()
}

现在当我输入:

let resultsVC = segue.destination as? CollectionViewController
resultsVC?.books = self.books

外部函数，在目标 Controller 中，我在第一次单击时得到一个空数组作为输出，但在下一次单击时我得到正确的数据。当我尝试将其放入函数 "getTitles" 中时，CollectionViewController 中的输出每次都是 "nil" 。值得一提的是，我定义了 "books" 变量，如下所示:

主 Controller :

var books: [Book]? = []

集合 Controller :

var books: [Book]?

我创建了类型 [Book]，它基本上是一个对象，在单独的结构中包含 3 个字符串变量。

上面的所有代码都封装在

override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
    if segue.identifier == "ShowResults" {

任何帮助/指南将不胜感激!

Answer 1

当您进行 api 调用时，它将在后台执行，意味着异步，其中 prepare(for:sender:) 将同步调用。

现在，从您的问题来看，您似乎已经在 Storyboard中从 Button 到 ViewController 创建了 segue，因此在您从 api 获得响应之前，您已移至您的目标 Controller ，要解决您的问题，您需要创建从 Source ViewController 到 Destination ViewController 的 Segue 并设置其标识符。之后，在 for 循环在主线程上执行 segue 后，在 getTitles(catId: Int) 方法中。

for (index, value) in titlesArray.enumerated() {
    var book = Book()
    book.bookTitle = value["title"] as? String
    book.bookAuthor = value["author"] as? String
    if let imageSource = value["_links"] as? Array<AnyObject> {
        book.bookImageSource = imageSource[1]["href"] as? String
    }
    self.books?.append(book)
}

DispatchQueue.main.async {
    self.performSegue(withIdentifier: "ShowResults", sender: nil)
}

之后，在您的 prepare(for:sender:) 中进行如下更改。

override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
     if segue.identifier == "ShowResults" {
          let resultsVC = segue.destination as? CollectionViewController
          resultsVC?.books = self.books
     }
}