解析 json 时使用未解析的标识符

Question

let url = URL(string: "https://api.coindesk.com/v1/bpi/currentprice.json")
    let task = URLSession.shared.dataTask(with: url!) { (data, response, error) in
        if error != nil {
            print ("Error!")
        } else {
            if let content = data {
                do {
                    let myJson = try JSONSerialization.jsonObject(with: content) as! [String:Any]
                    if let rates = myJson["bpi"] as? [String:Any] {
                        if let currency = rates["USD"] as? [String:Any] {
                            if let btc = currency["rate"] as? String {
                                DispatchQueue.main.async{
                                    self.bitcoinlabel.text = "$" + btc

                                }
                            }
                        }
                    }
                }
                catch{
                    print(error)
                }
            }
        }
    }
    task.resume()

    let btcprice: Double = btc

问题出在最后一行（使用未解析的标识符“btc”）。如何导出从数据中获取的值，以便在代码块之外或其他函数中使用。

Answer 1

btc 在外部范围内不可访问。你应该只在它被定义的范围内做任何你想做的事情。

像这样：

if let btc = currency["rate"] as? String {
    DispatchQueue.main.async{
        self.bitcoinlabel.text = "$" + btc
        let btcprice: Double = btc
        // Do Stuff here with btcprice
        // OR Call another func from here like someFunctionWhichUsesTheBtcprice()
        // OR Add CompletionHandler in the Method definition like : completion: @escaping (()->()) and pass the function you want to call after you get the response with the method and call that function here like: completion()
    }
}

此外，如果您将 btcprice 声明为像 var btcprice: Double? 这样的类变量，那么您应该只在该块中分配 self.btcprice = btc。但是无论你想用它做什么，都应该在 API 返回响应之后完成，否则 btcprice 将是 nil。

更新代码：

你可以这样使用：

func callAPI(completion: @escaping (Double) -> ()) {
    // Call Your API Here...
    let url = URL(string: "https://api.coindesk.com/v1/bpi/currentprice.json")
    let task = URLSession.shared.dataTask(with: url!) { (data, response, error) in
        if error != nil {
            print ("Error!")
        } else {
            if let content = data {
                do {
                    let myJson = try JSONSerialization.jsonObject(with: content) as! [String:Any]
                    if let rates = myJson["bpi"] as? [String:Any] {
                        if let currency = rates["USD"] as? [String:Any] {
                            if let btc = currency["rate"] as? String {
                                DispatchQueue.main.async{
                                    self.bitcoinlabel.text = "$" + btc
                                    if let btcDoubleValue = Double(btc) {
                                        // This will be called when we will get the response and btc value
                                        completion(btcDoubleValue)
                                    }
                                }
                            }
                        }
                    }
                }
                catch{
                    print(error)
                }
            }
        }
    }
    task.resume()
}

这样称呼：

self.callAPI() { (btcprice) in
   // Do your stuff here
}

Answer 2

您可以将其声明为实例变量，但请记住对 API 的调用是异步的，因此在响应返回之前它的值不会是真实的，请在类范围内声明 btc

 var btc: Double?

然后

                if let btc = currency["rate"] as? String {
                            DispatchQueue.main.async{
                                self.bitcoinlabel.text = "$" + btc
                                self.btc = btc 
                            }
                        }