在展开可选值时意外发现 nil，即使它已分配了值

Question

我正在使用 Swift 从冠状病毒 API 获取 JSON。但是，当我尝试运行代码时，出现此错误。

致命错误：在展开可选值时意外发现 nil：第 22 行

我的部分代码是

override func viewDidLoad() {
        super.viewDidLoad()
        
        let url = "https://api.coronavirus.data.gov.uk/v1/data?filters=areaType=nation;areaName=england&structure={%22date%22:%22date%22,%22areaName%22:%22areaName%22,%22areaCode%22:%22areaCode%22,%22newCasesByPublishDate%22:%22newCasesByPublishDate%22,%22cumCasesByPublishDate%22:%22cumCasesByPublishDate%22,%22newDeathsByDeathDate%22:%22newDeathsByDeathDate%22,%22cumDeathsByDeathDate%22:%22cumDeathsByDeathDate%22}"
        getData(from: url)
        // Do any additional setup after loading the view.
    }
    
    private func getData(from url: String) {
        
        let getfromurl = URLSession.shared.dataTask(with: URL(string: url)!, completionHandler: {data, response, error in
            guard let data = data, error == nil else{
                print("Something Went Wrong")
                return
            }
            
            //Have data
            var result: Response?
            do {
                result = try JSONDecoder().decode(Response.self, from: data)
            }
            catch{
                print("failed to convert \(error.localizedDescription)")
            }
            
            guard let json = result else {
                return
            }
            
            print(json.data.date)
        })
        getfromurl.resume()
    
    }

第 22 行是：

let getfromurl = URLSession.shared.dataTask(with: URL(string: url)!, completionHandler: {data, response, error in

我很困惑，因为我认为这意味着 url 没有分配任何东西，但即使调试器也认为它有。

更新：

我可以获取数据，但一旦获取数据就会出错。错误是：

无法转换 valueNotFound(Swift.Int, Swift.DecodingError.Context(codingPath: [CodingKeys(stringValue: "data", intValue: nil), _JSONKey(stringValue: "Index 0", intValue: 0), CodingKeys( stringValue: "newDeathsByDeathDate", intValue: nil)], debugDescription: "预期的 Int 值，但发现为 null。"，underlyingError: nil))

转换失败表明它是在解码 JSON 和值时出错。

Answer 1

异常并不意味着url是nil，而是URL(string:url)是nil

您需要检查url 字符串是否为有效的url：

private func getData(from url: String) {
    guard let theURL = URL(string: url) else { print ("oops"); return }
    let getfromurl = URLSession.shared.dataTask(with: theURL, completionHandler: {data, response, error in
       /* ... */
    }
}

更新

既然现在给出了 url 字符串：问题是花括号；它们在RFC1738 中被标记为不安全，应替换为%7b（打开）和%7d（关闭），因此：

let url = "https://api.coronavirus.data.gov.uk/v1/data?filters=areaType=nation;areaName=england&structure=%7b%22date%22:%22date%22,%22areaName%22:%22areaName%22,%22areaCode%22:%22areaCode%22,%22newCasesByPublishDate%22:%22newCasesByPublishDate%22,%22cumCasesByPublishDate%22:%22cumCasesByPublishDate%22,%22newDeathsByDeathDate%22:%22newDeathsByDeathDate%22,%22cumDeathsByDeathDate%22:%22cumDeathsByDeathDate%22%7d"

let theUrl = URL(string: url)
print (theUrl)

这也是official API documentation中的一个例子：

curl -si 'https://api.coronavirus.data.gov.uk/v1/data?filters=areaType=nation;areaName=england&structure=%7B%22name%22:%22areaName%22%7D'

Answer 2

网址编码不正确。浏览器有自己的方式来对 URL 进行编码，但 URL(string: 根本不进行任何编码。

对于这种复杂的 URL，建议使用URLComponents/URLQueryItem 创建它。 URLComponents 的好处是它代表您处理编码

let structure = """
{"date":"date","areaName":"areaName","areaCode":"areaCode","newCasesByPublishDate":"newCasesByPublishDate","cumCasesByPublishDate":"cumCasesByPublishDate","newDeathsByDeathDate":"newDeathsByDeathDate","cumDeathsByDeathDate":"cumDeathsByDeathDate"}
"""

var components = URLComponents(string: "https://api.coronavirus.data.gov.uk")!
components.path = "/v1/data"
components.queryItems = [URLQueryItem(name: "filters", value: "areaType=nation;areaName=england"),
                         URLQueryItem(name: "structure", value: structure)]

if let url = components.url {
    print(url)
}

旁注：

从不在 JSONDecoder 捕获块中打印 error.localizedDescription。它只向您显示一条无意义的通用错误消息。永远print(error)

Answer 3

对 getData 方法进行了更改，请尝试一次

private func getData(from url: String) {
    
    guard let validUrl  = URL(string: url) else {
        return
    }
    let getfromurl = URLSession.shared.dataTask(with: validUrl, completionHandler: {data, response, error in
        guard let data = data, error == nil else{
            print("Something Went Wrong")
            return
        }
        
        //Have data
        var result: Response?
        do {
            result = try JSONDecoder().decode(Response.self, from: data)
        }
        catch{
            print("failed to convert \(error.localizedDescription)")
        }
        
        guard let json = result else {
            return
        }
        
        print(json.data.date)
    })
    getfromurl.resume()

}