无法在 swift 中检查具有"null"值的切换案例

Question

要检查多个条件，我使用switch statements

代码:这里所有的情况都很好，但是。如果user_details?.rating = ""是空的，那么我需要显示默认情况，我的意思是所有的星星都是空的，但它会去case _ where user_details?.rating ?? "" <= "1.9"为什么?

如果我强制打开case _ where (user_details?.rating)! <= "1.9":，那么当到达这个

时应用程序会崩溃user_details ?。评级= "那么如何显示所有的星星为空

请指路

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
}

Answer 1

应用程序崩溃，因为你是force unwrappinguser_details?.rating基本上说:这个属性持有的值不是nil，如果我错了，崩溃的应用程序.

尝试在switch语句之前安全地打开user_details?.rating:

1. Usingif let:

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

2. Usingguard let:

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

3. 提供默认值:

   let rating = user_details?.rating ?? ""
   

你的代码看起来像:

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
}