斯威夫特/ NSURLSession JSON下載/意外發現零而展開的可選值

Question

我下載數據，並在viewDidLoad()使用下面的代碼處理它：

NSURLSession.sharedSession().dataTaskWithURL(NSURL(string: "http://api.fixer.io/latest?base=USD")!, completionHandler: { (data, response, error) -> Void in 

     // Check if data was received successfully 
     if error == nil && data != nil { 
      do { 
      // Convert NSData to Dictionary where keys are of type String, and values are of any type 
      let json = try NSJSONSerialization.JSONObjectWithData(data!, options: NSJSONReadingOptions.MutableContainers) as! [String:AnyObject] 
      // Access specific key with value of type String 
      let dollarToDollarDict = json["rates"] as! NSDictionary 
      SpeedLog.print(dollarToDollarDict) 
      let dollarToDollarNSNumber = dollarToDollarDict.valueForKey("EUR") 
      let dollarToDollarString = String(dollarToDollarNSNumber) 
      SpeedLog.print(dollarToDollarString) 
      self.dollarExchangeRateStr = dollarToDollarString 
      } catch { 
       // Something went wrong 
      } 
      } 
     }).resume() 

print(dollarToDollarDict)打印出正確的解釋。 print(dollarToDollarString)打印出正確的期望字符串我想要爲：「可選（1.1303）」

但是，當我想有一個按鈕被按下，與使用String在dollarToDollarString：

func dollarBtnFunc(button: UIButton!) { 

    if dollarExchangeRateStr != "" { 
     SpeedLog.print("DOLLAR downloaded", dollarExchangeRateStr)    
     let dollarExchangeRateDouble = Double(dollarExchangeRateStr)! 
     tableCounterData = ["\(numberInt) $", "\(numberInt * dollarExchangeRateDouble) €"] 

    } else if dollarExchangeRateStr == "" { 
     SpeedLog.print("DOLLAR not downloaded, Dollar = 0.89") 
     tableCounterData = ["\(numberInt) $", "\(numberInt * 0.89) €"] 
    } 
    currentButton = "dollarBtnFunc" 
} 

我得到unexpectedly found nil while unwrapping an Optional value on let dollarExchangeRateDouble = Double(dollarExchangeRateStr)!。

我錯過了什麼？爲什麼字符串nil？我早些時候打印了正確的值...爲什麼一旦我想稍後使用它，爲什麼它不再是我的字符串的一部分？非常感謝幫助。

Answer 1

您的問題是在這兩條線：

let dollarToDollarNSNumber = dollarToDollarDict.valueForKey("EUR") 
let dollarToDollarString = String(dollarToDollarNSNumber) 

dollarToDollarDict.valueForKey("EUR")返回一個可選的，因此dollarToDollarNSNumber是可選的。

調用String初始化一個可選的給串"Optional(...)"所以當你以後該字符串傳遞給Double初始化失敗，因爲它在它的字母。

你需要做這樣的事情來解開你從valueForKey獲取的價值：

if let dollarToDollarNSNumber = dollarToDollarDict.valueForKey("EUR") { 
    let dollarToDollarString = String(dollarToDollarNSNumber) 
    SpeedLog.print(dollarToDollarString) 
    self.dollarExchangeRateStr = dollarToDollarString 
} else { 
    self.dollarExchangeRateStr = "" 
} 

Answer 2

我相信你的問題在於將dollarExchangeRateStr轉換爲Double。 Double（「foo」）返回一個可選項，可能會導致轉換失敗，因此它爲零，當解壓縮時會失敗。

變化

func dollarBtnFunc(button: UIButton!) { 

    if let exchangeRateStr = dollarExchangeRateStr { 
     SpeedLog.print("DOLLAR downloaded", exchangeRateStr)    
     let dollarExchangeRateDouble = Double(exchangeRateStr) { 
     tableCounterData = ["\(numberInt) $", "\(numberInt * dollarExchangeRateDouble) €"] 
     } 
     else { 
     print("your string to double conversion failed") 
     } 
    } else if dollarExchangeRateStr == "" { 
     SpeedLog.print("DOLLAR not downloaded, Dollar = 0.89") 
     tableCounterData = ["\(numberInt) $", "\(numberInt * 0.89) €"] 
    } 
    currentButton = "dollarBtnFunc" 
} 

它看起來從你的評論認爲，dollarExchangeRateStr也是可選的，這意味着你將需要打開那也請注意，如果讓exchangeRateStr = dollarExchangeRateStr

乾杯！