swift - 如何在再次运行之前等待 Swift 的 URLSession 完成？

Question

可能是一个愚蠢的问题，但我是这方面的初学者。

下面的代码应该通过关键字搜索从 Google 图书中获取图书信息。然后它会检查结果并检查我在 Firebase 数据库中是否有匹配的 ISBN。它有效，但目前只能搜索 40 本书，因为这是每次搜索的 Google Books API 最大值。

幸运的是，我可以指定从哪里开始索引，并且还可以搜索接下来的 40 本书。不幸的是，我已经尝试了几个小时来了解 URLSession 是如何工作的。我尝试过的所有方法都向我表明，URLSession 块之后的代码不一定要等待会话完成。因此，如果我随后检查是否找到任何匹配项，甚至可能无法完成搜索。

我怀疑答案是在完成处理中，但到目前为止我的尝试都没有成功。下面是我的带有 URL 设置的代码，用于获取各种起始索引值。

var startingIndex = 0

        //encode keyword(s) to be appended to URL
        let query = query.addingPercentEncoding(withAllowedCharacters: .urlQueryAllowed)!
        let url = "https://www.googleapis.com/books/v1/volumes?q=\(query)&&maxResults=40&startIndex=\(startingIndex)"

        URLSession.shared.dataTask(with: URL(string: url)!) { (data, response, error) in
            if error != nil {
                print(error!.localizedDescription)
            }else{

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

                if let items = json["items"] as? [[String: AnyObject]] {

                    //for each result make a book and add title
                    for item in items {
                        if let volumeInfo = item["volumeInfo"] as? [String: AnyObject] {
                            let book = Book()
                            //default values
                            book.isbn13 = "isbn13"
                            book.isbn10 = "isbn10"
                            book.title = volumeInfo["title"] as? String

                            //putting all authors into one string
                            if let temp = volumeInfo["authors"] as? [String] {
                                var authors = ""
                                for i in 0..<temp.count {
                                    authors = authors + temp[i]
                                }
                                book.author = authors
                            }

                            if let imageLinks = volumeInfo["imageLinks"] as? [String: String] {
                                book.imageURL = imageLinks["thumbnail"]
                            }

                            //assign isbns
                            if let isbns = volumeInfo["industryIdentifiers"] as? [[String: String]] {

                                for i in 0..<isbns.count {

                                    let firstIsbn = isbns[i]
                                    if firstIsbn["type"] == "ISBN_10" {
                                        book.isbn10 = firstIsbn["identifier"]
                                    }else{
                                        book.isbn13 = firstIsbn["identifier"]
                                    }
                                }
                            }

                            //adding book to an array of books
                            myDatabase.child("listings").child(book.isbn13!).observeSingleEvent(of: .value, with: { (snapshot) in
                                if snapshot.exists() {
                                    if listings.contains(book) == false{
                                        listings.append(book)
                                    }
                                    DispatchQueue.main.async { self.tableView.reloadData() }
                                }
                            })
                            myDatabase.child("listings").child(book.isbn10!).observeSingleEvent(of: .value, with: { (snapshot) in
                                if snapshot.exists() {
                                    if listings.contains(book) == false{
                                        listings.append(book)
                                    }
                                    DispatchQueue.main.async { self.tableView.reloadData() }
                                }
                            })
                        }
                    }
                }
            }

            SVProgressHUD.dismiss()
            }.resume()

以下是我修改后的代码：

 func searchForSale(query: String, startingIndex: Int) {

        directionsTextLabel.isHidden = true
        tableView.isHidden = false
        listings.removeAll()
        DispatchQueue.main.async { self.tableView.reloadData() }
        SVProgressHUD.show(withStatus: "Searching")

        //clear previous caches of textbook images
        cache.clearMemoryCache()
        cache.clearDiskCache()
        cache.cleanExpiredDiskCache()


        let url = "https://www.googleapis.com/books/v1/volumes?q=\(query)&&maxResults=40&startIndex=\(startingIndex)"

        URLSession.shared.dataTask(with: URL(string: url)!) { (data, response, error) in
            if error != nil {
                print(error!.localizedDescription)
            }else{

                var needToContinueSearch = true

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

                if json["error"] == nil {

                    let totalItems = json["totalItems"] as? Int
                    if totalItems == 0 {
                        SVProgressHUD.showError(withStatus: "No matches found")
                        return
                    }

                    if let items = json["items"] as? [[String: AnyObject]] {

                        //for each result make a book and add title
                        for item in items {

                            if let volumeInfo = item["volumeInfo"] as? [String: AnyObject] {

                                let book = Book()
                                //default values
                                book.isbn13 = "isbn13"
                                book.isbn10 = "isbn10"
                                book.title = volumeInfo["title"] as? String

                                //putting all authors into one string
                                if let temp = volumeInfo["authors"] as? [String] {
                                    var authors = ""
                                    for i in 0..<temp.count {
                                        authors = authors + temp[i]
                                    }
                                    book.author = authors
                                }

                                if let imageLinks = volumeInfo["imageLinks"] as? [String: String] {
                                    book.imageURL = imageLinks["thumbnail"]
                                }

                                //assign isbns
                                if let isbns = volumeInfo["industryIdentifiers"] as? [[String: String]] {

                                    for i in 0..<isbns.count {

                                        let firstIsbn = isbns[i]
                                        //checks if isbns have invalid characters
                                        let isImproperlyFormatted = firstIsbn["identifier"]!.contains {".$#[]/".contains($0)}

                                        if isImproperlyFormatted == false {
                                            if firstIsbn["type"] == "ISBN_10" {
                                                book.isbn10 = firstIsbn["identifier"]
                                            }else{
                                                book.isbn13 = firstIsbn["identifier"]
                                            }
                                        }
                                    }
                                }

                                //adding book to an array of books
                                myDatabase.child("listings").child(book.isbn13!).observeSingleEvent(of: .value, with: { (snapshot) in
                                    if snapshot.exists() {
                                        if listings.contains(book) == false{
                                            listings.append(book)
                                            needToContinueSearch = false
                                        }
                                        DispatchQueue.main.async { self.tableView.reloadData() }
                                    }
                                })
                                myDatabase.child("listings").child(book.isbn10!).observeSingleEvent(of: .value, with: { (snapshot) in
                                    if snapshot.exists() {
                                        if listings.contains(book) == false{
                                            listings.append(book)
                                            needToContinueSearch = false
                                        }
                                        DispatchQueue.main.async { self.tableView.reloadData() }
                                        return
                                    }
                                    if startingIndex < 500 {
                                        if needToContinueSearch {
                                            let nextIndex = startingIndex + 40
                                            self.searchForSale(query: query, startingIndex: nextIndex)
                                        }
                                    }
                                })
                            }
                        }
                    }
                }else{
                    return
                }
            }

            SVProgressHUD.dismiss()
            }.resume()

        //hide keyboard
        self.searchBar.endEditing(true)
    }

标签： swiftfirebasensurlsessiongoogle-books

Answer 1

将 bool 变量声明为 isLoading，如果该函数正在加载，则不要触发 urlsession。希望下面的示例对您有所帮助。

var isLoading : Bool = false

func loadMore(with pageCount: Int){
    if isLoading { return }
    isLoading = true
    // call the network
    URLSession.shared.dataTask(with: URL(string: "xxxxx")!) { (data, response, error) in

        // after updating the data set isloding to false again
        // do the api logic here
        // 
        DispatchQueue.main.async {
            // self.items = downloadedItems 
            self.tableView.reloadData()
            self.isLoading = false
        }

    }.resume()

}