無法在swift中檢查具有“null”值的Switch案例

檢查我正在使用的多個條件switch statements

代碼：這里所有情況都很好，但是。如果user_details?.rating = ""是空的，那么我需要顯示默認情況，我的意思是所有的星星都是空的，但它會case _ where user_details?.rating ?? "" <= "1.9"為什么？

如果我確實強制展開case _ where (user_details?.rating)! <= "1.9":，那么應用程式在到達此位置時會崩潰

user_details?.rating = "" 那么如何將所有星星顯示為空

請指導我

switch user_details?.rating{

case "1.0":

    star1.image = UIImage(named: "star")

    star2.image = UIImage(named: "staremp")
    star3.image = UIImage(named: "staremp")
    star4.image = UIImage(named: "staremp")
    star5.image = UIImage(named: "staremp")
break
case _ where user_details?.rating ?? "" <= "1.9":

    star1.image = UIImage(named: "star")
    star2.image = UIImage(named: "star-half")

    star3.image = UIImage(named: "staremp")
    star4.image = UIImage(named: "staremp")
    star5.image = UIImage(named: "staremp")
break

case "2.0":

    star1.image = UIImage(named: "star")
    star2.image = UIImage(named: "star")

    star3.image = UIImage(named: "staremp")
    star4.image = UIImage(named: "staremp")
    star5.image = UIImage(named: "staremp")

 break

case _ where user_details?.rating ?? "" <= "2.9":

    star1.image = UIImage(named: "star")
    star2.image = UIImage(named: "star")
    star3.image = UIImage(named: "star-half")

    star4.image = UIImage(named: "staremp")
    star5.image = UIImage(named: "staremp")
 break


default:
    star1.image = UIImage(named: "staremp")
    star2.image = UIImage(named: "staremp")
    star3.image = UIImage(named: "staremp")
    star4.image = UIImage(named: "staremp")
    star5.image = UIImage(named: "staremp")

break

}

uj5u.com熱心網友回復：

該應用程式正在崩潰，因為您force unwrapping user_details?.rating基本上說：此屬性包含的值不是nil，如果我錯了，則崩潰應用程式。

user_details?.rating在 switch 陳述句之前嘗試安全地展開：

1. 使用if let：

   if let rating = user_details?.rating {
   }else {
   }
   

2. 使用guard let：

   guard let rating = user_details?.rating else {
       return
   }
   

3. 提供默認值：

   let rating = user_details?.rating ?? ""
   

然后您的代碼將如下所示：

guard let rating = user_details?.rating else {
    //do something if it is nil
    return
}

switch rating {
case "1.0":
    star1.image = UIImage(named: "star")
    star2.image = UIImage(named: "staremp")
    star3.image = UIImage(named: "staremp")
    star4.image = UIImage(named: "staremp")
    star5.image = UIImage(named: "staremp")
    break
case _ where user_details?.rating ?? "" <= "1.9":
    star1.image = UIImage(named: "star")
    star2.image = UIImage(named: "star-half")
    star3.image = UIImage(named: "staremp")
    star4.image = UIImage(named: "staremp")
    star5.image = UIImage(named: "staremp")
    break
case "2.0":
    star1.image = UIImage(named: "star")
    star2.image = UIImage(named: "star")
    star3.image = UIImage(named: "staremp")
    star4.image = UIImage(named: "staremp")
    star5.image = UIImage(named: "staremp")
    break
case _ where user_details?.rating ?? "" <= "2.9":
    star1.image = UIImage(named: "star")
    star2.image = UIImage(named: "star")
    star3.image = UIImage(named: "star-half")
    star4.image = UIImage(named: "staremp")
    star5.image = UIImage(named: "staremp")
    break
default:
    star1.image = UIImage(named: "staremp")
    star2.image = UIImage(named: "staremp")
    star3.image = UIImage(named: "staremp")
    star4.image = UIImage(named: "staremp")
    star5.image = UIImage(named: "staremp")
    break
}

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